Xiaohui Fan

Interests

No activities entered.

Courses

Scholarly Contributions

Journals/Publications

• Barnett, R., Warren, S., Cross, N., Mortlock, D., Fan, X., Wang, F., & Hewett, P. (2021). "A complete search for redshift z {ensuremath{gtrsim} 6.5 quasars in the VIKING survey}". mnras, 501(2), 1663-1676.
• Ba{~nados}, E., Mazzucchelli, C., Momjian, E., Eilers, A., Wang, F., Schindler, J., Connor, T., Andika, I. T., Barth, A. J., Carilli, C., Davies, F. B., Decarli, R., Fan, X., Farina, E. P., Hennawi, J. F., Pensabene, A., Stern, D., Venemans, B. P., Wenzl, L., & Yang, J. (2021). "The Discovery of a Highly Accreting, Radio-loud Quasar at z = 6.82". apj, 909(1), 80.
• Jiang, L., Kashikawa, N., Wang, S., Walth, G., Ho, L. C., Cai, Z., Egami, E., Fan, X., Ito, K., Liang, Y., Schaerer, D., & Stark, D. P. (2021). "Evidence for GN-z11 as a luminous galaxy at redshift 10.957". Nature Astronomy, 5, 256-261.
• Li, J., Wang, F., Yang, J., Bregman, J. N., Fan, X., & Zhang, Y. (2021). "A Chandra survey of z {ensuremath{geq} 4.5 quasars}". mnras, 504(2), 2767-2782.
• Li, J., Wang, F., Yang, J., Zhang, Y., Fu, Y., Bian, F., Bregman, J. N., Fan, X., Li, Q., Wu, X., & Yu, X. (2021). "Chandra Detection of Three X-Ray Bright Quasars at z > 5". apj, 906(2), 135.
• Li, Q., Wang, R., Dannerbauer, H., Cai, Z., Emonts, B., Prochaska, J. X., Battaia, F. A., Neri, R., Zhang, C., Fan, X., Jin, S., Yoon, I., & Bechtel, S. (2021). "Discovery of a Protocluster Core Associated with an Enormous Lya Nebula at z = 2.3". apj, 922(2), 236.
• Liang, Y., Kashikawa, N., Cai, Z., Fan, X., Prochaska, J. X., Shimasaku, K., Tanaka, M., Uchiyama, H., Ito, K., Shimakawa, R., Nagamine, K., Shimizu, I., Onoue, M., & Toshikawa, J. (2021). "Statistical Correlation between the Distribution of Ly{ensuremath{alpha} Emitters and Intergalactic Medium H I at z ensuremath{sim} 2.2 Mapped by the Subaru/Hyper Suprime-Cam}". apj, 907(1), 3.
• Neeleman, M., Novak, M., Venemans, B. P., Walter, F., Decarli, R., Kaasinen, M., Schindler, J., Ba{~nados}, E., Carilli, C. L., Drake, A. B., Fan, X., & Rix, H. (2021). "The Kinematics of z {ensuremath{gtrsim} 6 Quasar Host Galaxies}". apj, 911(2), 141.
• Pensabene, A., Decarli, R., Ba{~nados}, E., Venemans, B., Walter, F., Bertoldi, F., Fan, X., Farina, E., Li, J., Mazzucchelli, C., Novak, M., Riechers, D., Rix, H. -., Strauss, M., Wang, R., Wei{ss}, A., Yang, J., & Yang, Y. (2021). "ALMA multiline survey of the ISM in two quasar host-companion galaxy pairs at z > 6". aap, 652, A66.
• Schindler, J., Fan, X., Novak, M., Venemans, B., Walter, F., Wang, F., Yang, J., Yue, M., Ba{~nados}, E., & Huang, Y. (2021). "A Closer Look at Two of the Most Luminous Quasars in the Universe". apj, 906(1), 12.
• Shi, D. D., Cai, Z., Fan, X., Zheng, X. Z., Huang, Y., & Xu, J. (2021). "Spectroscopic Confirmation of Two Extremely Massive Protoclusters, BOSS1244 and BOSS1542, at z = 2.24". apj, 915(1), 32.
• Wang, F., Fan, X., Yang, J., Mazzucchelli, C., Wu, X., Li, J., Ba{~nados}, E., Farina, E. P., Nanni, R., Ai, Y., Bian, F., Davies, F. B., Decarli, R., Hennawi, J. F., Schindler, J., Venemans, B., & Walter, F. (2021). "Revealing the Accretion Physics of Supermassive Black Holes at Redshift z {ensuremath{sim} 7 with Chandra and Infrared Observations}". apj, 908(1), 53.
• Wang, F., Yang, J., Fan, X., Hennawi, J. F., Barth, A. J., Banados, E., Bian, F., Boutsia, K., Connor, T., Davies, F. B., Decarli, R., Eilers, A., Farina, E. P., Green, R., Jiang, L., Li, J., Mazzucchelli, C., Nanni, R., Schindler, J., , Venemans, B., et al. (2021). "A Luminous Quasar at Redshift 7.642". apjl, 907(1), L1.
• Wenzl, L., Schindler, J., Fan, X., Andika, I. T., Ba{~nados}, E., Decarli, R., Jahnke, K., Mazzucchelli, C., Onoue, M., Venemans, B. P., Walter, F., & Yang, J. (2021). "Random Forests as a Viable Method to Select and Discover High-redshift Quasars". aj, 162(2), 72.
• Wu, Y., Cai, Z., Neeleman, M., Finlator, K., Zhang, S., Prochaska, J. X., Wang, R., Emonts, B. H., Fan, X., Keating, L. C., Wang, F., Yang, J., Hennawi, J. F., & Wang, J. (2021). "A [C II] 158 {ensuremath{mu}m emitter associated with an O I absorber at the end of the reionization epoch}". Nature Astronomy, 5, 1110-1117.
• Yang, J., Wang, F., Fan, X., Barth, A. J., Hennawi, J. F., Nanni, R., Bian, F., Davies, F. B., Farina, E. P., Schindler, J., Ba{~nados}, E., Decarli, R., Eilers, A., Green, R., Guo, H., Jiang, L., Li, J., Venemans, B., Walter, F., , Wu, X., et al. (2021). "Probing Early Supermassive Black Hole Growth and Quasar Evolution with Near-infrared Spectroscopy of 37 Reionization-era Quasars at 6.3 < z {ensuremath{leq} 7.64}". apj, 923(2), 262.
• Yue, M., Fan, X., Yang, J., & Wang, F. (2021). "A Candidate Kiloparsec-scale Quasar Pair at z = 5.66". apjl, 921(2), L27.
• Yue, M., Yang, J., Fan, X., Wang, F., Spilker, J., Georgiev, I. Y., Keeton, C. R., Litke, K. C., Marrone, D. P., Walter, F., Wang, R., Wu, X., Venemans, B. P., & Zabludoff, A. (2021). "ALMA Observations of the Sub-kpc Structure of the Host Galaxy of a z = 6.5 Lensed Quasar: A Rotationally Supported Hyper-Starburst System at the Epoch of Reionization". apj, 917(2), 99.
• Zheng, X. Z., Cai, Z., An, F. X., Fan, X., & Shi, D. D. (2021). "MAMMOTH: confirmation of two massive galaxy overdensities at z = 2.24 with H{ensuremath{alpha} emitters}". mnras, 500(4), 4354-4364.
• Zhu, Y., Becker, G. D., Bosman, S. E., Keating, L. C., Christenson, H. M., Ba{~nados}, E., Bian, F., Davies, F. B., D'Odorico, V., Eilers, A., Fan, X., Haehnelt, M. G., Kulkarni, G., Pallottini, A., Qin, Y., Wang, F., & Yang, J. (2021). "Chasing the Tail of Cosmic Reionization with Dark Gap Statistics in the Ly{ensuremath{alpha} Forest over 5 < z < 6}". apj, 923(2), 223.
• Zou, S., Jiang, L., Shen, Y., Wu, J., Ba{~nados}, E., Fan, X., Ho, L. C., Riechers, D. A., Venemans, B., Vestergaard, M., Walter, F., Wang, F., Willott, C. J., Joshi, R., Wu, X., & Yang, J. (2021). "Strong Mg II and Fe II Absorbers at 2.2 < z < 6.0". apj, 906(1), 32.
• Ahumada, R., Prieto, C. A., Almeida, A., Anders, F., Anderson, S. F., Andrews, B. H., Anguiano, B., Arcodia, R., Armengaud, E., Aubert, M., Avila, S., Avila-Reese, V., Badenes, C., Balland, C., Barger, K., Barrera-Ballesteros, J. K., Basu, S., Bautista, J., Beaton, R. L., , Beers, T. C., et al. (2020). "The 16th Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra". apjs, 249(1), 3.
• Bian, F., & Fan, X. (2020). "Lyman continuum escape fraction in Ly {ensuremath{alpha} emitters at z ≃ 3.1}". mnras, 493(1), L65-L69.
• Connor, T., Ba{~nados}, E., Mazzucchelli, C., Stern, D., Decarli, R., Fan, X., Farina, E. P., Lusso, E., Neeleman, M., & Walter, F. (2020). "X-Ray Observations of a [C II]-bright, z = 6.59 Quasar/Companion System". apj, 900(2), 189.
• Ding, J., Cai, Z., Prochaska, J. X., Finley, H., Fan, X., Zheng, Z., Fathivavsari, H., & Petitjean, P. (2020). "Deep Hubble Space Telescope Imaging on the Extended Ly{ensuremath{alpha} Emission of a QSO at z = 2.19 with a Damped Lyman Alpha System as a Natural Coronagraph}". apjl, 889(1), L12.
• Eilers, A., Hennawi, J. F., Decarli, R., Davies, F. B., Venemans, B., Walter, F., Ba{~nados}, E., Fan, X., Farina, E. P., Mazzucchelli, C., Novak, M., Schindler, J., Simcoe, R. A., Wang, F., & Yang, J. (2020). "Detecting and Characterizing Young Quasars. I. Systemic Redshifts and Proximity Zone Measurements". apj, 900(1), 37.
• Kinemuchi, K., Hall, P. B., McGreer, I., Kochanek, C., Grier, C. J., Trump, J., Shen, Y., Brandt, W., Wood-Vasey, W., Fan, X., Peterson, B. M., Schneider, D. P., Hern{'andez, S., Horne, K., Chen, Y., Eftekharzadeh, S., Guo, Y., Jia, S., Li, F., , Li, Z., et al. (2020). "The Sloan Digital Sky Survey Reverberation Mapping Project: Photometric g and i Light Curves". apjs, 250(1), 10.
• Li, J., Wang, R., Cox, P., Gao, Y. u., Walter, F., Wagg, J., Menten, K. M., Bertoldi, F., Shao, Y., Venemans, B. P., Decarli, R., Riechers, D., Neri, R., Fan, X., Omont, A., & Narayanan, D. (2020). "Ionized and Atomic Interstellar Medium in the z = 6.003 Quasar SDSS J2310+1855". apj, 900(2), 131.
• Li, J., Wang, R., Riechers, D., Walter, F., Decarli, R., Venamans, B. P., Neri, R., Shao, Y., Fan, X., Gao, Y. u., Carilli, C. L., Omont, A., Cox, P., Menten, K. M., Wagg, J., Bertoldi, F., & Narayanan, D. (2020). "Probing the Full CO Spectral Line Energy Distribution (SLED) in the Nuclear Region of a Quasar-starburst System at z = 6.003". apj, 889(2), 162.
• Li, Q., Wang, R., Fan, X., Wu, X., Jiang, L., Ba{~nados}, E., Venemans, B., Shao, Y., Li, J., Zhang, Y., Zhang, C., Wagg, J., Decarli, R., Mazzucchelli, C., Omont, A., & Bertoldi, F. (2020). "SCUBA2 High Redshift Bright Quasar Survey: Far-infrared Properties and Weak-line Features". apj, 900(1), 12.
• Marshall, M., Mechtley, M., Windhorst, R., Cohen, S., Jansen, R., Jiang, L., Jones, V., Wyithe, J., Fan, X., Hathi, N., Jahnke, K., Keel, W., Koekemoer, A., Marian, V., Ren, K., Robinson, J., R{"ottgering}, H., Ryan Jr., ., Scannapieco, E., , Schneider, D., et al. (2020). "Limits to Rest-frame Ultraviolet Emission from Far-infrared-luminous z ≃ 6 Quasar Hosts". apj, 900(1), 21.
• Ning, Y., Jiang, L., Zheng, Z., Wu, J., Bian, F., Egami, E., Fan, X., Ho, L. C., Shen, Y., Wang, R., & Wu, X. (2020). "The Magellan M2FS Spectroscopic Survey of High-redshift Galaxies: A Sample of 260 Ly{ensuremath{alpha} Emitters at Redshift z ≍ 5.7}". apj, 903(1), 4.
• Novak, M., Venemans, B. P., Walter, F., Neeleman, M., Kaasinen, M., Liang, L., Feldmann, R., Ba{~nados}, E., Carilli, C., Decarli, R., Drake, A. B., Fan, X., Farina, E. P., Mazzucchelli, C., Rix, H., & Wang, R. (2020). "No Evidence for [C II] Halos or High-velocity Outflows in z {ensuremath{gtrsim} 6 Quasar Host Galaxies}". apj, 904(2), 131.
• Onken, C. A., Bian, F., Fan, X., Wang, F., Wolf, C., & Yang, J. (2020). "A thirty-four billion solar mass black hole in SMSS J2157-3602, the most luminous known quasar". mnras, 496(2), 2309-2314.
• Schindler, J., Farina, E. P., Ba{~nados}, E., Eilers, A., Hennawi, J. F., Onoue, M., Venemans, B. P., Walter, F., Wang, F., Davies, F. B., Decarli, R., Rosa, G. D., Drake, A., Fan, X., Mazzucchelli, C., Rix, H., Worseck, G., & Yang, J. (2020). "The X-SHOOTER/ALMA Sample of Quasars in the Epoch of Reionization. I. NIR Spectral Modeling, Iron Enrichment, and Broad Emission Line Properties". apj, 905(1), 51.
• Venemans, B. P., Walter, F., Neeleman, M., Novak, M., Otter, J., Decarli, R., Ba{~nados}, E., Drake, A., Farina, E. P., Kaasinen, M., Mazzucchelli, C., Carilli, C., Fan, X., Rix, H., & Wang, R. (2020). "Kiloparsec-scale ALMA Imaging of [C II] and Dust Continuum Emission of 27 Quasar Host Galaxies at z {ensuremath{sim} 6}". apj, 904(2), 130.
• Wang, F., Davies, F. B., Yang, J., Hennawi, J. F., Fan, X., Barth, A. J., Jiang, L., Wu, X., Mudd, D. M., Ba{~nados}, E., Bian, F., Decarli, R., Eilers, A., Farina, E. P., Venemans, B., Walter, F., & Yue, M. (2020). "A Significantly Neutral Intergalactic Medium Around the Luminous z = 7 Quasar J0252-0503". apj, 896(1), 23.
• Yang, J., Wang, F., Fan, X., Hennawi, J. F., Davies, F. B., Yue, M., Banados, E., Wu, X., Venemans, B., Barth, A. J., Bian, F., Boutsia, K., Decarli, R., Farina, E. P., Green, R., Jiang, L., Li, J., Mazzucchelli, C., & Walter, F. (2020). "P{={o}niu={a}'ena: A Luminous z = 7.5 Quasar Hosting a 1.5 Billion Solar Mass Black Hole}". apjl, 897(1), L14.
• Yang, J., Wang, F., Fan, X., Hennawi, J. F., Davies, F. B., Yue, M., Eilers, A., Farina, E. P., Wu, X., Bian, F., Pacucci, F., & Lee, K. (2020). "Measurements of the z {textasciitilde 6 Intergalactic Medium Optical Depth and Transmission Spikes Using a New z > 6.3 Quasar Sample}". apj, 904(1), 26.
• Yi, W., Zuo, W., Yang, J., Wang, F., Timlin, J., Grier, C., Wu, X., Fan, X., & Bai, J. (2020). "Spectroscopy of Broad Absorption Line Quasars at 3 {ensuremath{lesssim} Z ensuremath{lesssim} 5. I. Evidence for Quasar Winds Shaping Broad/Narrow Emission Line Regions}". apj, 893(2), 95.
• Zuo, W., Wu, X., Fan, X., Green, R., Yi, W., Schulze, A., Wang, R., & Bian, F. (2020). "C IV Emission-line Properties and Uncertainties in Black Hole Mass Estimates of z {ensuremath{sim} 3.5 Quasars}". apj, 896(1), 40.
• Ba{~nados}, E., Novak, M., Neeleman, M., Walter, F., Decarli, R., Venemans, B. P., Mazzucchelli, C., Carilli, C., Wang, F., Fan, X., Farina, E. P., & Rix, H. (2019). The z = 7.54 Quasar ULAS J1342+0928 Is Hosted by a Galaxy Merger. apjl, 881(1), L23.
• Connor, T., Ba{~nados}, E., Stern, D., Decarli, R., Schindler, J., Fan, X., Farina, E. P., Mazzucchelli, C., Mulchaey, J. S., & Walter, F. (2019). X-Ray Observations of a z ensuremath{sim} 6.2 Quasar/Galaxy Merger. apj, 887(2), 171.
• Decarli, R., Dotti, M., Ba{~nados}, E., Farina, E. P., Walter, F., Carilli, C., Fan, X., Mazzucchelli, C., Neeleman, M., Novak, M., Riechers, D., Strauss, M. A., Venemans, B. P., Yang, Y., & Wang, R. (2019). ALMA and HST Kiloparsec-scale Imaging of a Quasar-galaxy Merger at Z ensuremath{approx} 6.2. apj, 880(2), 157.
• Dey, A., Schlegel, D. J., Lang, D., Blum, R., Burleigh, K., Fan, X., Findlay, J. R., Finkbeiner, D., Herrera, D., Juneau, S., Landriau, M., Levi, M., McGreer, I., Meisner, A., Myers, A. D., Moustakas, J., Nugent, P., Patej, A., Schlafly, E. F., , Walker, A. R., et al. (2019). Overview of the DESI Legacy Imaging Surveys. aj, 157(5), 168.
• Fan, X., Wang, F., Yang, J., Keeton, C. R., Yue, M., Zabludoff, A., Bian, F., Bonaglia, M., Georgiev, I. Y., Hennawi, J. F., Li, J., McGreer, I. D., Naidu, R., Pacucci, F., Rabien, S., Thompson, D., Venemans, B., Walter, F., Wang, R., & Wu, X. (2019). The Discovery of a Gravitationally Lensed Quasar at z = 6.51. apjl, 870(2), L11.
• Farina, E. P., Arrigoni-Battaia, F., Costa, T., Walter, F., Hennawi, J. F., Drake, A. B., Decarli, R., Gutcke, T. A., Mazzucchelli, C., Neeleman, M., Georgiev, I., Eilers, A., Davies, F. B., Ba{~nados}, E., Fan, X., Onoue, M., Schindler, J., Venemans, B. P., Wang, F., , Yang, J., et al. (2019). The REQUIEM Survey. I. A Search for Extended Lyensuremath{alpha} Nebular Emission Around 31 z > 5.7 Quasars. apj, 887(2), 196.
• Lu, K., Zhao, Y., Bai, J., & Fan, X. (2019). Reddening of the BLR and NLR in AGNs from a systematic analysis of Balmer decrement. mnras, 483(2), 1722-1730.
• Mazzucchelli, C., Decarli, R., Farina, E., Ba{~nados}, E., Venemans, B., Strauss, M., Walter, F., Neeleman, M., Bertoldi, F., Fan, X., Riechers, D., Rix, H. -., & Wang, R. (2019). Spectral Energy Distributions of Companion Galaxies to z ̃ 6 Quasars. apj, 881(2), 163.
• Neeleman, M., Ba{~nados}, E., Walter, F., Decarli, R., Venemans, B. P., Carilli, C. L., Fan, X., Farina, E. P., Mazzucchelli, C., Novak, M., Riechers, D. A., Rix, H., & Wang, R. (2019). Resolved [C II] Emission from z > 6 Quasar Host-Companion Galaxy Pairs. apj, 882(1), 10.
• Novak, M., Ba{~nados}, E., Decarli, R., Walter, F., Venemans, B., Neeleman, M., Farina, E. P., Mazzucchelli, C., Carilli, C., Fan, X., Rix, H., & Wang, F. (2019). An ALMA Multiline Survey of the Interstellar Medium of the Redshift 7.5 Quasar Host Galaxy J1342+0928. apj, 881(1), 63.
• Schindler, J., Fan, X., Huang, Y., Yue, M., Yang, J., Hall, P. B., Wenzl, L., Hughes, A., Litke, K. C., & Rees, J. M. (2019). The Extremely Luminous Quasar Survey in the Pan-STARRS 1 Footprint (PS-ELQS). apjs, 243(1), 5.
• Schindler, J., Fan, X., McGreer, I. D., Yang, J., Wang, F., Green, R., Fynbo, J. P., Krogager, J., Green, E. M., Huang, Y., Kadowaki, J., Patej, A., Wu, Y., & Yue, M. (2019). The Extremely Luminous Quasar Survey in the Sloan Digital Sky Survey Footprint. III. The South Galactic Cap Sample and the Quasar Luminosity Function at Cosmic Noon. apj, 871(2), 258.
• Shao, Y., Wang, R., Carilli, C. L., Wagg, J., Walter, F., Li, J., Fan, X., Jiang, L., Riechers, D. A., Bertoldi, F., Strauss, M. A., Cox, P., Omont, A., & Menten, K. M. (2019). Star Formation and ISM Properties in the Host Galaxies of Three Far-infrared Luminous Quasars at z ̃ 6. apj, 876(2), 99.
• Shen, Y., Wu, J., Jiang, L., Ba{~nados}, E., Fan, X., Ho, L. C., Riechers, D. A., Strauss, M. A., Venemans, B., Vestergaard, M., Walter, F., Wang, F., Willott, C., Wu, X., & Yang, J. (2019). Gemini GNIRS Near-infrared Spectroscopy of 50 Quasars at z ensuremath{gtrsim} 5.7. apj, 873(1), 35.
• Wang, F., Wang, R., Fan, X., Wu, X., Yang, J., Neri, R., & Yue, M. (2019). Spatially Resolved Interstellar Medium and Highly Excited Dense Molecular Gas in the Most Luminous Quasar at z = 6.327. apj, 880(1), 2.
• Wang, F., Yang, J., Fan, X., Wu, X., Yue, M., Li, J., Bian, F., Jiang, L., Ba{~nados}, E., Schindler, J., Findlay, J. R., Davies, F. B., Decarli, R., Farina, E. P., Green, R., Hennawi, J. F., Huang, Y., Mazzuccheli, C., McGreer, I. D., , Venemans, B., et al. (2019). Exploring Reionization-era Quasars. III. Discovery of 16 Quasars at 6.4 ensuremath{lesssim} z ensuremath{lesssim} 6.9 with DESI Legacy Imaging Surveys and the UKIRT Hemisphere Survey and Quasar Luminosity Function at z ̃ 6.7. apj, 884(1), 30.
• Wang, R., Shao, Y., Carilli, C. L., Jones, G. C., Walter, F., Fan, X., Riechers, D. A., Decarli, R., Bertoldi, F., Wagg, J., Strauss, M. A., Omont, A., Cox, P., Jiang, L., Narayanan, D., Menten, K. M., & Venemans, B. P. (2019). Resolving the Interstellar Medium in the Nuclear Region of Two z = 5.78 Quasar Host Galaxies with ALMA. apj, 887(1), 40.
• Yang, J., Venemans, B., Wang, F., Fan, X., Novak, M., Decarli, R., Walter, F., Yue, M., Momjian, E., Keeton, C. R., Wang, R., Zabludoff, A., Wu, X., & Bian, F. (2019). Far-infrared Properties of the Bright, Gravitationally Lensed Quasar J0439+1634 at z = 6.5. apj, 880(2), 153.
• Yang, J., Wang, F., Fan, X., Wu, X., Bian, F., Ba{~nados}, E., Yue, M., Schindler, J., Yang, Q., Jiang, L., McGreer, I. D., Green, R., & Dye, S. (2019). Filling in the Quasar Redshift Gap at z ̃ 5.5. II. A Complete Survey of Luminous Quasars in the Post-reionization Universe. apj, 871(2), 199.
• Yang, J., Wang, F., Fan, X., Yue, M., Wu, X., Li, J., Bian, F., Jiang, L., Ba{~nados}, E., & Beletsky, Y. (2019). Exploring Reionization-era Quasars. IV. Discovery of Six New z ensuremath{gtrsim} 6.5 Quasars with DES, VHS, and unWISE Photometry. aj, 157(6), 236.
• Yue, M., Fan, X., Schindler, J., McGreer, I. D., & Huang, Y. (2019). Quasars Have Fewer Close Companions than Normal Galaxies. apj, 883(2), 141.
• Zou, H. u., Zhou, X. u., Fan, X., Zhang, T., Zhou, Z., Peng, X., Nie, J., Jiang, L., McGreer, I., Cai, Z., Chen, G., Chen, X., Dey, A., Fan, D., Findlay, J. R., Gao, J., Gu, Y., Guo, Y., He, B., , Jiang, Z., et al. (2019). The Third Data Release of the Beijing─Arizona Sky Survey. apjs, 245(1), 4.
• Abolfathi, B., Aguado, D., Aguilar, G., Allende Prieto, C., Almeida, A., Ananna, T., Anders, F., Anderson, S., Andrews, B., Anguiano, B., & al., e. (2018). The Fourteenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the Extended Baryon Oscillation Spectroscopic Survey and from the Second Phase of the Apache Point Observatory Galactic Evolution Experiment. apjs, 235, 42.
• Ba{~nados}, E., Connor, T., Stern, D., Mulchaey, J., Fan, X., Decarli, R., Farina, E., Mazzucchelli, C., Venemans, B., Walter, F., Wang, F., & Yang, J. (2018). Chandra X-Rays from the Redshift 7.54 Quasar ULAS J1342+0928. apjl, 856, L25.
• Ba{~nados}, E., Venemans, B., Mazzucchelli, C., Farina, E., Walter, F., Wang, F., Decarli, R., Stern, D., Fan, X., Davies, F., Hennawi, J., Simcoe, R., Turner, M., Rix, H., Yang, J., Kelson, D., Rudie, G., & Winters, J. (2018). An 800-million-solar-mass black hole in a significantly neutral Universe at a redshift of 7.5. nat, 553, 473-476.
• Bosman, S., Fan, X., Jiang, L., Reed, S., Matsuoka, Y., Becker, G., & Haehnelt, M. (2018). New constraints on Lyman-$alpha$ opacity with a sample of 62 quasarsat z $gt$ 5.7. mnras, 479, 1055-1076.
• Champagne, J., Decarli, R., Casey, C., Venemans, B., Ba{~nados}, E., Walter, F., Bertoldi, F., Fan, X., Farina, E., Mazzucchelli, C., Riechers, D., Strauss, M., Wang, R., & Yang, Y. (2018). No Evidence for Millimeter Continuum Source Overdensities in the Environments of znbspgsimnbsp6 Quasars. apj, 867, 153.
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• {Plotkin}, R., {Shemmer}, O., {Trakhtenbrot}, B., {Anderson}, S., {Brandt}, W., {Fan}, X., {Gallo}, E., {Lira}, P., {Luo}, B., {Richards}, G., {Schneider}, D., {Strauss}, M., , J. (2015). "{Detection of Rest-frame Optical Lines from X-shooter Spectroscopy of Weak Emission Line Quasars}". apj, 805, 123.
• {Richards}, G., {Myers}, A., {Peters}, C., {Krawczyk}, C., {Chase}, G., {Ross}, N., {Fan}, X., {Jiang}, L., {Lacy}, M., {McGreer}, I., {Trump}, J., , R. (2015). "{Bayesian High-redshift Quasar Classification from Optical and Mid-IR Photometry}". apjs, 219, 39.
• {Shen}, Y., {Brandt}, W., {Dawson}, K., {Hall}, P., {McGreer}, I., {Anderson}, S., {Chen}, Y., {Denney}, K., {Eftekharzadeh}, S., {Fan}, X., {Gao}, Y., {Green}, P., {Greene}, J., {Ho}, L., {Horne}, K., {Jiang}, L., {Kelly}, B., {Kinemuchi}, K., {Kochanek}, C., , {P{^a}ris}, I., et al. (2015). "{The Sloan Digital Sky Survey Reverberation Mapping Project: Technical Overview}". apjs, 216, 4.
• {Stefan}, I., {Carilli}, C., {Wagg}, J., {Walter}, F., {Riechers}, D., {Bertoldi}, F., {Green}, D., {Fan}, X., {Menten}, K., , R. (2015). "{Imaging the cold molecular gas in SDSS J1148 + 5251 at z = 6.4}". mnras, 451, 1713-1718.
• {Venemans}, B., {Ba{~n}ados}, E., {Decarli}, R., {Farina}, E., {Walter}, F., {Chambers}, K., {Fan}, X., {Rix}, H., {Schlafly}, E., {McMahon}, R., {Simcoe}, R., {Stern}, D., {Burgett}, W., {Draper}, P., {Flewelling}, H., {Hodapp}, K., {Kaiser}, N., {Magnier}, E., {Metcalfe}, N., , {Morgan}, J., et al. (2015). "{The Identification of Z-dropouts in Pan-STARRS1: Three Quasars at 6.5$lt$ z$lt$ 6.7}". apjl, 801, L11.
• {Wang}, F., {Wu}, X., {Fan}, X., {Yang}, J., {Cai}, Z., {Yi}, W., {Zuo}, W., {Wang}, R., {McGreer}, I., {Ho}, L., {Kim}, M., {Yang}, Q., {Bian}, F., , L. (2015). "{An Ultra-luminous Quasar at z = 5.363 with a Ten Billion Solar Mass Black Hole and a Metal-rich DLA at z {tilde} 5}". apjl, 807, L9.
• {Wu}, X., {Wang}, F., {Fan}, X., {Yi}, W., {Zuo}, W., {Bian}, F., {Jiang}, L., {McGreer}, I., {Wang}, R., {Yang}, J., {Yang}, Q., {Thompson}, D., , Y. (2015). "{An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30}". nat, 518, 512-515.
• {Wu}, X., {Wang}, F., {Fan}, X., {Yi}, W., {Zuo}, W., {Bian}, F., {Jiang}, L., {McGreer}, I., {Wang}, R., {Yang}, J., {Yang}, Q., {Thompson}, D., , Y. (2015). "{Discovery of a 12 billion solar mass black hole at redshift 6.3 and its challenge to the black hole/galaxy co-evolution at cosmic dawn}". IAU General Assembly, 22, 2251223.
• {Zou}, H., {Jiang}, Z., {Zhou}, X., {Wu}, Z., {Ma}, J., {Fan}, X., {Fan}, Z., {He}, B., {Jing}, Y., {Lesser}, M., {Li}, C., {Nie}, J., {Shen}, S., {Wang}, J., {Zhang}, T., , Z. (2015). "{South Galactic Cap u-band Sky Survey (SCUSS): Data Reduction}". aj, 150, 104.
• {Zou}, H., {Wu}, X., {Zhou}, X., {Wang}, S., {Jiang}, L., {Fan}, X., {Fan}, Z., {Jiang}, Z., {Jing}, Y., {Lesser}, M., {Li}, C., {Ma}, J., {Nie}, J., {Shen}, S., {Wang}, J., {Wu}, Z., {Zhang}, T., , Z. (2015). "{Capability of Quasar Selection by Combining SCUSS and SDSS Observations}". pasp, 127, 94-101.
• {Zuo}, W., {Wu}, X., {Fan}, X., {Green}, R., {Wang}, R., , F. (2015). "{Black Hole Mass Estimates and Rapid Growth of Supermassive Black Holes in Luminous z ~{} 3.5 Quasars}". apj, 799, 189.
• Ahn, C. P., Alexandroff, R., Allende Prieto, C., Anders, F., Anderson, S. F., Anderton, T., Andrews, B. H., Aubourg, \., Bailey, S., Bastien, F. A., & al., e. (2014). The Tenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Apache Point Observatory Galactic Evolution Experiment. \apjs, 211, 17.
• Annis, J., Soares-Santos, M., Strauss, M. A., Becker, A. C., Dodelson, S., Fan, X., Gunn, J. E., Hao, J., Ivezi\'c, \. Z., Jester, S., Jiang, L., Johnston, D. E., Kubo, J. M., Lampeitl, H., Lin, H., Lupton, R. H., Miknaitis, G., Seo, H., Simet, M., & Yanny, B. (2014). The Sloan Digital Sky Survey Coadd: 275 deg$^2$ of Deep Sloan Digital Sky Survey Imaging on Stripe 82. \apj, 794, 120.
• Ba\~nados, E., Venemans, B. P., Morganson, E., Decarli, R., Walter, F., Chambers, K. C., Rix, H., Farina, E. P., Fan, X., Jiang, L., McGreer, I., De Rosa, G., Simcoe, R., Wei\ss, A., Price, P. A., Morgan, J. S., Burgett, W. S., Greiner, J., Kaiser, N., , Kudritzki, R., et al. (2014). Discovery of Eight z \~ 6 Quasars from Pan-STARRS1. \aj, 148, 14.
• Cai, Z., Fan, X., Noterdaeme, P., Wang, R., McGreer, I., Carithers, B., Bian, F., Miralda-Escud\'e, J., Finley, H., P\^aris, I., Schneider, D. P., Zakamska, N. L., Ge, J., Petitjean, P., & Slosar, A. (2014). A Glimpse at Quasar Host Galaxy Far-UV Emission Using Damped Ly$\alpha$'s as Natural Coronagraphs. \apj, 793, 139.
• Jia, Y., Du, C., Wu, Z., Peng, X., Ma, J., Zhou, X., Fan, X., Fan, Z., Jing, Y., Jiang, Z., Lesser, M., Nie, J., Olszewski, E., Shen, S., Wang, J., Zou, H., Zhang, T., & Zhou, Z. (2014). Estimation of absolute magnitude-dependent Galactic model parameters in intermediate latitude with SDSS and SCUSS. \mnras, 441, 503-512.
• Jiang, L., Fan, X., Bian, F., McGreer, I. D., Strauss, M. A., Annis, J., Buck, Z., Green, R., Hodge, J. A., Myers, A. D., Rafiee, A., & Richards, G. (2014). The Sloan Digital Sky Survey Stripe 82 Imaging Data: Depth-optimized Co-adds over 300 deg$^2$ in Five Filters. \apjs, 213, 12.
• Leipski, C., Meisenheimer, K., Walter, F., Klaas, U., Dannerbauer, H., De Rosa, G., Fan, X., Haas, M., Krause, O., & Rix, H. (2014). Spectral Energy Distributions of QSOs at z \gt 5: Common Active Galactic Nucleus-heated Dust and Occasionally Strong Star-formation. \apj, 785, 154.
• McGreer, I. D., Fan, X., Strauss, M. A., Haiman, Z., Richards, G. T., Jiang, L., Bian, F., & Schneider, D. P. (2014). Close Companions to Two High-redshift Quasars. \aj, 148, 73.
• P\^aris, I., Petitjean, P., Aubourg, \., Ross, N. P., Myers, A. D., Streblyanska, A., Bailey, S., Hall, P. B., Strauss, M. A., Anderson, S. F., Bizyaev, D., Borde, A., Brinkmann, J., Bovy, J., Brandt, W. N., Brewington, H., Brownstein, J. R., Cook, B. A., Ebelke, G., , Fan, X., et al. (2014). The Sloan Digital Sky Survey quasar catalog: tenth data release. \aap, 563, A54.
• Paris, I., Petitjean, P., Aubourg, E., Ross, N. P., Myers, A. D., Strblyanska, A., Bailey, S., Hall, P. B., Strauss, M. A., Anderson, S. F., Bizyaev, D., Borde, A., Brinkmann, J., Bovy, J., Brandt, W. N., Brewington, H., Browstein, J. R., Cook, B. A., Ebelke, G., , Fan, X., et al. (2014). VizieR Online Data Catalog: SDSS quasar catalog: tenth data release (Paris+, 2014). VizieR Online Data Catalog, 7270, 0.
• Yi, W., Wang, F., Wu, X., Yang, J., Bai, J., Fan, X., Brandt, W. N., Ho, L. C., Zuo, W., Kim, M., Wang, R., Yang, Q., Zhang, J., Wang, F., Wang, J., Ai, Y., Fan, Y., Chang, L., Wang, C., , Lun, B., et al. (2014). SDSS J013127.34-032100.1: A Newly Discovered Radio-loud Quasar at z = 5.18 with Extremely High Luminosity. \apjl, 795, L29.
• Yuan, W., Zhou, H., Dou, L., Dong, X. -., Fan, X., & Wang, T. -. (2014). Chandra and MMT observations of low-mass black hole active galactic nuclei accreting at low rates in dwarf galaxies. Astrophysical Journal, 782(1).
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  Abstract: We report on Chandra X-ray observations of four candidate low-mass black hole (Mbh ≲ 106 M ) active galactic nuclei (AGNs) that have the estimated Eddington ratios among the lowest (10-2) found for this class. The aims are to validate the nature of their AGNs and to confirm the low Eddington ratios that are derived from the broad Hα line, and to explore this poorly studied regime in the AGN parameter space. Among them, two objects with the lowest significance of the broad lines are also observed with the Multi-Mirror Telescope, and the high-quality optical spectra taken confirm them as Seyfert 1 AGNs and as having small black hole masses. X-ray emission is detected from the nuclei of two of the galaxies, which is variable on timescales of 103 s, whereas no significant (or only marginal at best) detection is found for the remaining two. The X-ray luminosities are on the order of 1041 erg s-1 or even lower, on the order of 1040 erg s-1 for non-detections, which are among the lowest regimes ever probed for Seyfert galaxies. The low X-ray luminosities, compared to their black hole masses derived from Hα, confirm their low accretion rates assuming typical bolometric corrections. Our results hint at the existence of a possibly large population of under-luminous low-mass black holes in the local universe. An off-nucleus ultra-luminous X-ray source in one of the dwarf galaxies is detected serendipitously, with a luminosity (6-9)× 1039 erg s-1 in 2-10 keV. © 2014. The American Astronomical Society. All rights reserved..
• Bian, F., Fan, X., Jiang, L., McGreer, I., Dey, A., Green, R. F., Maiolino, R., Walter, F., Lee, K., & Davé, R. (2013). The LBT boötes field survey. I. the rest-frame ultraviolet and near-infrared luminosity functions and clustering of bright lyman break galaxies at Z ∼ 3. Astrophysical Journal, 774(1).
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  Abstract: We present a deep LBT/LBC Uspec-band imaging survey (9 deg 2) covering the NOAO Boötes field. A total of 14,485 Lyman break galaxies (LBGs) at z ∼ 3 are selected, which are used to measure the rest-frame UV luminosity function (LF). The large sample size and survey area reduce the LF uncertainties due to Poisson statistics and cosmic variance by ≥3 compared to previous studies. At the bright end, the LF shows excess power compared to the best-fit Schechter function, which can be attributed to the contribution of z ∼ 3 quasars. We compute the rest-frame near-infrared LF and stellar mass function (SMF) of z ∼ 3 LBGs based on the R-band and [4.5 μm]-band flux relation. We investigate the evolution of the UV LFs and SMFs between z ∼ 7 and z ∼ 3, which supports a rising star formation history in the LBGs. We study the spatial correlation function of two bright LBG samples and estimate their average host halo mass. We find a tight relation between the host halo mass and the galaxy star formation rate (SFR), which follows the trend predicted by the baryonic accretion rate onto the halo, suggesting that the star formation in LBGs is fueled by baryonic accretion through the cosmic web. By comparing the SFRs with the total baryonic accretion rates, we find that cosmic star formation efficiency is about 5%-20% and it does not evolve significantly with redshift, halo mass, or galaxy luminosity. © 2013. The American Astronomical Society. All rights reserved.
• Dawson, K. S., Schlegel, D. J., Ahn, C. P., Anderson, S. F., Aubourg, É., Bailey, S., Barkhouser, R. H., Bautista, J. E., Beifiori, A., Berlind, A. A., Bhardwaj, V., Bizyaev, D., Blake, C. H., Blanton, M. R., Blomqvist, M., Bolton, A. S., Borde, A., Bovy, J., Brandt, W. N., , Brewington, H., et al. (2013). The baryon oscillation spectroscopic survey of SDSS-III. Astronomical Journal, 145(1).
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  Abstract: The Baryon Oscillation Spectroscopic Survey (BOSS) is designed to measure the scale of baryon acoustic oscillations (BAO) in the clustering of matter over a larger volume than the combined efforts of all previous spectroscopic surveys of large-scale structure. BOSS uses 1.5 million luminous galaxies as faint as i = 19.9 over 10,000deg2 to measure BAO to redshifts z < 0.7. Observations of neutral hydrogen in the Lyα forest in more than 150,000quasar spectra (g < 22) will constrain BAO over the redshift range 2.15 < z < 3.5. Early results from BOSS include the first detection of the large-scale three-dimensional clustering of the Lyα forest and a strong detection from the Data Release 9 data set of the BAO in the clustering of massive galaxies at an effective redshift z = 0.57. We project that BOSS will yield measurements of the angular diameter distance dA to an accuracy of 1.0% at redshifts z = 0.3 and z = 0.57 and measurements of H(z) to 1.8% and 1.7% at the same redshifts. Forecasts for Lyα forest constraints predict a measurement of an overall dilation factor that scales the highly degenerate DA (z) and H -1(z) parameters to an accuracy of 1.9% at z ∼ 2.5 when the survey is complete. Here, we provide an overview of the selection of spectroscopic targets, planning of observations, and analysis of data and data quality of BOSS. © 2013. The American Astronomical Society. All rights reserved.
• Jiang, L., Bian, F., Fan, X., Krug, H. B., McGreer, I. D., Stark, D. P., Clément, B., & Egami, E. (2013). Deep LBT/LUCI spectroscopy of an Lyα emitter candidate at z ≃ 7.7. Astrophysical Journal Letters, 771(1).
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  Abstract: We present deep spectroscopic observations of an Lyα emitter (LAE) candidate at z ≃ 7.7 using the infrared spectrograph LUCI on the 2 × 8.4 m Large Binocular Telescope (LBT). The candidate is the brightest among the four z ≃ 7.7 LAE candidates found in a narrowband imaging survey by Krug et al. Our spectroscopic data include a total of 7.5 hr of integration with LBT/LUCI and are deep enough to significantly (3.2σ-4.9σ) detect the Lyα emission line of this candidate based on its Lyα flux 1.2 × 10-17 erg s-1 cm-2 estimated from the narrowband photometry. However, we do not find any convincing signal at the expected position of its Lyα emission line, suggesting that this source is not an LAE at z ≃ 7.7. The non-detection in this work, together with the previous studies of z ≃ 7.7 LAEs, puts a strong constraint on the bright-end Lyα luminosity function (LF) at z ≃ 7.7. We find a rapid evolution of the Lyα LF from z ≃ 6.5 to 7.7: the upper limit of the z ≃ 7.7 LF is more than five times lower than the z ≃ 6.5 LF at the bright end (f≥ 1.0 × 10-17 erg s-1 cm -2 or L≥ 6.9 × 1042 erg s-1). This is likely caused by an increasing neutral fraction in the intergalactic medium that substantially attenuates Lyα emission at z ≃ 7.7. © 2013. The American Astronomical Society. All rights reserved.
• Jiang, L., Egami, E., Fan, X., Windhorst, R. A., Cohen, S. H., Davé, R., Finlator, K., Kashikawa, N., Mechtley, M., Ouchi, M., & Shimasaku, K. (2013). Physical properties of spectroscopically confirmed galaxies at z ≥ 6. II. Morphology of the rest-frame UV continuum and Lyα emission. Astrophysical Journal, 773(2).
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  Abstract: We present a detailed structural and morphological study of a large sample of spectroscopically confirmed galaxies at z ≥ 6 using deep Hubble Space Telescope (HST) near-IR broad-band images and Subaru Telescope optical narrow-band images. The galaxy sample consists of 51 Lyα emitters (LAEs) at z ≃ 5.7, 6.5, and 7.0, and 16 Lyman break galaxies (LBGs) at 5.9 ≤ z ≤ 6.5. These galaxies exhibit a wide range of rest-frame UV continuum morphology in the HST images, from compact features to multiple component systems. The fraction of merging/interacting galaxies reaches 40%-50% at the brightest end of M 1500 ≤ -20.5 mag. The intrinsic half-light radii r hl, in, after correcting for point-spread function (PSF) broadening, are roughly between r hl, in ≃ 0.″05 (0.3 kpc) and 0.″3 (1.7 kpc) at M 1500 ≤ -19.5 mag. The median r hl, in value is 0.″16 (∼0.9 kpc). This is consistent with the sizes of bright LAEs and LBGs at z ≥ 6 found in previous studies. In addition, more luminous galaxies tend to be larger and exhibit a weak size-luminosity relation, r hl, in∝L 0.14 at M 1500 ≤ -19.5 mag. The slope of 0.14 is significantly flatter than those in fainter LBG samples. We discuss the morphology of z ≥ 6 galaxies with nonparametric methods, including the concentration, asymmetry, and smoothness system and the Gini and M 20 parameters, and demonstrate their validity through simulations. We search for extended Lyα emission halos around LAEs at z ≃ 5.7 and 6.5 by stacking a number of narrow-band images. We do not find evidence of extended Lyα halos predicted by cosmological simulations. Such halos, if they exist, could be weaker than predicted. Finally, we investigate positional misalignment between the UV continuum and Lyα emissions in LAEs. While the two positions are generally consistent, several merging galaxies show significant positional differences. This is likely caused by a disturbed interstellar medium distribution due to merging activity. © 2013. The American Astronomical Society. All rights reserved.
• Jiang, L., Egami, E., Mechtley, M., Fan, X., Cohen, S. H., Windhorst, R. A., Davé, R., Finlator, K., Kashikawa, N., Ouchi, M., & Shimasaku, K. (2013). Physical properties of spectroscopically confirmed galaxies at z ≥ 6. I. basic characteristics of the rest-frame UV continuum and Lyα emission. Astrophysical Journal, 772(2).
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  Abstract: We present deep Hubble Space Telescope near-IR and Spitzer mid-IR observations of a large sample of spectroscopically confirmed galaxies at z ≥ 6. The sample consists of 51 Lyα emitters (LAEs) at z ≃ 5.7, 6.5, and 7.0, and 16 Lyman break galaxies (LBGs) at 5.9 ≤ z ≤ 6.5. The near-IR images were mostly obtained with WFC3 in the F125W and F160W bands, and the mid-IR images were obtained with IRAC in the 3.6 μm and 4.5 μm bands. Our galaxies also have deep optical imaging data from Subaru Suprime-Cam. We utilize the multi-band data and secure redshifts to derive their rest-frame UV properties. These galaxies have steep UV-continuum slopes roughly between β ≃ -1.5 and -3.5, with an average value of β ≃ -2.3, slightly steeper than the slopes of LBGs in previous studies. The slope shows little dependence on UV-continuum luminosity except for a few of the brightest galaxies. We find a statistically significant excess of galaxies with slopes around β ≃ -3, suggesting the existence of very young stellar populations with extremely low metallicity and dust content. Our galaxies have moderately strong rest-frame Lyα equivalent width (EW) in a range of ∼10 to ∼200 Å. The star formation rates are also moderate, from a few to a few tens of solar masses per year. The LAEs and LBGs in this sample share many common properties, implying that LAEs represent a subset of LBGs with strong Lyα emission. Finally, the comparison of the UV luminosity functions between LAEs and LBGs suggests that there exists a substantial population of faint galaxies with weak Lyα emission (EW < 20 Å) that could be the dominant contribution to the total ionizing flux at z ≥ 6. © 2013. The American Astronomical Society. All rights reserved.
• Leipski, C., Meisenheimer, K., Walter, F., Besel, M. -., Dannerbauer, H., Fan, X., Haas, M., Klaas, U., Krause, O., & Rix, H. -. (2013). Complete infrared spectral energy distributions of millimeter detected quasars at z > 5. Astrophysical Journal, 772(2).
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  Abstract: We present Herschel far-infrared (FIR) photometry of 11 quasars at redshift z > 5 that have previously been detected at 1.2 mm. We perform full spectral energy distribution (SED) fits over the wavelength range λ rest ∼ 0.1-400 μm for those objects with good Herschel detections. These fits reveal the need for an additional FIR component besides the emission from a dusty active galactic nucleus (AGN)-powered torus. This additional FIR component has temperatures of T FIR ∼ 40-60 K with luminosities of L 8-1000μm ∼ 1013 LO (accounting for 25%-60% of the bolometric FIR luminosity). If the FIR dust emission is due to star formation it would suggest star formation rates in excess of 1000 solar masses per year. We show that at long wavelengths (λrest ≳ 50 μm) the contribution of the AGN-powered torus emission is negligible. This explains how previous FIR studies of high-redshift quasars that relied on single-component fits to (ground-based) observations at λobs ≳ 350 μm reached T FIR and LFIR values similar to our complete SED fits. Stacking the Herschel data of four individually undetected sources reveals a significant average signal in the PACS bands but not in SPIRE. The average SED of sources with individual Herschel detections shows a striking surplus in near- and mid-infrared (MIR) emission when compared to common AGN templates. The comparison between two average SEDs (sources with and without individual Herschel detections) matched in the UV/optical indicates that for these objects the strength of the MIR emission may correlate with the strength of the FIR emission. © 2013. The American Astronomical Society. All rights reserved.
• Matheson, T., Fan, X., Green, R., McConnachie, A., Newman, J., Olsen, K., Szkody, P., & Wood-Vasey, W. M. (2013). Spectroscopy in the Era of LSST. ArXiv e-prints.
• McGreer, I. D., Jiang, L., Fan, X., Richards, G. T., Strauss, M. A., Ross, N. P., White, M., Shen, Y., Schneider, D. P., Myers, A. D., Brandt, W. N., Degraf, C., Glikman, E., Jian, G. e., & Streblyanska, A. (2013). The z = 5 quasar luminosity function from SDSS stripe 82. Astrophysical Journal, 768(2).
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  Abstract: We present a measurement of the Type I quasar luminosity function at z = 5 using a large sample of spectroscopically confirmed quasars selected from optical imaging data. We measure the bright end (M 1450 < -26) with Sloan Digital Sky Survey (SDSS) data covering ∼6000 deg2, then extend to lower luminosities (M 1450 < -24) with newly discovered, faint z ∼ 5 quasars selected from 235 deg2 of deep, coadded imaging in the SDSS Stripe 82 region (the celestial equator in the Southern Galactic Cap). The faint sample includes 14 quasars with spectra obtained as ancillary science targets in the SDSS-III Baryon Oscillation Spectroscopic Survey, and 59 quasars observed at the MMT and Magellan telescopes. We construct a well-defined sample of 4.7 < z < 5.1 quasars that is highly complete, with 73 spectroscopic identifications out of 92 candidates. Our color selection method is also highly efficient: of the 73 spectra obtained, 71 are high-redshift quasars. These observations reach below the break in the luminosity function (). The bright-end slope is steep (β ≲ -4), with a constraint of β < -3.1 at 95% confidence. The break luminosity appears to evolve strongly at high redshift, providing an explanation for the flattening of the bright-end slope reported previously. We find a factor of ∼2 greater decrease in the number density of luminous quasars (M 1450 < -26) from z = 5 to z = 6 than from z = 4 to z = 5, suggesting a more rapid decline in quasar activity at high redshift than found in previous surveys. Our model for the quasar luminosity function predicts that quasars generate ∼30% of the ionizing photons required to keep hydrogen in the universe ionized at z = 5. © 2013. The American Astronomical Society. All rights reserved..
• Palanque-Delabrouille, N., Magneville, C., Yèche, C., Eftekharzadeh, S., Myers, A. D., Petitjean, P., Pâris, I., Aubourg, E., McGreer, I., Fan, X., Dey, A., Schlegel, D., Bailey, S., Bizayev, D., Bolton, A., Dawson, K., Ebelke, G., Ge, J., Malanushenko, E., , Malanushenko, V., et al. (2013). Luminosity function from dedicated SDSS-III and MMT data of quasars in 0.7 < z < 4.0 selected with a new approach. Astronomy and Astrophysics, 551.
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  Abstract: We present a measurement of the quasar luminosity function in the range 0.68 < z < 4 down to extinction corrected magnitude gdered = 22.5, using a simple and well understood target selection technique based on the time-variability of quasars. The completeness of our sample was derived directly from a control sample of quasars, without requiring complex simulations of quasar light-curves or colors. A total of 1877 quasar spectra were obtained from dedicated programs on the Sloan telescope (as part of the SDSS-III/BOSS survey) and on the Multiple Mirror Telescope. They allowed us to derive the quasar luminosity function. It agrees well with results previously published in the redshift range 0.68 < z < 2.6. Our deeper data allow us to extend the measurement to z = 4. We measured quasar densities to gdered < 22.5, obtaining 30 QSO per deg2 at z < 1, 99 QSO per deg 2 for 1 < z < 2.15, and 47 QSO per deg2 at z > 2.15. Using pure luminosity evolution models, we fitted our LF measurements and predicted quasar number counts as a function of redshift and observed magnitude. These predictions are useful inputs for future cosmology surveys such as those relying on the observation of quasars to measure baryon acoustic oscillations. © 2013 ESO.
• Ross, N. P., McGreer, I. D., White, M., Richards, G. T., Myers, A. D., Palanque-Delabrouille, N., Strauss, M. A., Anderson, S. F., Shen, Y., Brandt, W. N., Yèche, C., E., M., Aubourg, É., Bailey, S., Bizyaev, D., Bovy, J., Brewington, H., Brinkmann, J., Degraf, C., , Matteo, T. D., et al. (2013). The sdss-iii baryon oscillation spectroscopic survey: The quasar luminosity function from data release nine. Astrophysical Journal, 773(1).
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  Abstract: We present a new measurement of the optical quasar luminosity function (QLF), using data from the Sloan Digital Sky Survey-III: Baryon Oscillation Spectroscopic Survey (SDSS-III: BOSS). From the SDSS-III Data Release Nine, a uniform sample of 22,301 i ≲ 21.8 quasars are selected over an area of 2236 deg2, with confirmed spectroscopic redshifts between 2.2 < z < 3.5, filling in a key part of the luminosity-redshift plane for optical quasar studies. The completeness of the survey is derived through simulated quasar photometry, and this completeness estimate is checked using a sample of quasars selected by their photometric variability within the BOSS footprint. We investigate the level of systematics associated with our quasar sample using the simulations, in the process generating color-redshift relations and a new quasar K-correction. We probe the faint end of the QLF to Mi (z = 2.2) ≈ -24.5 and see a clear break in the QLF at all redshifts up to z = 3.5. A log-linear relation (in log Φ*-M*) for a luminosity evolution and density evolution model is found to adequately describe our data within the range 2.2 < z < 3.5; across this interval the break luminosity increases by a factor of ∼2.6 while Φ* declines by a factor of ∼8. At z ≲ 2.2 our data are reasonably well fit by a pure luminosity evolution model, and only a weak signature of "AGN downsizing" is seen, in line with recent studies of the hard X-ray luminosity function. We compare our measured QLF to a number of theoretical models and find that models making a variety of assumptions about quasar triggering and halo occupation can fit our data over a wide range of redshifts and luminosities. © 2013. The American Astronomical Society. All rights reserved.
• Wang, R., Wagg, J., Carilli, C. L., Walter, F., Lentati, L., Fan, X., Riechers, D. A., Bertoldi, F., Narayanan, D., Strauss, M. A., Cox, P., Omont, A., Menten, K. M., Knudsen, K. K., Neri, R., & Jiang, L. (2013). Star formation and gas kinematics of quasar host galaxies at z ∼ 6: New insights from ALMA. Astrophysical Journal, 773(1).
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  Abstract: We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [C II] 158 μm fine structure line and dust continuum emission from the host galaxies of five redshift 6 quasars. We also report complementary observations of 250 GHz dust continuum and CO (6-5) line emission from the z = 6.00 quasar SDSS J231038.88+185519.7 using the IRAM facilities. The ALMA observations were carried out in the extended array at 0.″7 resolution. We have detected the line and dust continuum in all five objects. The derived [C II] line luminosities are 1.6 × 109 to 8.7 × 10 9 L ⊙ and the [C II]-to-FIR luminosity ratios are 2.9-5.1 × 10-4, which is comparable to the values found in other high-redshift quasar-starburst systems and local ultra-luminous infrared galaxies. The sources are marginally resolved and the intrinsic source sizes (major axis FWHM) are constrained to be 0.″3-0.″6 (i.e., 1.7-3.5 kpc) for the [C II] line emission and 0.″2-0.″4 (i.e., 1.2-2.3 kpc) for the continuum. These measurements indicate that there is vigorous star formation over the central few kpc in the quasar host galaxies. The ALMA observations also constrain the dynamical properties of the star-forming gas in the nuclear region. The intensity-weighted velocity maps of three sources show clear velocity gradients. Such velocity gradients are consistent with a rotating, gravitationally bound gas component, although they are not uniquely interpreted as such. Under the simplifying assumption of rotation, the implied dynamical masses within the [C II]-emitting regions are of order 10 10-1011 M ⊙. Given these estimates, the mass ratios between the supermassive black holes and the spheroidal bulge are an order of magnitude higher than the mean value found in local spheroidal galaxies, which is in agreement with results from previous CO observations of high redshift quasars. © 2013. The American Astronomical Society. All rights reserved.
• Ahn, C. P., Alexandroff, R., Prieto, C. A., Anderson, S. F., Anderton, T., Andrews, B. H., Aubourg, É., Bailey, S., Balbinot, E., Barnes, R., Bautista, J., Beers, T. C., Beifiori, A., Berlind, A. A., Bhardwaj, V., Bizyaev, D., Blake, C. H., Blanton, M. R., Blomqvist, M., , Bochanski, J. J., et al. (2012). The ninth data release of the sloan digital sky survey: First spectroscopic data from the sdss-iii baryon oscillation spectroscopic survey. Astrophysical Journal, Supplement Series, 203(2).
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  Abstract: The Sloan Digital Sky Survey III (SDSS-III) presents the first spectroscopic data from the Baryon Oscillation Spectroscopic Survey (BOSS). This ninth data release (DR9) of the SDSS project includes 535,995 new galaxy spectra (median z 0.52), 102,100 new quasar spectra (median z 2.32), and 90,897 new stellar spectra, along with the data presented in previous data releases. These spectra were obtained with the new BOSS spectrograph and were taken between 2009 December and 2011 July. In addition, the stellar parameters pipeline, which determines radial velocities, surface temperatures, surface gravities, and metallicities of stars, has been updated and refined with improvements in temperature estimates for stars with T eff < 5000 K and in metallicity estimates for stars with [Fe/H] > -0.5. DR9 includes new stellar parameters for all stars presented in DR8, including stars from SDSS-I and II, as well as those observed as part of the SEGUE-2. The astrometry error introduced in the DR8 imaging catalogs has been corrected in the DR9 data products. The next data release for SDSS-III will be in Summer 2013, which will present the first data from the APOGEE along with another year of data from BOSS, followed by the final SDSS-III data release in 2014 December. © 2012. The American Astronomical Society. All rights reserved..
• Bian, F., Fan, X., Jiang, L., Dey, A., Green, R. F., Maiolino, R., Walter, F., McGreer, I., Wang, R., & Lin, Y. (2012). An ultraviolet ultra-luminous lyman break galaxy at Z = 2.78 in NDWFS boötes field. Astrophysical Journal, 757(2).
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  Abstract: We present one of the most ultraviolet (UV) luminous Lyman break galaxies (LBGs; J1432+3358) at z = 2.78, discovered in the NOAO Deep Wide-Field Survey Boötes field. The R-band magnitude of J1432+3358 is 22.29AB, more than two magnitudes brighter than typical L* LBGs at this redshift. The deep z-band image reveals two components of J1432+3358 separated by 10 with a flux ratio of 3:1. The high signal-to-noise ratio rest-frame UV spectrum shows Lyα emission line and interstellar medium absorption lines. The absence of N V and C IV emission lines, and the non-detection in X-ray and radio wavelengths and mid-infrared (MIR) colors indicates weak or no active galactic nuclei (
• Brammer, G. B., G., P., Franx, M., Fumagalli, M., Patel, S., Rix, H., Skelton, R. E., Kriek, M., Nelson, E., Schmidt, K. B., Bezanson, R., Cunha, E. D., Erb, D. K., Fan, X., Schreiber, N. F., Illingworth, G. D., Labbé, I., Leja, J., Lundgren, B., , Magee, D., et al. (2012). 3D-HST: A wide-field grism spectroscopic survey with the hubble space telescope. Astrophysical Journal, Supplement Series, 200(2).
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  Abstract: We present 3D-HST, a near-infrared spectroscopic Treasury program with the Hubble Space Telescope for studying the physical processes that shape galaxies in the distant universe. 3D-HST provides rest-frame optical spectra for a sample of ∼7000 galaxies at 1 < z < 3.5, the epoch when 60% of all star formation took place, the number density of quasars peaked, the first galaxies stopped forming stars, and the structural regularity that we see in galaxies today must have emerged. 3D-HST will cover three quarters (625arcmin 2) of the CANDELS Treasury survey area with two orbits of primary WFC3/G141 grism coverage and two to four orbits with the ACS/G800L grism in parallel. In the IR, these exposure times yield a continuum signal-to-noise ratio of 5per resolution element at H 140 ∼ 23.1 and a 5σ emission-line sensitivity of ∼5 × 10 -17 erg s -1 cm -2 for typical objects, improving by a factor of ∼2 for compact sources in images with low sky background levels. The WFC3/G141 spectra provide continuous wavelength coverage from 1.1 to 1.6 μm at a spatial resolution of ∼0″.13, which, combined with their depth, makes them a unique resource for studying galaxy evolution. We present an overview of the preliminary reduction and analysis of the grism observations, including emission-line and redshift measurements from combined fits to the extracted grism spectra and photometry from ancillary multi-wavelength catalogs. The present analysis yields redshift estimates with a precision of σ(z) = 0.0034(1 + z), or σ(v) 1000kms -1. We illustrate how the generalized nature of the survey yields near-infrared spectra of remarkable quality for many different types of objects, including a quasar at z = 4.7, quiescent galaxies at z ∼ 2, and the most distant T-type brown dwarf star known. The combination of the CANDELS and 3D-HST surveys will provide the definitive imaging and spectroscopic data set for studies of the 1 < z < 3.5 universe until the launch of the James Webb Space Telescope. © 2012. The American Astronomical Society. All rights reserved..
• D'Onofrio, M., Marziani, P., Sulentic, J. W., Dultzin, D., Richards, G., Knapen, J., Shlosman, I., Morganti, R., Falomo, R., Hawkins, M., Cavaliere, A., McLure, R., Shields, G., Netzer, H., Proga, D., Franceschini, A., Fan, X., & Elvis, M. (2012). Quasars in the Cosmic Environment. Fifty Years of Quasars: From Early Observations and Ideas to Future Research, 386, 439.
• Decarli, R., Walter, F., Yang, Y., Carilli, C. L., Fan, X., Hennawi, J. F., Kurk, J., Riechers, D., Rix, H., Strauss, M. A., & Venemans, B. P. (2012). Hubble Space Telescope narrowband search for extended Lyα emission around two z > 6 quasars. Astrophysical Journal, 756(2).
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  Abstract: We search for extended Lyα emission around two z > 6 quasars, SDSS J1030+0524 (z = 6.309) and SDSS J1148+5251 (z = 6.419) using Wide Field Camera 3 narrowband filters on board the Hubble Space Telescope. For each quasar, we collected two deep, narrowband images, one sampling the Lyα line+continuum at the quasar redshifts and one of the continuum emission redward of the line. After carefully modeling the point-spread function, we find no evidence for extended Lyα emission. These observations set 2σ limits of L(Lyα, extended)
• Dong, X., Ho, L. C., Yuan, W., Wang, T., Fan, X., Zhou, H., & Jiang, N. (2012). A uniformly selected sample of low-mass black holes in seyfert1 galaxies. Astrophysical Journal, 755(2).
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  Abstract: We have conducted a systematic search of low-mass black holes (BHs) in active galactic nuclei (AGNs) with broad Hα emission lines, aiming at building a homogeneous sample that is more complete than previous ones for fainter, less highly accreting sources. For this purpose, we developed a set of elaborate, automated selection procedures and applied it uniformly to the Fourth Data Release of the Sloan Digital Sky Survey. Special attention is given to AGN-galaxy spectral decomposition and emission-line deblending. We define a sample of 309 type 1 AGNs with BH masses in the range 8 × 10 4-2 × 10 6 M ⊙ (with a median of 1.2 × 10 6 M ⊙), using the virial mass estimator based on the broad Hα line. About half of our sample of low-mass BHs differs from that of Greene & Ho, with 61 of them discovered here for the first time. Our new sample picks up more AGNs with low accretion rates: the Eddington ratios of the present sample range from ≲ 0.01 to 1, with 30% below 0.1. This suggests that a significant fraction of low-mass BHs in the local universe are accreting at low rates. The host galaxies of the low-mass BHs have luminosities similar to those of L* field galaxies, optical colors of Sbc spirals, and stellar spectral features consistent with a continuous star formation history with a mean stellar age of less than 1Gyr. © 2012. The American Astronomical Society. All rights reserved.
• Fan, X. (2012). Observations of the first light and the epoch of reionization. Research in Astronomy and Astrophysics, 12(8), 865-890.
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  Abstract: Studying the first generation of stars, galaxies and supermassive black holes as well as the epoch of reionization is one of the fundamental questions of modern astrophysics. The last few years have witnessed the first confirmation of the discoveries of galaxies, quasars and Gamma-Ray Bursts at z > 7, with possible detections at z10. There is also mounting evidence that cosmic reionization is a prolonged process that peaks around z10 and ends at z6-7. Observations of the highest redshift intergalactic medium and the most metal-poor stars in the Galaxy begin to constrain the earliest chemical enrichment processes in the Universe. These observations provide a glimpse of cosmic history over the first billion years after the Big Bang. In this review, we will present recent results on the observations of the high-redshift Universe over the past decade, highlight key challenges and uncertainties in these observations, and preview what is possible with the next generation facilities in studying the first light and mapping the history of reionization. © 2012 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd.
• Fan, X. (2012). Observations of the first light and the epoch of reionization. Research in Astronomy and Astrophysics, 12, 865-890.
• Huang, S. Y., Fan, X., Qu, D., Chen, Y. P., Wang, W. G., Wu, J., Chen, T. Y., Xiao, J. Q., & Chien, C. L. (2012). Transport Magnetic Proximity Effects in Platinum. Physical Review Letters, 109(10), 107204.
• Lundgren, B. F., Brammer, G., Dokkum, P. V., Bezanson, R., Franx, M., Fumagalli, M., Momcheva, I., Nelson, E., Skelton, R. E., Wake, D., Whitaker, K., Cunha, E. D., Erb, D. K., Fan, X., Kriek, M., Labbé, I., Marchesini, D., Patel, S., Rix, H. W., , Schmidt, K., et al. (2012). Large-scale star-formation-driven outflows at 1 < z < 2 in the 3D-HST survey. Astrophysical Journal, 760(1).
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  Abstract: We present evidence of large-scale outflows from three low-mass (log(M */M ·) 9.75) star-forming (SFR>4 M ·yr -1) galaxies observed at z = 1.24, z = 1.35, and z = 1.75 in the 3D-HST Survey. Each of these galaxies is located within a projected physical distance of 60kpc around the sight line to the quasar SDSSJ123622.93+621526.6, which exhibits well-separated strong (W λ2796 r g∼ 0.8 Å) Mg II absorption systems matching precisely to the redshifts of the three galaxies. We derive the star formation surface densities from the Hα emission in the WFC3 G141 grism observations for the galaxies and find that in each case the star formation surface density well exceeds 0.1 M ·yr -1kpc -2, the typical threshold for starburst galaxies in the local universe. From a small but complete parallel census of the 0.65 < z < 2.6 galaxies with H 140 ≲ 24 proximate to the quasar sight line, we detect Mg II absorption associated with galaxies extending to physical distances of 130kpc. We determine that the W r > 0.8Å Mg II covering fraction of star-forming galaxies at 1 < z < 2 may be as large as unity on scales extending to at least 60kpc, providing early constraints on the typical extent of starburst-driven winds around galaxies at this redshift. Our observations additionally suggest that the azimuthal distribution of W r > 0.4Å Mg II absorbing gas around star-forming galaxies may evolve from z ∼ 2 to the present, consistent with recent observations of an increasing collimation of star-formation-driven outflows with time from z 3. © 2012. The American Astronomical Society. All rights reserved.
• Mechtley, M., Windhorst, R. A., Ryan, R. E., Schneider, G., Cohen, S. H., Jansen, R. A., Fan, X., Hathi, N. P., Keel, W. C., Koekemoer, A. M., Röttgering, H., Scannapieco, E., Schneider, D. P., Strauss, M. A., & Yan, H. J. (2012). Near-infrared imaging of A z = 6.42 quasar host galaxy with the hubble space telescope wide field camera 3. Astrophysical Journal Letters, 756(2).
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  Abstract: We report on deep near-infrared F125W (J) and F160W (H) Hubble Space Telescope Wide Field Camera 3 images of the z = 6.42 quasar J1148+5251 to attempt to detect rest-frame near-ultraviolet emission from the host galaxy. These observations included contemporaneous observations of a nearby star of similar near-infrared colors to measure temporal variations in the telescope and instrument point-spread function (PSF). We subtract the quasar point source using both this direct PSF and a model PSF. Using direct subtraction, we measure an upper limit for the quasar host galaxy of mJ > 22.8 and m H > 23.0 AB mag (2 σ). After subtracting our best model PSF, we measure a limiting surface brightness from 03 to 05 radius of μJ > 23.5 and μH > 23.7 AB mag arcsec -2 (2 σ). We test the ability of the model subtraction method to recover the host galaxy flux by simulating host galaxies with varying integrated magnitude, effective radius, and Sérsic index, and conducting the same analysis. These models indicate that the surface brightness limit (μJ > 23.5 AB mag arcsec-2) corresponds to an integrated upper limit of mJ > 22-23 AB mag, consistent with the direct subtraction method. Combined with existing far-infrared observations, this gives an infrared excess log (IRX) > 1.0 and corresponding ultraviolet spectral slope β > -1.2 ± 0.2. These values match those of most local luminous infrared galaxies, but are redder than those of almost all local star-forming galaxies and z ≃ 6 Lyman break galaxies. © 2012 The American Astronomical Society. All rights reserved.
• Morganson, E., Rosa, G. D., Decarli, R., Walter, F., Chambers, K., McGreer, I., Fan, X., Burgett, W., Flewelling, H., Greiner, J., Hodapp, K., Kaiser, N., Magnier, E., Price, P., Rix, H., Sweeney, B., & Waters, C. (2012). The first high-redshift quasar from pan-starrs. Astronomical Journal, 143(6).
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  Abstract: We present the discovery of the first high-redshift (z > 5.7) quasar from the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1 or PS1). This quasar was initially detected as an i P1 dropout in PS1, confirmed photometrically with the SAO Wide-field InfraRed Camera at Arizona's Multiple Mirror Telescope (MMT) and the Gamma-Ray Burst Optical/Near-Infrared Detector at the MPG 2.2m telescope in La Silla. The quasar was verified spectroscopically with the MMT Spectrograph, Red Channel and the Cassegrain Twin Spectrograph at the Calar Alto 3.5m telescope. Its near-infrared spectrum was taken at the Large Binocular Telescope Observatory (LBT) with the LBT Near-Infrared Spectroscopic Utility with Camera and Integral Field Unit for Extragalactic Research. It has a redshift of 5.73, an AB z P1 magnitude of 19.4, a luminosity of 3.8 × 10 47ergs -1, and a black hole mass of 6.9 × 10 9 M .It is a broad absorption line quasar with a prominent Lyβ peak and a very blue continuum spectrum. This quasar is the first result from the PS1 high-redshift quasar search that is projected to discover more than 100i P1 dropout quasars and could potentially find more than 10 z P1 dropout (z > 6.8) quasars. © 2012 The American Astronomical Society. All rights reserved.
• Pĝris, I., Petitjean, P., Aubourg, É., Bailey, S., Ross, N. P., Myers, A. D., Strauss, M. A., Anderson, S. F., Arnau, E., Bautista, J., Bizyaev, D., Bolton, A. S., Bovy, J., Brandt, W. N., Brewington, H., Browstein, J. R., Busca, N., Capellupo, D., Carithers, W., , Croft, R. A., et al. (2012). The Sloan Digital Sky Survey quasar catalog: Ninth data release. Astronomy and Astrophysics, 548.
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  Abstract: We present the Data Release 9 Quasar (DR9Q) catalog from the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey III. The catalog includes all BOSS objects that were targeted as quasar candidates during the survey, are spectrocopically confirmed as quasars via visual inspection, have luminosities Mi[z = 2] 2.15 (61 931) is ~2.8 times larger than the number of z > 2.15 quasars previously known. Redshifts and FWHMs are provided for the strongest emission lines (C iv, C iii], Mg ii). The catalog identifies 7533 broad absorption line quasars and gives their characteristics. For each object the catalog presents five-band (u, g, r, i, z) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains X-ray, ultraviolet, near-infrared, and radio emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3600-10 500 at a spectral resolution in the range 1300 < R < 2500; the spectra can be retrieved from the SDSS Catalog Archive Server. We also provide a supplemental list of an additional 949 quasars that have been identified, among galaxy targets of the BOSS or among quasar targets after DR9 was frozen. © 2012 ESO.
• Wang, R., Wagg, J., Carilli, C. L., Walter, F., Fan, X., Bertoldi, F., Riechers, D. A., Omont, A., Menten, K. M., Cox, P., Strauss, M. A., & Narayanan, D. (2012). Star formation in quasar host galaxies at redshift 6: Millimeter surveys and new insights from ALMA. Proceedings of the International Astronomical Union, 8(S292), 184-187.
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  Abstract: We have been carrying out a systematic survey of the star formation and ISM properties in the host galaxies of z∼6 quasars. Our 250 GHz observations, together with available data from the literature, yield a sample of 14 z∼6 quasars that are bright in millimeter dust continuum emission with estimated FIR luminosities of a few 1012 to 1013 L⊠™. Most of these millimeter-detected z∼6 quasars have also been detected in molecular CO line emission, indicating molecular gas masses on order of 1010 M⊙. We have searched for [C II] 158 micron fine structure line emission toward four of the millimeter bright z∼6 quasars with ALMA and all of them have been detected. All these results suggest massive star formation at rates of about 600 to 2000 M⊙ yr-1 over the central few kpc region of these quasar host galaxies. © 2013 International Astronomical Union.
• White, M., Myers, A. D., Ross, N. P., Schlegel, D. J., Hennawi, J. F., Shen, Y., McGreer, I., Strauss, M. A., Bolton, A. S., Bovy, J., Fan, X., Miralda-Escude, J., Palanque-Delabrouille, N., Paris, I., Petitjean, P., Schneider, D. P., Viel, M., Weinberg, D. H., Yeche, C., , Zehavi, I., et al. (2012). The clustering of intermediate-redshift quasars as measured by the Baryon Oscillation Spectroscopic Survey. Monthly Notices of the Royal Astronomical Society, 424(2), 933-950.
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  Abstract: We measure the quasar two-point correlation function over the redshift range 2.2 < z < 2.8 using data from the Baryon Oscillation Spectroscopic Survey. We use a homogeneous subset of the data consisting of 27129 quasars with spectroscopic redshifts - by far the largest such sample used for clustering measurements at these redshifts to date. The sample covers 3600deg 2, corresponding to a comoving volume of 9.7(h -1Gpc) 3 assuming a fiducial Λ cold dark matter cosmology, and it has a median absolute i-band magnitude of -26, k-corrected to z= 2. After accounting for redshift errors we find that the redshift-space correlation function is fitted well by a power law of slope -2 and amplitude s 0= (9.7 ± 0.5)h -1Mpc over the range 3 < s < 25h -1Mpc. The projected correlation function, which integrates out the effects of peculiar velocities and redshift errors, is fitted well by a power law of slope -1 and r 0= (8.4 ± 0.6)h -1Mpc over the range 4 < R < 16h -1Mpc. There is no evidence for strong luminosity or redshift dependence to the clustering amplitude, in part because of the limited dynamic range in our sample. Our results are consistent with, but more precise than, previous measurements at similar redshifts. Our measurement of the quasar clustering amplitude implies a bias factor of b≃ 3.5 for our quasar sample. We compare the data to models to constrain the manner in which quasars occupy dark matter haloes at z∼ 2.4 and infer that such quasars inhabit haloes with a characteristic mass of 〈M〉≃ 10 12h -1M ⊙ with a duty cycle for the quasar activity of 1 per cent. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.
• Zhang, Z. H., Gui, Y. S., Fu, L., Fan, X. L., Cao, J. W., Xue, D. S., Freitas, P. P., Houssameddine, D., Hemour, S., Wu, K., & Hu, C. (2012). Seebeck Rectification Enabled by Intrinsic Thermoelectrical Coupling in Magnetic Tunneling Junctions. Physical Review Letters, 109(3), 037206.
• Aihara, H., Prieto, C. A., Deokkeun, A. n., Anderson, S. F., Aubourg, É., Balbinot, E., Beers, T. C., Berlind, A. A., Bickerton, S. J., Bizyaev, D., Blanton, M. R., Bochanski, J. J., Bolton, A. S., Bovy, J., Brandt, W. N., Brinkmann, J., Brown, P. J., Brownstein, J. R., Busca, N. G., , Campbell, H., et al. (2011). The eighth data release of the sloan digital sky survey: First data from SDSS-III. Astrophysical Journal, Supplement Series, 193(2).
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  Abstract: The Sloan Digital Sky Survey (SDSS) started a new phase in 2008 August, with new instrumentation and new surveys focused on Galactic structure and chemical evolution, measurements of the baryon oscillation feature in the clustering of galaxies and the quasar Lyα forest, and a radial velocity search for planets around 8000 stars. This paper describes the first data release of SDSS-III (and the eighth counting from the beginning of the SDSS). The release includes five-band imaging of roughly 5200 deg2 in the southern Galactic cap, bringing the total footprint of the SDSS imaging to 14,555 deg2, or over a third of the Celestial Sphere. All the imaging data have been reprocessed with an improved sky-subtraction algorithm and a final, self-consistent photometric recalibration and flat-field determination. This release also includes all data from the second phase of the Sloan Extension for Galactic Understanding and Exploration (SEGUE-2), consisting of spectroscopy of approximately 118,000 stars at both high and low Galactic latitudes. All the more than half a million stellar spectra obtained with the SDSS spectrograph have been reprocessed through an improved stellar parameter pipeline, which has better determination of metallicity for high-metallicity stars. © 2011. The American Astronomical Society. All rights reserved.
• Aihara, H., Prieto, C. A., Deokkeun, A. n., Anderson, S. F., Aubourg, É., Balbinot, E., Beers, T. C., Berlind, A. A., Bickerton, S. J., Bizyaev, D., Blanton, M., Bochanski, J. J., Bolton, A. S., Bovy, J., Brandt, W. N., Brinkmann, J., Brown, P., Brownstein, J. R., Busca, N. G., , Campbell, H., et al. (2011). Erratum: The eighth data release of the sloan digital sky survey: First data from SDSS-III (The Astrophysical Journal Supplement Series (2011) 193 (29)). Astrophysical Journal, Supplement Series, 195(2).
• Dong, X., Wang, J., Ho, L. C., Wang, T., Fan, X., Wang, H., Zhou, H., & Yuan, W. (2011). What controls the Fe II strength in active galactic nuclei?. Astrophysical Journal, 736(2).
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  Abstract: We used a large, homogeneous sample of 4178 z ≤ 0.8 Seyfert 1 galaxies and QSOs selected from the Sloan Digital Sky Survey to investigate the strength of Fe II emission and its correlation with other emission lines and physical parameters of active galactic nuclei. We find that the strongest correlations of almost all the emission-line intensity ratios and equivalent widths (EWs) are with the Eddington ratio (L/LEdd), rather than with the continuum luminosity at 5100 Å (L5100) or black hole mass (M BH); the only exception is the EW of ultraviolet Fe II emission, which does not correlate at all with broad-line width, L5100, M BH, or L/LEdd. By contrast, the intensity ratios of both the ultraviolet and optical Fe II emission to Mg II λ2800 correlate quite strongly with L/LEdd. Interestingly, among all the emission lines in the near-UV and optical studied in this paper (including Mg II λ2800, Hβ, and [O III] λ5007), the EW of narrow optical Fe II emission has the strongest correlation with L/LEdd. We hypothesize that the variation of the emission-line strength in active galaxies is regulated by L/LEdd because it governs the global distribution of the hydrogen column density of the clouds gravitationally bound in the line-emitting region, as well as its overall gas supply. The systematic dependence on L/L Edd must be corrected when using the Fe II/Mg II intensity ratio as a measure of the Fe/Mg abundance ratio to study the history of chemical evolution in QSO environments. © 2011. The American Astronomical Society. All rights reserved.
• G., P., Brammer, G., Fumagalli, M., Nelson, E., Franx, M., Rix, H., Kriek, M., Skelton, R. E., Patel, S., Schmidt, K. B., Bezanson, R., Bian, F., Cunha, E. D., Erb, D. K., Fan, X., Schreiber, N. F., Illingworth, G. D., Labbé, I., Lundgren, B., , Magee, D., et al. (2011). First results from the 3D-HST survey: The striking diversity of massive galaxies at z > 1. Astrophysical Journal Letters, 743(1).
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  Abstract: We present first results from the 3D-HST program, a near-IR spectroscopic survey performed with the Wide Field Camera 3 (WFC3) on the HST. We have used 3D-HST spectra to measure redshifts and Hα equivalent widths (EW Hα) for a complete, stellar mass-limited sample of 34 galaxies at 1 < z < 1.5 with M star > 1011M ⊙ in the COSMOS, GOODS, and AEGIS fields. We find that a substantial fraction of massive galaxies at this epoch are forming stars at a high rate: the fraction of galaxies with EWHα >10 is 59%, compared to 10% among Sloan Digital Sky Survey galaxies of similar masses at z = 0.1. Galaxies with weak Hα emission show absorption lines typical of 2-4Gyr old stellar populations. The structural parameters of the galaxies, derived from the associated WFC3 F140W imaging data, correlate with the presence of Hα; quiescent galaxies are compact with high Sérsic index and high inferred velocity dispersion, whereas star-forming galaxies are typically large two-armed spiral galaxies, with low Sérsic index. Some of these star-forming galaxies might be progenitors of the most massive S0 and Sa galaxies. Our results challenge the idea that galaxies at fixed mass form a homogeneous population with small scatter in their properties. Instead, we find that massive galaxies form a highly diverse population at z > 1, in marked contrast to the local universe. © 2011. The American Astronomical Society. All rights reserved.
• Jang, M., Myungshin, I. m., Lee, I., Urata, Y., Huang, K., Hirashita, H., Fan, X., & Jiang, L. (2011). Dust properties in the afterglow of GRB 071025 at z∼5. Astrophysical Journal Letters, 741(1).
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  Abstract: At high redshift, the universe is so young that core-collapse supernovae (SNe) are suspected to be the dominant source of dust production. However, some observations indicate that the dust production by SNe is an inefficient process, casting doubts on the existence of abundant SNe-dust in the early universe. Recently, Perley et al. reported that the afterglow of GRB 071025 - an unusually red gamma-ray burst (GRB) at z 5 - shows evidence for SNe-produced dust. Since this is perhaps the only high-redshift GRB exhibiting compelling evidence for SNe-dust but the result could easily be affected by small systematics in photometry, we re-examined the extinction properties of GRB 071025 using our own optical/near-infrared data at a different epoch. In addition, we tested SNe-dust models with different progenitor masses and dust destruction efficiencies to constrain the dust formation mechanisms. By searching for the best-fit model of the afterglow spectral energy distribution, we confirm the previous claim that the dust in GRB 071025 is most likely to originate from SNe. We also find that the SNe-dust model of 13 or 25 M⊙ without dust destruction fits the extinction property of GRB 071025 best, while pair-instability SNe models with a 170 M⊙ progenitor poorly fit the data. Our results indicate that, at least in some systems at high redshift, SNe with intermediate initial masses within 10-30 M⊙ were the main contributors for the dust enrichment, and the dust destruction effect due to reverse shock was negligible. © 2011. The American Astronomical Society. All rights reserved.
• Lane, R. A., Shemmer, O., Diamond-Stanic, A. M., Fan, X., Anderson, S. F., Brandt, W. N., Plotkin, R. M., Richards, G. T., Schneider, D. P., & Strauss, M. A. (2011). The ultraviolet-to-mid-infrared spectral energy distribution of weak emission line quasars. Astrophysical Journal, 743(2).
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  Abstract: We present Spitzer Space Telescope photometry of 18 Sloan Digital Sky Survey (SDSS) quasars at 2.7 ≤ z ≤ 5.9 which have weak or undetectable high-ionization emission lines in their rest-frame ultraviolet (UV) spectra (hereafter weak-lined quasars, or WLQs). The Spitzer data are combined with SDSS spectra and ground-based, near-infrared (IR) photometry of these sources to produce a large inventory of spectral energy distributions (SEDs) of WLQs across the rest-frame 0.1-5 μm spectral band. The SEDs of our sources are inconsistent with those of BL Lacertae objects which are dominated by synchrotron emission due to a jetaligned close to our line of sight, but are consistent with the SED of ordinary quasars with similar luminosities and redshifts that exhibit a near-to-mid-IR "bump," characteristic of hot dust emission. This indicates that broad emission lines in WLQs are intrinsically weak, rather than suffering continuum dilution from a jet, and that such sources cannot be selected efficiently from traditional photometric surveys. © 2011. The American Astronomical Society. All rights reserved.
• McGreer, I. D., Mesinger, A., & Fan, X. (2011). The first (nearly) model-independent constraint on the neutral hydrogen fraction at. Monthly Notices of the Royal Astronomical Society, 415(4), 3237-3246.
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  Abstract: Cosmic reionization is expected to be complex, extended and very inhomogeneous. Existing constraints atz~ 6 on the volume-averaged neutral hydrogen fraction,, are highly model-dependent and controversial. Constraints atz< 6, suggesting that the Universe is highly ionized, are also model-dependent, but more fundamentally are invalid in the context of inhomogeneous reionization. As such, it has recently been pointed out that there is no conclusive evidence that reionization has completed byz~ 5-6, a fact that has important ramifications on the interpretation of high-redshift observations and theoretical models. We present the first direct upper limits onatz~ 5-6 using the simple and robust statistic of the covering fraction of dark pixels in the Lyα/β forests of high-redshift quasars. With a sample of 13 Keck Echellette Spectrograph and Imager spectra we constrainat 5.0 ≲z≲ 5.5, rising toatz~ 6.1. We also find tentative evidence for a break in the redshift evolution of the dark covering fraction atz~ 5.5. A subsample of two deep spectra provides a more stringent constraint of, when combined with conservative estimates of cosmic variance. This upper limit is comparable to existing results atz~ 6 but is more robust. The results presented here do not rely on assumptions about quasar continua, intergalactic medium density, the morphology of Hii regions or ionizing background fields, and thus are a good starting point for future interpretation of high-redshift observations. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.
• McLinden, E. M., Finkelstein, S. L., Rhoads, J. E., Malhotra, S., Hibon, P., L., M., Cresci, G., Quirrenbach, A., Pasquali, A., Bian, F., Fan, X., & Woodward, C. E. (2011). First spectroscopic measurements of [O III] emission from Lyα selected field galaxies at z ∼ 3.1. Astrophysical Journal, 730(2).
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  Abstract: We present the first spectroscopic measurements of the [O III] 5007 line in two z3.1 Lyα emitting galaxies (LAEs) using the new near-infrared instrument LUCIFER1 on the 8.4 m Large Binocular Telescope. We also describe the optical imaging and spectroscopic observations used to identify these LAEs. Using the [O III] line we have measured accurate systemic redshifts for these two galaxies, and discovered a velocity offset between the [O III] and Lyα lines in both, with the Lyα line peaking 342 and 125 km s-1 redward of the systemic velocity. These velocity offsets imply that there are powerful outflows in high-redshift LAEs. They also ease the transmission of Lyα photons through the interstellar medium and intergalactic medium around the galaxies. By measuring these offsets directly, we can refine both Lyα-based tests for reionization, and Lyα luminosity function measurements where the Lyα forest affects the blue wing of the line. Our work also provides the first direct constraints on the strength of the [O III] line in high-redshift LAEs. We find [O III] fluxes of 7 and 36 ×10 -17 erg s-1 cm-2 in two z3.1 LAEs. These lines are strong enough to dominate broadband flux measurements that include the line (in this case, Ks -band photometry). Spectral energy distribution fits that do not account for the lines would therefore overestimate the 4000 (and/or Balmer) break strength in such galaxies, and hence also the ages and stellar masses of such high-z galaxies. © 2011. The American Astronomical Society. All rights reserved.
• Rosa, G. D., Decarli, R., Walter, F., Fan, X., Jiang, L., Kurk, J., Pasquali, A., & Rix, H. W. (2011). Evidence for non-evolving fe II/Mg II ratios in rapidly accreting z ∼ 6 QSOs. Astrophysical Journal, 739(2).
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  Abstract: Quasars (QSOs) at the highest known redshift (z 6) are unique probes of the early growth of supermassive black holes (BHs). Until now, only the most luminous QSOs have been studied, often one object at a time. Here we present the most extensive consistent analysis to date of 4 < z < 6.5 QSOs with observed near-infrared spectra, combining three new z 6 objects from our ongoing Very Large Telescope-Infrared Spectrometer And Array Camera program with nineteen sources from the literature. The new sources extend the existing Sloan Digital Sky Survey (SDSS) sample toward the faint end of the QSO luminosity function. Using a maximum likelihood fitting routine optimized for our spectral decomposition, we estimate the BH mass (M BH), the Eddington ratio (defined as L bol/L Edd), and the Fe II/Mg II line ratio, a proxy for the chemical abundance, to characterize both the central object and the broad-line region gas. The QSOs in our sample host BHs with masses of 109 M⊙ that are accreting close to the Eddington luminosity, consistent with earlier results. We find that the distribution of observed Eddington ratios is significantly different than that of a luminosity-matched comparison sample of SDSS QSOs at lower redshift (0.35 < z < 2.25): the average 〈log (L bol/L Edd)〉 = -0.37 (L bol/L Edd 0.43) with a scatter of 0.20dex for the z > 4 sample and the 〈log (L bol/L Edd)〉 = -0.80 (L bol/L Edd 0.16) with a scatter of 0.24dex for the 0.35 < z < 2.25 sample. This implies that, at a given luminosity, the M BH at high-z is typically lower than the average M BH of the lower-redshift population, i.e., the z > 4 sources are accreting significantly faster than the lower-redshift ones. We show that the derived Fe II/Mg II ratios depend sensitively on the analysis performed: our self-consistent, homogeneous analysis significantly reduces the Fe II/Mg II scatter found in previous studies. The measured Fe II/Mg II line ratios show no sign of evolution with cosmic time in the redshift range 4 < z < 6.5. If the Fe II/Mg II line ratio is used as a secondary proxy of the Fe/Mg abundance ratio, this implies that the QSOs in our sample have undergone a major episode of Fe enrichment in the few 100 Myr preceding the cosmic age at which they are observed. © 2011. The American Astronomical Society. All rights reserved..
• Ryan Jr., R. E., Thorman, P. A., Yan, H., Fan, X., Yan, L., Mechtley, M. R., Hathi, N. P., Cohen, S. H., Windhorst, R. A., McCarthy, P. J., & Wittman, D. M. (2011). Hubble space telescope observations of field ultracool dwarfs at high galactic latitude. Astrophysical Journal, 739(2).
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  Abstract: We present a sample of 17 newly discovered ultracool dwarf candidates later than M8, drawn from 231.90arcmin2 of Hubble Space Telescope Wide Field Camera 3 infrared imaging. By comparing the observed number counts for 17.5 ≤ J125 ≤ 25.5ABmag to an exponential disk model, we estimate a vertical scale height of zscl = 290 25 (random) 31 (systematic)pc for a binarity fraction of fb = 0. While our estimate is roughly consistent with published results, we suggest that the differences can be attributed to sample properties, with the present sample containing far more substellar objects than previous work. We predict the object counts should peak at J125 24ABmag due to the exponentially declining number density at the edge of the disk. We conclude by arguing that trend in scale height with spectral type may breakdown for brown dwarfs since they do not settle onto the main sequence. © 2011 The American Astronomical Society. All rights reserved.
• Simcoe, R. A., Cooksey, K. L., Matejek, M., Burgasser, A. J., Bochanski, J., Lovegrove, E., Bernstein, R. A., Pipher, J. L., Forrest, W. J., McMurtry, C., Fan, X., & O'Meara, J. (2011). Constraints on the universal C IV mass density at z ∼ 6 from early infrared spectra obtained with the Magellan fire spectrograph. Astrophysical Journal, 743(1).
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  Abstract: We present a new determination of the intergalactic C IV mass density at 4.3 < z < 6.3. Our constraints are derived from high signal-to-noise spectra of seven quasars at z > 5.8 obtained with the newly commissioned Folded-Port Infrared Echellette (FIRE) spectrograph on the Magellan Baade telescope, coupled with six observations of northern objects taken from the literature. We confirm the presence of a downturn in the C IV abundance at 〈z〉 = 5.66 by a factor of 4.1 relative to its value at 〈z〉 = 4.96, as measured in the same sight lines. In the FIRE sample, a strong system previously reported in the literature as C IV at z = 5.82 is re-identified as Mg II at z = 2.78, leading to a substantial downward revision in for these prior studies. Additionally, we confirm the presence of at least two systems with low-ionization C II, Si II, and O I absorption but relatively weak signal from C IV. The latter systems may be of interest if the downward trend in at high redshift is driven in part by ionization effects. © 2011 The American Astronomical Society. All rights reserved.
• Wang, R., Wagg, J., Carilli, C. L., Neri, R., Walter, F., Omont, A., Riechers, D. A., Bertoldi, F., Menten, K. M., Cox, P., Strauss, M. A., Fan, X., & Jiang, L. (2011). Far-infrared and molecular co emission from the host galaxies of faint quasars at z ∼ 6. Astronomical Journal, 142(4).
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  Abstract: We present new millimeter and radio observations of nine z ∼ 6 quasars discovered in deep optical and near-infrared surveys. We observed the 250GHz continuum in eight of the nine objects and detected three of them. New 1.4GHz radio continuum data have been obtained for four sources, and one has been detected. We searched for molecular CO (6-5) line emission in the three 250GHz detections and detected two of them. Combined with previous millimeter and radio observations, we study the far-infrared (FIR) and radio emission and quasar-host galaxy evolution with a sample of 18 z ∼ 6 quasars that are faint at UV and optical wavelengths (rest-frame 1450 magnitudes of m 1450 ≥ 20.2). The average FIR-to-active galactic nucleus (AGN) UV luminosity ratio of this faint quasar sample is about two times higher than that of the bright quasars at z ∼ 6 (m 1450 < 20.2). A fit to the average FIR and AGN bolometric luminosities of both the UV/optically faint and bright z ∼ 6 quasars, and the average luminosities of samples of submillimeter/millimeter-observed quasars at z ∼ 2-5, yields a relationship of L FIR L bol0.62. Five of the 18 faint z ∼ 6 quasars have been detected at 250GHz. These 250GHz detections, as well as most of the millimeter-detected optically bright z ∼ 6 quasars, follow a shallower trend of ∼ L FIR L bol0.45 defined by the starburst-AGN systems in local and high-z universe. The millimeter continuum detections in the five objects and molecular CO detections in three of them reveal a few × 108 M ⊙ of FIR-emitting warm dust and 1010 M ⊙ of molecular gas in the quasar host galaxies. All these results argue for massive star formation in the quasar host galaxies, with estimated star formation rates of a few hundred M ⊙ yr-1. Additionally, the higher FIR-to-AGN luminosity ratio found in these 250GHz detected faint quasars also suggests a higher ratio between star formation rate and supermassive black hole accretion rate than the UV/optically most luminous quasars at z ∼ 6. © 2011. The American Astronomical Society. All rights reserved.
• Wang, R., Wagg, J., Carilli, C. L., Walter, F., Riechers, D. A., Willott, C., Bertoldi, F., Omont, A., Beelen, A., Cox, P., Strauss, M. A., Bergeron, J., Forveille, T., Menten, K. M., & Fan, X. (2011). CO (2-1) line emission in redshift 6 quasar host galaxies. Astrophysical Journal Letters, 739(1).
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  Abstract: We report new observations of CO (2-1) line emission toward five z 6 quasars using the Ka-band receiver system on the Expanded Very Large Array (EVLA). Strong detections were obtained in two of them, SDSSJ092721.82+200123.7 and CFHQS J142952.17+544717.6, and a marginal detection was obtained in another source, SDSS J084035.09+562419.9. Upper limits of the CO (2-1) line emission have been obtained for the other two objects. The CO (2-1) line detection in J0927+2001 together with previous measurements of the CO (6-5) and (5-4) lines reveal important constraints on the CO excitation in the central 10kpc region of the quasar host galaxy. The CO (2-1) line emission from J1429+5447 is resolved into two distinct peaks separated by 12 (6.9kpc), indicating a possible gas-rich, major merging system, and the optical quasar position is consistent with the west peak. This result is in good agreement with the picture in which intense host galaxy star formation is coeval with rapid supermassive black hole (SMBH) accretion in the most distant universe. The two EVLA detections are ideal targets for further high-resolution imaging (e.g., with ALMA or EVLA observations) to study the gas distribution, dynamics, and SMBH-bulge-mass relation in these earliest quasar host galaxy systems. © 2011. The American Astronomical Society. All rights reserved.
• Wu, X., Wang, R., Schmidt, K. B., Bian, F., Jiang, L., & Fan, X. (2011). Discovering the missing 2.2 < z < 3 quasars by combining optical variability and optical/near-infrared colors. Astronomical Journal, 142(3).
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  Abstract: The identification of quasars in the redshift range 2.2 < z < 3 is known to be very inefficient because the optical colors of such quasars are indistinguishable from those of stars. Recent studies have proposed using optical variability or near-infrared (near-IR) colors to improve the identification of the missing quasars in this redshift range. Here we present a case study combining both methods. We select a sample of 70 quasar candidates from variables in Sloan Digital Sky Survey (SDSS) Stripe 82, which are non-ultraviolet excess sources and have UKIDSS near-IR public data. They are clearly separated into two parts on the Y - K/g - z color-color diagram, and 59 of them meet or lie close to a newly proposed Y - K/g - z selection criterion for z < 4 quasars. Of these 59 sources, 44 were previously identified as quasars in SDSS DR7, and 35 of them are quasars at 2.2 < z < 3. We present spectroscopic observations of 14 of 15 remaining quasar candidates using the Bok 2.3m telescope and the MMT 6.5m telescope, and successfully identify all of them as new quasars at z= 2.36-2.88. We also apply this method to a sample of 643 variable quasar candidates with SDSS-UKIDSS nine-band photometric data selected from 1875 new quasar candidates in SDSS Stripe 82 given by Butler & Bloom based on the time-series selections, and find that 188 of them are probably new quasars with photometric redshifts at 2.2 < z < 3. Our results indicate that the combination of optical variability and optical/near-IR colors is probably the most efficient way to find 2.2 < z < 3 quasars and is very helpful for constructing a complete quasar sample. We discuss its implications for ongoing and upcoming large optical and near-IR sky surveys. © 2011. The American Astronomical Society. All rights reserved..
• Yan, H., Yan, L., Zamojski, M. A., Windhorst, R. A., McCarthy, P. J., Fan, X., J., H., Koekemoer, A. M., Robertson, B. E., Davé, R., & Cai, Z. (2011). Probing very bright end of galaxy luminosity function at z ≳ 7 using Hubble Space Telescope Pure Parallel observations. Astrophysical Journal Letters, 728(1 PART II).
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  Abstract: We report the first results from the Hubble Infrared Pure Parallel Imaging Extragalactic Survey, which utilizes the pure parallel orbits of the Hubble Space Telescope to do deep imaging along a large number of random sightlines. To date, our analysis includes 26 widely separated fields observed by the Wide Field Camera 3, which amounts to 122.8 arcmin2 in total area. We have found three bright Y098-dropouts, which are candidate galaxies at z ≳ 7.4. One of these objects shows an indication of peculiar variability and its nature is uncertain. The other two objects are among the brightest candidate galaxies at these redshifts known to date (L > 2L *). Such very luminous objects could be the progenitors of the high-mass Lyman break galaxies observed at lower redshifts (up to z ∼ 5). While our sample is still limited in size, it is much less subject to the uncertainty caused by "cosmic variance" than other samples because it is derived using fields along many random sightlines. We find that the existence of the brightest candidate at z ≈ 7.4 is not well explained by the current luminosity function (LF) estimates at z ≈ 8. However, its inferred surface density could be explained by the prediction from the LFs at z ≈ 7 if it belongs to the high-redshift tail of the galaxy population at z ≈ 7. © 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
• Carilli, C. L., Wang, R., Fan, X., Walter, F., Kurk, J., Riechers, D., Wagg, J., Hennawi, J., Jiang, L., Menten, K. M., Bertoldi, F., Strauss, M. A., & Cox, P. (2010). Ionization near zones associated with quasars at z ∼ 6. Astrophysical Journal Letters, 714(1), 834-839.
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  Abstract: We analyze the size evolution of H II regions around 27 quasars between z = 5.7 and 6.4 ("quasar near zones" or NZs). We include more sources than previous studies, and we use more accurate redshifts for the host galaxies, with eight CO molecular line redshifts and nine Mg II redshifts. We confirm the trend for an increase in NZ size with decreasing redshift, with the luminosity-normalized proper size evolving as R NZ,corrected = (7.4 ± 0.3) - (8.0 ± 1.1) × (z - 6) Mpc. While derivation of the absolute neutral fraction remains difficult with this technique, the evolution of the NZ sizes suggests a decrease in the neutral fraction of intergalactic hydrogen by a factor ∼ 9.4 from z = 6.4 to 5.7, in its simplest interpretation. Alternatively, recent numerical simulations suggest that this rapid increase in NZ size from z = 6.4 to 5.7 is due to the rapid increase in the background photo-ionization rate at the end of the percolation or overlap phase, when the average mean-free path of ionizing photons increases dramatically. In either case, the results are consistent with the idea that z ∼ 6-7 corresponds to the tail end of cosmic reionization. The scatter in the normalized NZ sizes is larger than expected simply from measurement errors, and likely reflects intrinsic differences in the quasars or their environments. We find that the NZ sizes increase with quasar UV luminosity, as expected for photo-ionization dominated by quasar radiation. © 2010. The American Astronomical Society. All rights reserved.
• Dong, X., Ho, L. C., Wang, J., Wang, T., Wang, H., Fan, X., & Zhou, H. (2010). The prevalence of narrow optical Fe II emission lines in type 1 active galactic nuclei. Astrophysical Journal Letters, 721(2 PART 2), L143-L147.
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  Abstract: From detailed spectral analysis of a large sample of low-redshift active galactic nuclei (AGNs) selected from the Sloan Digital Sky Survey, we demonstrate - statistically for the first time - that narrow optical Fe II emission lines, both permitted and forbidden, are prevalent in type 1 AGNs. Remarkably, these optical lines are completely absent in type 2 AGNs, across a wide luminosity range, from Seyfert 2 galaxies to type 2 quasars. We suggest that the narrow Fe II-emitting gas is confined to a disk-like geometry in the innermost regions of the narrow-line region on physical scales smaller than the obscuring torus. © 2010. The American Astronomical Society. All rights reserved.
• Fan, X. (2010). The highest-redshift quasars. AIP Conference Proceedings, 1279, 44-51.
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  Abstract: The discovery of luminous quasars at z > 6 indicates the existence billion-solar-mass black holes at the end of reionization epoch. directly probing the early growth of supermassive black holes in the universe and the relation between the formation of early galaxies and black holes. Absorption spectra of these quasars trace the evolution of intergalactic medium (IGM). Currently, more than 60 quasars have been discovered at z > 5.5, including about 30 at z > 6. In this proceeding, I review the recent observational results studies of the highest redshift quasars, including the evolution of quasar density and luminosity function, the evolution of their spectral properties and chemical enrichment history and the evolution of AGN structure, and the evolution of Gunn-Peterson optical depth to probe the end of reionization epoch. © 2010 American Institute of Physics.
• Gallerani, S., Maiolino, R., Juarez, Y., Nagao, T., Marconi, A., Bianchi, S., Schneider, R., Mannucci, F., Oliva, T., Willott, C. J., Jiang, L., & Fan, X. (2010). The extinction law at high redshift and its implications. Astronomy and Astrophysics, 523(5).
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  Abstract: We analyze the optical-near infrared spectra of 33 quasars with redshifts 3.9 ≤ z ≤ 6.4 to investigate the properties of dust extinction at these cosmic epochs. The SMC extinction curve has been shown to reproduce the dust reddening of most quasars at z < 2.2; we investigate whether this curve also provides a good description of dust extinction at higher redshifts. We fit the observed spectra with synthetic absorbed quasar templates obtained by varying the intrinsic slope (αλ), the absolute extinction (A3000), and by using a grid of empirical and theoretical extinction curves. We find that seven quasars in our sample are affected by substantial extinction (A3000 ≥ 0.8), and characterized by very steep intrinsic slopes (αλ ≤ -2.3). All of the individual quasars require extinction curve deviating from that of the SMC, with a tendency to flatten at λ ≤ 2000 Å (in the rest frame of the source). However, due to the uncertainties in the individual extinction curves the SMC is still (marginally) consistent with the data in most cases. We obtain a mean extinction curve at z > 4, both by performing a simultaneous fit of all quasars and by averaging the extinction curves inferred for individual quasars. In the case of broad absorption line quasars (which are generally more absorbed by dust and possibly in a younger evolutionary stage), the mean extinction curve deviates from the SMC at a confidence level ≥95%. The difference between extinction curves in quasars at z > 4 and in quasars at lower redshift is indicative of either a different dust production mechanism at high redshift, or a different mechanism for processing dust into the ISM. We suggest that the same transitions may also apply to normal, star-forming galaxies at z > 4. In particular, the observed change in the average spectral slope of galaxies at z > 4 may be partially ascribed to a variation in the extinction curve, rather than a lower dust content at high redshift. In this scenario, the extinction curve inferred at z > 4 would imply a cosmic star-formation corrected for dust attenuation a factor of ∼2 higher than estimated in the past. © ESO 2010.
• Hennawi, J. F., Myers, A. D., Shen, Y., Strauss, M. A., Djorgovski, S. G., Fan, X., Glikman, E., Mahabal, A., Martin, C. L., Richards, G. T., Schneider, D. P., & Shankar, F. (2010). Binary quasars at high redshift. I. 24 new quasar pairs at z ∼ 3-4. Astrophysical Journal Letters, 719(2), 1672-1692.
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  Abstract: The clustering of quasars on small scales yields fundamental constraints on models of quasar evolution and the buildup of supermassive black holes. This paper describes the first systematic survey to discover high-redshift binary quasars. Using color-selection and photometric redshift techniques, we searched 8142 deg2 of Sloan Digital Sky Survey imaging data for binary quasar candidates, and confirmed them with follow-up spectroscopy. Our sample of 27 high-redshift binaries (24 of them new discoveries) atredshifts 2.9 < z < 4.3 with proper transverse separations 10 kpc < R- < 650 kpc increases the number of such objects known by an order of magnitude. Eight members of this sample are very close pairs with- < 100 kpc, and of these close systems four are at z > 3.5. The completeness and efficiency of our well-defined selection algorithm are quantified using simulated photometry and we find that our sample is ∼50% complete. Our companion paper uses this knowledge to make the first measurement of the small-scale clustering (R < 1 h-1 Mpc comoving) of high-redshift quasars. High-redshift binaries constitute exponentially rare coincidences of two extreme (M ≳ 109 M⊙) supermassive black holes. At z ∼ 4, there is about one close binary per 10 Gpc3, thus these could be the highest sigma peaks, the analogs of superclusters, in the early universe. © 2010. The American Astronomical Society. All rights reserved.
• Jiang, L., Fan, X., Brandt, W. N., Carilli, C. L., Egami, E., Hines, D. C., Kurk, J. D., Richards, G. T., Shen, Y., Strauss, M. A., Vestergaard, M., & Walter, F. (2010). Dust-free quasars in the early Universe. Nature, 464(7287), 380-383.
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  PMID: 20237563;Abstract: The most distant quasars known, at redshifts zφ6, generally have properties indistinguishable from those of lower-redshift quasars in the rest-frame ultraviolet/optical and X-ray bands. This puzzling result suggests that these distant quasars are evolved objects even though the Universe was only seven per cent of its current age at these redshifts. Recently one z 6 quasar was shown not to have any detectable emission from hot dust, but it was unclear whether that indicated different hot-dust properties at high redshift or if it is simply an outlier. Here we report the discovery of a second quasar without hot-dust emission in a sample of 21 zφ6 quasars. Such apparently hot-dust-free quasars have no counterparts at low redshift. Moreover, we demonstrate that the hot-dust abundance in the 21 quasars builds up in tandem with the growth of the central black hole, whereas at low redshift it is almost independent of the black hole mass. Thus zφ6 quasars are indeed at an early evolutionary stage, with rapid mass accretion and dust formation. The two hot-dust-free quasars are likely to be first-generation quasars born in dust-free environments and are too young to have formed a detectable amount of hot dust around them. © 2010 Macmillan Publishers Limited. All rights reserved.
• Kelly, B. C., Vestergaard, M., Fan, X., Hopkins, P., Hernquist, L., & Siemiginowska, A. (2010). Constraints on black hole growth, quasar lifetimes, and Eddington ratio distributions from the SDSS broad-line quasar black hole mass function. Astrophysical Journal Letters, 719(2), 1315-1334.
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  Abstract: We present an estimate of the black hole mass function of broad-line quasars (BLQSOs) that self-consistently corrects for incompleteness and the statistical uncertainty in the mass estimates, based on a sample of 9886 quasars at 1 < z < 4.5 drawn from the Sloan Digital Sky Survey (SDSS). We find evidence for "cosmic downsizing" of black holes in BLQSOs, where the peak in their number density shifts to higher redshift with increasing black hole mass. The cosmic mass density for black holes seen as BLQSOs peaks at z ∼ 2. We estimate the completeness of the SDSS as a function of the black hole mass and Eddington ratio, and find that at z > 1 it is highly incomplete at Mbh ≲ 109 M⊙ and L/LEdd ≲ 0.5. We estimate a lower limit on the lifetime of a single BLQSO phase to be tbl > 150 ± 15 Myr for black holes at z = 1 with a mass of Mbh = 109 M⊙, and we constrain the maximum mass of a black hole in a BLQSO to be ∼3 × 1010 M ⊙. Our estimated distribution of BLQSO Eddington ratios peaks at L/LEdd ∼ 0.05 and has a dispersion of ∼0.4 dex, implying that most BLQSOs are not radiating at or near the Eddington limit; however, the location of the peak is subject to considerable uncertainty. The steep increase in number density of BLQSOs toward lower Eddington ratios is expected if the BLQSO accretion rate monotonically decays with time. Furthermore, our estimated lifetime and Eddington ratio distributions imply that the majority of the most massive black holes spend a significant amount of time growing in an earlier obscured phase, a conclusion which is independent of the unknown obscured fraction. These results are consistent with models for self-regulated black hole growth, at least for massive systems at z > 1, where the BLQSO phase occurs at the end of a fueling event when black hole feedback unbinds the accreting gas, halting the accretion flow. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
• Leipski, C., Meisenheimer, K., Klaas, U., Walter, F., Nielbock, M., Krause, O., Dannerbauer, H., Bertoldi, F., Besel, M. -., Rosa, G. D., Fan, X., Haas, M., Hutsemekers, D., Jean, C., Lemke, D., Rix, H. -., & Stickel, M. (2010). Herschel -PACS far-infrared photometry of two z > 4 quasars. Astronomy and Astrophysics, 518(7-8).
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  Abstract: We present Herschel far-infrared (FIR) observations of two sub-mm bright quasars at high redshift: SDSS J1148+5251 (z = 6.42) and BR 1202-0725 (z = 4.69) obtained with the PACS instrument. Both objects are detected in the PACS photometric bands. The Herschel measurements provide additional data points that constrain the FIR spectral energy distributions (SEDs) of both sources, and they emphasise a broad range of dust temperatures in these objects. For λrest ≲ 20 μm, the two SEDs are very similar to the average SEDs of quasars at low redshift. In the FIR, however, both quasars show excess emission compared to low-z QSO templates, most likely from cold dust powered by vigorous star formation in the QSO host galaxies. For SDSS J1148+5251 we detect another object at 160 μm with a distance of ∼10″ from the QSO. Although no physical connection between the quasar and this object can be shown with the available data, it could potentially confuse low-resolution measurements, thus resulting in an overestimate of the FIR luminosity of the z = 6.42 quasar. © 2010 ESO.
• Leipski, C., Meisenheimer, K., Klaas, U., Walter, F., Nielbock, M., Krause, O., Dannerbauer, H., Bertoldi, F., Besel, M. -., Rosa, G. D., Fan, X., Haas, M., Hutsemekers, D., Jean, C., Lemke, D., Rix, H. -., & Stickel, M. (2010). Herschel-PACS far-infrared photometry of two z > 4 quasars. Astronomy and Astrophysics, 518(2).
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  Abstract: We present Herschel far-infrared (FIR) observations of two sub-mm bright quasars at high redshift: SDSS J1148+5251 (z = 6.42) and BR 1202-0725 (z = 4.69) obtained with the PACS instrument. Both objects are detected in the PACS photometric bands. The Herschel measurements provide additional data points that constrain the FIR spectral energy distributions (SEDs) of both sources, and they emphasise a broad range of dust temperatures in these objects. For λrestλrest 20 μm, the two SEDs are very similar to the average SEDs of quasars at low redshift. In the FIR, however, both quasars show excess emission compared to low-z QSO templates, most likely from cold dust powered by vigorous star formation in the QSO host galaxies. For SDSS J1148+5251 we detect another object at 160 μm with a distance of ∼10 from the QSO. Although no physical connection between the quasar and this object can be shown with the available data, it could potentially confuse low-resolution measurements, thus resulting in an overestimate of the FIR luminosity of the z = 6.42 quasar. © 2010 ESO.
• McGreer, I. D., Hall, P. B., Fan, X., Bian, F., Inada, N., Oguri, M., Strauss, M. A., Schneider, D. P., & Farnsworth, K. (2010). SDSS J094604.90+183541.8: A gravitationally lensed quasar at z = 4.8. Astronomical Journal, 140(2), 370-378.
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  Abstract: We report the discovery of a gravitationally lensed quasar identified serendipitously in the Sloan Digital Sky Survey (SDSS). The object, SDSS J094604.90+183541.8, was initially targeted for spectroscopy as a luminous red galaxy, but the SDSS spectrum has the features of both a z = 0.388 galaxy and a z = 4.8 quasar. We have obtained additional imaging that resolves the system into two quasar images separated by 3'.'06 and a bright galaxy that is strongly blended with one of the quasar images. We confirm spectroscopically that the two quasar images represent a single-lensed source at z = 4.8 with a total magnification of 3.2, and we derive a model for the lensing galaxy. This is the highest redshift lensed quasar currently known. We examine the issues surrounding the selection of such an unusual object from existing data and briefly discuss implications for lensed quasar surveys. © 2010. The American Astronomical Society. All rights reserved.
• Plotkin, R. M., Anderson, S. F., Brandt, W. N., Diamond-Stanic, A. M., Fan, X., MacLeod, C. L., Schneider, D. P., & Shemmer, O. (2010). Multiwavelength observations of radio-quiet quasars with weak emission lines. Astrophysical Journal Letters, 721(1), 562-575.
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  Abstract: We present radio and X-ray observations, as well as optical light curves, for a subset of 26 BL Lac candidates from the Sloan Digital Sky Survey (SDSS) lacking strong radio emission and with z < 2.2. Half of these 26 objects are shown to be stars, galaxies, or absorbed quasars. We conclude that the other 13 objects are active galactic nuclei (AGNs) with abnormally weak emission features; 10 of those 13 are definitively radio quiet, and, for those with available optical light curves, their level of optical flux variability is consistent with radio-quiet quasars. We cannot exclude the possibility that some of these 13 AGNs lie on the extremely radio-faint tail of the BL Lac distribution, but our study generally supports the notion that all BL Lac objects are radio-loud. These radio-quiet AGNs appear to have intrinsically weak or absent broad emission line regions (BELRs), and, based on their X-ray properties, we argue that some are low-redshift analogs to weak line quasars (WLQs). SDSS BL Lac searches are so far the only systematic surveys of the SDSS database capable of recovering such exotic low-redshift WLQs. There are 71 more z < 2.2 radio-quiet BL Lac candidates already identified in the SDSS, but not considered here, and many of those might be best unified with WLQs as well. Future studies combining low- and high-redshift WLQ samples will yield new insight on our understanding of the structure and formation of AGN BELRs. © 2010. The American Astronomical Society. All rights reserved.
• Schneider, D. P., Richards, G. T., Hall, P. B., Strauss, M. A., Anderson, S. F., Boroson, T. A., Ross, N. P., Shen, Y., Brandt, W. N., Fan, X., Inada, N., Jester, S., Knapp, G. R., Krawczyk, C. M., Thakar, A. R., E., D., Voges, W., Yanny, B., York, D. G., , Bahcall, N. A., et al. (2010). The sloan digital sky survey quasar catalog. V. Seventh data release. Astronomical Journal, 139(6), 2360-2373.
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  Abstract: We present the fifth edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog, which is based upon the SDSS Seventh Data Release. The catalog, which contains 105,783 spectroscopically confirmed quasars, represents the conclusion of the SDSS-I and SDSS-II quasar survey. The catalog consists of the SDSS objects that have luminosities larger than Mi = -22.0 (in a cosmology with H 0 = 70kms-1Mpc-1, ΩM = 0.3, and ΩΛ = 0.7), have at least one emission line with FWHM larger than 1000kms-1 or have interesting/complex absorption features, are fainter than i 15.0, and have highly reliable redshifts. The catalog covers an area of ≈ 9380 deg2. The quasar redshifts range from 0.065 to 5.46, with a median value of 1.49; the catalog includes 1248 quasars at redshifts greater than 4, of which 56 are at redshifts greater than 5. The catalog contains 9210 quasars with i < 18; slightly over half of the entries have i < 19. For each object the catalog presents positions accurate to better than 01rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03mag, and information on the morphology and selection method. The catalog also contains radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3800-9200 at a spectral resolution of ≃ 2000; the spectra can be retrieved from the SDSS public database using the information provided in the catalog. Over 96% of the objects in the catalog were discovered by the SDSS. We also include a supplemental list of an additional 207 quasars with SDSS spectra whose archive photometric information is incomplete. © 2010. The American Astronomical Society. All rights reserved.
• Sesar, B., Ivezić, Ž., Grammer, S. H., Morgan, D. P., Becker, A. C., Jurić, M., Lee, N. D., Annis, J., Beers, T. C., Fan, X., Lupton, R. H., Gunn, J. E., Knapp, G. R., Jiang, L., Jester, S., Johnston, D. E., & Lampeitl, H. (2010). Light curve templates and galactic distribution of RR lyrae stars from sloan digital sky survey stripe 82. Astrophysical Journal Letters, 708(1), 717-741.
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  Abstract: We present an improved analysis of halo substructure traced by RR Lyrae stars in the Sloan Digital Sky Survey (SDSS) stripe 82 region. With the addition of SDSS-II data, a revised selection method based on new ugriz light curve templates results in a sample of 483 RR Lyrae stars that is essentially free of contamination. The main result from our first study persists: the spatial distribution of halo stars at galactocentric distances 5-100 kpc is highly inhomogeneous. At least 20% of halo stars within ∼30 kpc from the Galactic center can be statistically associated with substructure. We present strong direct evidence, based on both RR Lyrae stars and main-sequence stars, that the halo stellar number density profile significantly steepens beyond a Galactocentric distance of 30 kpc, and a larger fraction of the stars are associated with substructure. By using a novel method that simultaneously combines data for RR Lyrae and main-sequence stars, and using photometric metallicity estimates for main-sequence stars derived from deep co-added u-band data, we measure the metallicity of the Sagittarius dSph tidal stream (trailing arm) toward R.A. ∼2h-3h and decl. ∼0° to be 0.3 dex higher ([Fe/H] = -1.2) than that of surrounding halo field stars. Together with a similar result for another major halo substructure, the Monoceros stream, these results support theoretical predictions that an early forming, smooth inner halo, is metal-poor compared to high surface brightness material that have been accreted onto a later-forming outer halo. The mean metallicity of stars in the outer halo that are not associated with detectable clumps may still be more metal-poor than the bulk of inner-halo stars, as has been argued from other data sets. © 2010. The American Astronomical Society. All rights reserved.
• Shemmer, O., Trakhtenbrot, B., Anderson, S. F., Brandt, W. N., Diamond-Stanic, A. M., Fan, X., Lira, P., Netzer, H., Plotkin, R. M., Richards, G. T., Schneider, D. P., & Strauss, M. A. (2010). Weak line quasars at high redshift: Extremely high accretion rates or anemic broad-line regions?. Astrophysical Journal Letters, 722(2 PART 2), L152-L156.
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  Abstract: We present Gemini-North K-band spectra of two representative members of the class of high-redshift quasars with exceptionally weak rest-frame ultraviolet emission lines (WLQs), SDSS J114153.34+021924.3 at z = 3.55 and SDSS J123743.08+630144.9 at z = 3.49. In both sources, we detect an unusually weak broad Hβ line and place tight upper limits on the strengths of their [Oiii] lines. Virial, Hβ-based black hole mass determinations indicate normalized accretion rates of L/LEdd= 0 .4 for these sources, which is well within the range observed for typical quasars with similar luminosities and redshifts.We also present high-quality XMM-Newton imaging spectroscopy of SDSS J114153.34+021924.3 and find a hard-X-ray photon index of Γ = 1.91 -0.22+0.24, which supports the virial L/LEdd determination in this source. Our results suggest that theweakness of the broad emission lines inWLQsis not a consequence of an extreme continuum-emission source but instead due to abnormal broad emission line region properties. © 2010 The American Astronomical Society. All rights reserved.
• Shen, Y., Hennawi, J. F., Shankar, F., Myers, A. D., Strauss, M. A., Djorgovski, S. G., Fan, X., Giocoli, C., Mahabal, A., Schneider, D. P., & Weinberg, D. H. (2010). Binary quasars at high redshift. II. Sub-Mpc clustering at z ∼ 3-4. Astrophysical Journal Letters, 719(2), 1693-1698.
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  Abstract: We present measurements of the small-scale (0.1 ≲ r ≲ 1 h -1 Mpc) quasar two-point correlation function at z > 2.9, for a flux-limited (i < 21) sample of 15 binary quasars compiled by Hennawi et al. The amplitude of the small-scale clustering increases from z ∼ 3 to z ∼ 4. The small-scale clustering amplitude is comparable to or lower than power-law extrapolations (assuming a fixed slope γ = 2) from the large-scale correlation function of the i < 20.2 quasar sample from the Sloan Digital Sky Survey. Using simple prescriptions relating quasars to dark matter halos, we model the observed small-scale clustering with halo occupation models. We found that the level of small-scale clustering favors an active fraction of black holes in (M ≳ 1013 h-1 M⊙) satellite halos fs ≳ 0.1 at z ≳ 3. © 2010. The American Astronomical Society. All rights reserved.
• Wang, R., Carilli, C. L., Neri, R., Riechers, D. A., Wagg, J., Walter, F., Bertoldi, F., Menten, K., Omont, A., Cox, P., & Fan, X. (2010). Molecular gas in z ∼ 6 quasar host galaxies. Astrophysical Journal Letters, 714(1), 699-712.
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  Abstract: We report our new observations of redshifted carbon monoxide emission from six z ∼ 6 quasars, using the IRAM Plateau de Bure Interferometer. CO (6-5) or (5-4) line emission was detected in all six sources. Together with two other previous CO detections, these observations provide unique constraints on the molecular gas emission properties in these quasar systems close to the end of the cosmic re-ionization. Complementary results are also presented for low-J CO lines observed at the Green Bank Telescope and the Very Large Array, and dust continuum from five of these sources with the SHARC-II bolometer camera at the Caltech Submillimeter Observatory. We then present a study of the molecular gas properties in our combined sample of eight CO-detected quasars at z 6. The detections of high-order CO line emission in these objects indicates the presence of highly excited molecular gas, with estimated masses on the order of 1010 M ⊙ within the quasar host galaxies. No significant difference is found in the gas mass and CO line width distributions between our z ∼ 6 quasars and samples of CO-detected 1.4 ≤ z ≤ 5 quasars and submillimeter galaxies. Most of the CO-detected quasars at z ∼ 6 follow the far-infrared-CO luminosity relationship defined by actively star-forming galaxies at low and high redshifts. This suggests that ongoing star formation in their hosts contributes significantly to the dust heating at FIR wavelengths. The result is consistent with the picture of galaxy formation co-eval with supermassive black hole (SMBH) accretion in the earliest quasar-host systems. We investigate the black hole-bulge relationships of our quasar sample, using the CO dynamics as a tracer for the dynamical mass of the quasar host. The median estimated black hole-bulge mass ratio is about 15 times higher than the present-day value of 0.0014. This places important constraints on the formation and evolution of the most massive SMBH-spheroidal host systems at the highest redshift. © 2010. The American Astronomical Society. All rights reserved.
• Abazajian, K. N., Adelman-Mccarthy, J. K., Agüeros, M. A., Allam, S. S., Prieto, C. A., Deokkeun, A. n., S., K., Anderson, S. F., Annis, J., Bahcall, N. A., Bailer-Jones, C., Barentine, J. C., Bassett, B. A., Becker, A. C., Beers, T. C., Bell, E. F., Belokurov, V., Berlind, A. A., Berman, E. F., , Bernardi, M., et al. (2009). The seventh data release of the sloan digital sky survey. Astrophysical Journal, Supplement Series, 182(2), 543-558.
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  Abstract: This paper describes the Seventh Data Release of the Sloan Digital Sky Survey (SDSS), marking the completion of the original goals of the SDSS and the end of the phase known as SDSS-II. It includes 11,663deg2 of imaging data, with most of the 2000deg2 increment over the previous data release lying in regions of low Galactic latitude. The catalog contains five-band photometry for 357million distinct objects. The survey also includes repeat photometry on a 120° long, 25 wide stripe along the celestial equator in the Southern Galactic Cap, with some regions covered by as many as 90individual imaging runs. We include a co-addition of the best of these data, going roughly 2mag fainter than the main survey over 250deg2. The survey has completed spectroscopy over 9380deg2; the spectroscopy is now complete over a large contiguous area of the Northern Galactic Cap, closing the gap that was present in previous data releases. There are over 1.6million spectra in total, including 930,000galaxies, 120,000quasars, and 460,000stars. The data release includes improved stellar photometry at low Galactic latitude. The astrometry has all been recalibrated with the second version of the USNO CCD Astrograph Catalog, reducing the rms statistical errors at the bright end to 45milliarcseconds per coordinate. We further quantify a systematic error in bright galaxy photometry due to poor sky determination; this problem is less severe than previously reported for the majority of galaxies. Finally, we describe a series of improvements to the spectroscopic reductions, including better flat fielding and improved wavelength calibration at the blue end, better processing of objects with extremely strong narrow emission lines, and an improved determination of stellar metallicities. © 2009. The American Astronomical Society. All rights reserved.
• Cimatti, A., Robberto, M., Baugh, C., Beckwith, S. V., Content, R., Daddi, E., Lucia, G. D., Garilli, B., Guzzo, L., Kauffmann, G., Lehnert, M., MacCagni, D., Martínez-Sansigre, A., Pasian, F., Reid, I. N., Rosati, P., Salvaterra, R., Stiavelli, M., Wang, Y., , Osorio, M. Z., et al. (2009). SPACE: The spectroscopic all-sky cosmic explorer. Experimental Astronomy, 23(1), 39-66.
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  Abstract: We describe the scientific motivations, the mission concept and the instrumentation of SPACE, a class-M mission proposed for concept study at the first call of the ESA Cosmic-Vision 2015-2025 planning cycle. SPACE aims to produce the largest three-dimensional evolutionary map of the Universe over the past 10 billion years by taking near-IR spectra and measuring redshifts for more than half a billion galaxies at 0∈
• Diamond-Stanic, A. M., Fan, X., Brandt, W. N., Shemmer, O., Strauss, M. A., Anderson, S. F., Carilli, C. L., Gibson, R. R., Jiang, L., Kim, J. S., Richards, G. T., Schmidt, G. D., Schneider, D. P., Shen, Y., Smith, P. S., Vestergaard, M., & Young, J. E. (2009). High-redshift sdss quasars with weak emission lines. Astrophysical Journal Letters, 699(1), 782-799.
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  Abstract: We identify a sample of 74 high-redshift quasars (z > 3) with weak emission lines from the Fifth Data Release of the Sloan Digital Sky Survey and present infrared, optical, and radio observations of a subsample of four objects at z > 4. These weak emission-line quasars (WLQs) constitute a prominent tail of the Lyα + N v equivalent width distribution, and we compare them to quasars with more typical emission-line properties and to low-redshift active galactic nuclei with weak/absent emission lines, namely BL Lac objects. We find that WLQs exhibit hot (T 1000K) thermal dust emission and have rest-frame 0.1-5 μm spectral energy distributions that are quite similar to those of normal quasars. The variability, polarization, and radio properties of WLQs are also different from those of BL Lacs, making continuum boosting by a relativistic jet an unlikely physical interpretation. The most probable scenario for WLQs involves broad-line region properties that are physically distinct from those of normal quasars. © 2009. The American Astronomical Society.
• Dong, X., Wang, T., Wang, J., Fan, X., Wang, H., Zhou, H., & Yuan, W. (2009). Eddington ratio governs the equivalent width of Mg II emission line in active galactic nuclei. Astrophysical Journal Letters, 703(1 PART 2), L1-L5.
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  Abstract: We have investigated the ensemble regularities of the equivalent widths (EWs) of Mg II λ2800 emission line of active galactic nuclei (AGNs), using a uniformly selected sample of 2092 Seyfert 1 galaxies and quasars at 0.45 ≤ z ≤ 0.8 in the spectroscopic data set of the Sloan Digital Sky Survey Fourth Data Release. We find a strong correlation between the EW of Mg II and the AGN Eddington ratio (L/L Edd): EW(Mg II) (L/L Edd)-0.4. Furthermore, for AGNs with the same L/L Edd, their EWs of Mg II show no correlation with luminosity, black hole mass, or line width, and the Mg II line luminosity is proportional to continuum luminosity, as expected by photoionization theory. Our result shows that Mg II EW is not dependent on luminosity, but is solely governed by L/L Edd. © 2009. The American Astronomical Society.
• Dong, X., Wang, T., Wang, J., Fan, X., Wang, H., Zhou, H., & Yuan, W. (2009). On the physical mechanism underlying PC1 and baldwin effect in quasars. Proceedings of the International Astronomical Union, 5(S267), 396-.
• Gibson, R. R., Jiang, L., Brandt, W. N., Hall, P. B., Shen, Y., Jianfeng, W. u., Anderson, S. F., Schneider, D. P., Berk, D. V., Gallagher, S. C., Fan, X., & York, D. G. (2009). A catalog of broad absorption line quasars in Sloan digital sky survey data release 5. Astrophysical Journal, 692(1), 758-777.
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  Abstract: We present a catalog of 5039 broad absorption line (BAL) quasars (QSOs) in the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) QSO catalog that have absorption troughs covering a continuous velocity range ≥2000 km s -1. We have fitted ultraviolet (UV) continua and line emission in each case, enabling us to report common diagnostics of BAL strengths and velocities in the range -25, 000 to 0 km s-1 for Si IV λ1400, C IV λ1549, Al III λ1857, and Mg II λ2799. We calculate these diagnostics using the spectrum listed in the DR5 QSO catalog, and also for spectra from additional SDSS observing epochs when available. In cases where BAL QSOs have been observed with Chandra or XMM-Newton, we report the X-ray monochromatic luminosities of these sources. We confirm and extend previous findings that BAL QSOs are more strongly reddened in the rest-frame UV than non-BAL QSOs, and that BAL QSOs are relatively X-ray weak compared to non-BAL QSOs. The observed BAL fraction is dependent on the spectral signal-to-noise ratio (S/N); for higher S/N sources, we find an observed BAL fraction of 15%. BAL QSOs show a similar Baldwin effect as for non-BAL QSOs, in that their C IV emission equivalent widths decrease with increasing continuum luminosity. However, BAL QSOs have weaker C IV emission in general than do non-BAL QSOs. Sources with higher UV luminosities are more likely to have higher-velocity outflows, and the BAL outflow velocity and UV absorption strength are correlated with relative X-ray weakness. These results are in qualitative agreement with models that depend on strong X-ray absorption to shield the outflow from overionization and enable radiative acceleration. In a scenario in which BAL trough shapes are primarily determined by outflow geometry, observed differences in Si IV and C IV trough shapes would suggest that some outflows have ion-dependent structure. © 2009. The American Astronomical Society. All rights reserved.
• Jiang, L., Fan, X., Bian, F., Annis, J., Chiu, K., Jester, S., Lin, H., Lupton, R., Richards, G. T., Strauss, M. A., Malanushenko, V., Malanushenko, E., & Schneider, D. P. (2009). A survey of z ∼ 6 quasars in the sloan digital sky survey deep stripe. II. Discovery of six quasars at z _AB>21. Astronomical Journal, 138(1), 305-311.
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  Abstract: We present the discovery of six new quasars at z ∼ 6 selected from the Sloan Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by repeatedly scanning a stripe along the celestial equator. The six quasars are about 2 mag fainter than the luminous z 6 quasars found in the SDSS main survey and 1 mag fainter than the quasars reported in Paper I. Four of them comprise a complete flux-limited sample at 21 < z AB < 21.8 over an effective area of 195 deg 2. The other two quasars are fainter than z AB = 22 and are not part of the complete sample. The quasar luminosity function at z 6 is well described as a single power law Φ(L 1450) L β1450 over the luminosity range -28 < M 1450 < -25. The best-fitting slope β varies from -2.6 to -3.1, depending on the quasar samples used, with a statistical error of 0.3-0.4. About 40% of the quasars discovered in the SDSS southern survey have very narrow Lyα emission lines, which may indicate small black hole masses and high Eddington luminosity ratios, and therefore short black hole growth timescales for these faint quasars at early epochs. © 2009. The American Astronomical Society. All rights reserved.
• Kelly, B. C., Vestergaard, M., & Fan, X. (2009). Determining quasar black hole mass functions from their broad emission lines: Application to the bright Quasar Survey. Astrophysical Journal, 692(2), 1388-1410.
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  Abstract: We describe a Bayesian approach to estimating quasar black hole mass functions (BHMF) using the broad emission lines to estimate black hole mass. We show how using the broad-line mass estimates in combination with statistical techniques developed for luminosity function estimation (e.g., the 1/V a correction) leads to statistically biased results. We derive the likelihood function for the BHMF based on the broad-line mass estimates, and derive the posterior distribution for the BHMF, given the observed data. We develop our statistical approach for a flexible model where the BHMF is modeled as a mixture of Gaussian functions. Statistical inference is performed using Markov chain Monte Carlo (MCMC) methods, and we describe a Metropolis-Hastings algorithm to perform the MCMC. The MCMC simulates random draws from the probability distribution of the BHMF parameters, given the data, and we use a simulated data set to show how these random draws may be used to estimate the probability distribution for the BHMF. In addition, we show how the MCMC output may be used to estimate the probability distribution of any quantities derived from the BHMF, such as the peak in the space density of quasars. Our method has the advantage that it is able to constrain the BHMF even beyond the survey detection limits at the adopted confidence level, accounts for measurement errors and the intrinsic uncertainty in broad-line mass estimates, and provides a natural way of estimating the probability distribution of any quantities derived from the BHMF. We conclude by using our method to estimate the local active BHMF using the z < 0.5 Bright Quasar Survey sources. At z ∼ 0.2, the quasar BHMF falls off approximately as a power law with slope ∼2 for MBH ≳ 108 M⊙. Our analysis implies that at a given MBH, z < 0.5 broad-line quasars have a typical Eddington ratio of ∼0.4 and a dispersion in Eddington ratio of ≲0.5 dex. © 2009. The American Astronomical Society. All rights reserved.
• Kelly, B. C., Vestergaard, M., Fan, X., Hernquist, L., Hopkins, P., & Siemiginowska, A. (2009). The distribution and evolution of black hole mass in broad line quasars. Proceedings of the International Astronomical Union, 5(S267), 263-.
• Kurk, J. D., Walter, F., Fan, X., Jiang, L., Jester, S., Rix, H. -., & Riechers, D. A. (2009). Near-infrared spectroscopy of sdss J0303 - 0019: A Low-luminosity, high-eddington-ratio quasar at z ∼ 6. Astrophysical Journal Letters, 702(2), 833-837.
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  Abstract: We present sensitive near-infrared (NIR) VLT ISAAC spectroscopic observations of the z = 6.08 quasar SDSS J030331.40-001912.9. This quasi-stellar object (QSO) is more than a magnitude fainter than other QSOs at z ∼ 6 for which NIR spectroscopy has been obtained to date and is therefore presumably more representative of the QSO population at the end of cosmic reionization. Combining rest-frame UV continuum luminosity with the width measurements of the Mg II and C IV lines, we derive a black hole mass of 2+1.0-0.5 × 108 M ⊙, the lowest mass observed for z ∼ 6 QSOs to date, and derive an Eddington ratio of 1.6 +0.4-0.6, among the highest value derived for QSOs at any redshift. The Spitzer 24 μm nondetection of this QSO does not leave space for a significant hot dust component in its optical/NIR spectral energy distribution, in common with one other faint QSO at z = 6, but in contrast to more than 20 more z = 6 QSOs and all known lower redshift QSOs with sufficiently deep multiwavelength photometry. We conclude that we have found evidence for differences in the intrinsic properties of at least one z ∼ 6 QSO as compared to the lower redshift population. © 2009 The American Astronomical Society. All rights reserved.
• Shemmer, O., Brandt, W. N., Anderson, S. F., Diamond-Stanic, A. M., Fan, X., Richards, G. T., Schneider, D. P., & Strauss, M. A. (2009). X-ray insights into the nature of weak emission-line quasars at high redshift. Astrophysical Journal, 696(1), 580-590.
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  Abstract: We present Chandra observations of nine high-redshift quasars (z = 2.7-5.9) discovered by the Sloan Digital Sky Survey with weak or undetectable high-ionization emission lines in their UV spectra (WLQs). Adding archival X-ray observations of six additional sources of this class has enabled us to place the strongest constraints yet on the X-ray properties of this remarkable class of active galactic nuclei (AGNs). Although our data cannot rule out the possibility that the emission lines are overwhelmed by a relativistically boosted continuum, as manifested by BL Lac objects, we find that WLQs are considerably weaker in the X-ray and radio bands than the majority of BL Lacs found at much lower redshifts. If WLQs are high-redshift BL Lacs, then it is difficult to explain the lack of a large parent population of X-ray and radio bright weak-lined sources at high redshift. We also consider the possibility that WLQs are quasars with extreme properties, and in particular that the emission lines are suppressed by high accretion rates. Using joint spectral fitting of the X-ray spectra of 11 WLQs, we find that the mean photon index in the hard X-ray band is consistent with those observed in typical radio-quiet AGNs with no hint of an unusually steep hard-X-ray spectrum. This result poses a challenge to the hypothesis that WLQs have extremely high accretion rates, and we discuss additional observations required to test this idea. © 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
• Shen, Y., Strauss, M. A., Ross, N. P., Hall, P. B., Lin, Y., Richards, G. T., Schneider, D. P., Weinberg, D. H., Connolly, A. J., Fan, X., Hennawi, J. F., Shankar, F., E., D., Bahcall, N. A., & Brunner, R. J. (2009). Quasar clustering from SDSS DR5: Dependences on physical properties. Astrophysical Journal Letters, 697(2), 1656-1673.
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  Abstract: Using a homogenous sample of 38,208 quasars with a sky coverage of 4000 deg2 drawn from the Sloan Digital Sky Survey Data Release Five quasar catalog, we study the dependence of quasar clustering on luminosity, virial black hole (BH) mass, quasar color, and radio loudness. At z < 2.5, quasar clustering depends weakly on luminosity and virial BH mass, with typical uncertainty levels 10% for the measured correlation lengths. These weak dependences are consistent with models in which substantial scatter between quasar luminosity, virial BH mass, and the host dark matter halo mass has diluted any clustering difference, where halo mass is assumed to be the relevant quantity that best correlates with clustering strength. However, the most luminous and most massive quasars are more strongly clustered (at the 2σ level) than the remainder of the sample, which we attribute to the rapid increase of the bias factor at the high-mass end of host halos. We do not observe a strong dependence of clustering strength on quasar colors within our sample. On the other hand, radio-loud quasars are more strongly clustered than are radio-quiet quasars matched in redshift and optical luminosity (or virial BH mass), consistent with local observations of radio galaxies and radio-loud type 2 active galactic nuclei. Thus, radio-loud quasars reside in more massive and denser environments in the biased halo clustering picture. Using the Sheth etal. (2001) formula for the linear halo bias, the estimated host halo mass for radio-loud quasars is 1013 h -1 M ⊙, compared to 2 × 1012 h -1 M ⊙ for radio-quiet quasar hosts at z 1.5. © 2009. The American Astronomical Society. All rights reserved.
• Stephens, D. C., Leggett, S. K., Cushing, M. C., Marley, M. S., Saumon, D., Geballe, T. R., Golimowski, D. A., Fan, X., & Noll, K. S. (2009). The 0.8-14.5 μm Spectra of mid-L to mid-T dwarfs: Diagnostics of effective temperature, grain sedimentation, gas transport, and surface gravity. Astrophysical Journal Letters, 702(1), 154-170.
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  Abstract: We present new 5.2-14.5 μm low-resolution spectra of 14 mid-L to mid-T dwarfs. We also present new 3.0-4.1 μm spectra for five of these dwarfs. These data are supplemented by existing red and near-infrared spectra (0.6-2.5μm), as well as red through mid-infrared spectroscopy of seven other L and T dwarfs presented by Cushing etal. We compare these spectra to those generated from the model atmospheres of Saumon & Marley. The models reproduce the observed spectra well, except in the case of one very red L3.5 dwarf, 2MASS J22244381-0158521. The broad wavelength coverage allows us to constrain almost independently the four parameters used to describe these photospheres in our models: effective temperature (T eff), surface gravity, grain sedimentation efficiency (f sed), and vertical gas transport efficiency (Kzz ). The CH4 bands centered at 2.2, 3.3, and 7.65 μm and the CO band at 2.3 μm are sensitive to K zz , and indicates that chemical mixing is important in all L and T dwarf atmospheres. The sample of L3.5 to T5.5 dwarfs spans the range 1800 K≳ T eff ≳1000 K, with an L-T transition (spectral types L7 to T4) that lies between 1400 and 1100 K for dwarfs with typical near-infrared colors; bluer and redder dwarfs can be 100 K warmer or cooler, respectively, when using infrared spectral types. When using optical spectral types, the bluer dwarfs have more typical T eff values as they tend to have earlier optical spectral types. In this model analysis, f sed increases rapidly between types T0 and T4, indicating that increased sedimentation can explain the rapid disappearance of clouds at this stage of brown dwarf evolution. There is a suggestion that the transition to dust-free atmospheres happens at lower temperatures for lower gravity dwarfs. © 2009. The American Astronomical Society. All rights reserved.
• Stephens, D. C., Leggett, S. K., Marley, M. S., Saumon, D., Cushing, M. C., Geballe, T. R., Golimowski, D. A., Fan, X., & Noll, K. S. (2009). The 0.8 to 14.5 μm spectral energy distributions of mid-L to mid-T dwarfs. AIP Conference Proceedings, 1094, 172-177.
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  Abstract: We fit theoretical model atmospheres to the spectral energy distribution of 21 L and T dwarfs recently observed with the Spitzer Space Telescope to identify and isolate four key physical parameters used in the model characterization of their atmospheres. The wide range of wavelengths observed (∼0.6 to 14.5 μm) lets us constrain almost independently the four model parameters used to describe these photospheres: effective temperature (T eff), grain sedimentation (f sed), vertical gas transport efficiency (K zz), and gravity. We find that the ratio of the mid-infrared to near-infrared flux is a good indicator of T eff, while the slope in the near-infrared is strongly dependent on f sed. The CH4 bands found at 2, 3 and 8 μm are sensitive to the timescale for vertical mixing, and gravity will influence the flux at 2 μm. © 2009 American Institute of Physics.
• Adelman-McCarthy, J. K., Agüeros, M. A., Allam, S. S., Prieto, C. A., S., K., Anderson, S. F., Annis, J., Bahcall, N. A., Bailer-Jones, C., Baldry, I. K., Barentine, J. C., Bassett, B. A., Becker, A. C., Beers, T. C., Bell, E. F., Berlind, A. A., Bernardi, M., Blanton, M. R., Bochanski, J. J., , Boroski, W. N., et al. (2008). The sixth data release of the sloan digital sky survey. Astrophysical Journal, Supplement Series, 175(2), 297-313.
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  Abstract: This paper describes the Sixth Data Release of the Sloan Digital Sky Survey. With this data release, the imaging of the northern Galactic cap is now complete. The survey contains images and parameters of roughly 287 million objects over 9583 deg2, including scans over a large range of Galactic latitudes and longitudes. The survey also includes 1.27 million spectra of stars, galaxies, quasars, and blank sky (for sky subtraction) selected over 7425 deg2. This release includes much more stellar spectroscopy than was available in previous data releases and also includes detailed estimates of stellar temperatures, gravities, and metallicities. The results of improved photometric calibration are now available, with uncertainties of roughly 1% in g, r, i, and z, and 2% in u, substantially better than the uncertainties in previous data releases. The spectra in this data release have improved wavelength and flux calibration, especially in the extreme blue and extreme red, leading to the qualitatively better determination of stellar types and radial velocities. The spectrophotometric fluxes are now tied to point-spread function magnitudes of stars rather than fiber magnitudes. This gives more robust results in the presence of seeing variations, but also implies a change in the spectrophotometric scale, which is now brighter by roughly 0.35 mag. Systematic errors in the velocity dispersions of galaxies have been fixed, and the results of two independent codes for determining spectral classifications and redshifts are made available. Additional spectral outputs are made available, including calibrated spectra from individual 15 minute exposures and the sky spectrum subtracted from each exposure. We also quantify a recently recognized underestimation of the brightnesses of galaxies of large angular extent due to poor sky subtraction; the bias can exceed 0.2 mag for galaxies brighter than r = 14 mag. © 2008. The American Astronomical Society. All rights reserved.
• Chiu, K., Liu, M. C., Jiang, L., Allers, K. N., Stark, D. P., Bunker, A., Fan, X., Glazebrook, K., & Dupuy, T. J. (2008). Four faint T dwarfs from the UKIRT infrared deep sky survey (UKIDSS) southern stripe. Monthly Notices of the Royal Astronomical Society: Letters, 385(1), L53-L57.
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  Abstract: We present the optical and near-infrared photometry and spectroscopy of four faint T dwarfs newly discovered from the UKIDSS first data release. The sample, drawn from an imaged area of ~136 deg2 to a depth of Y = 19.9 (5σ, Vega), is located in the Sloan Digital Sky Survey (SDSS) Southern Equatorial Stripe, a region of significant future deep imaging potential. We detail the selection and followup of these objects, three of which are spectroscopically confirmed brown dwarfs ranging from type T2.5 to T7.5, and one is photometrically identified as early T. Their magnitudes range from Y = 19.01 to 19.88 with derived distances from 34 to 98 pc, making these among the coldest and faintest brown dwarfs known. The T7.5 dwarf appears to be single based on 0.05-arcsec images from Keck laser guide star adaptive optics. The sample brings the total number of T dwarfs found or confirmed by UKIDSS data in this region to nine, and we discuss the projected numbers of dwarfs in the future survey data. We estimate that ~240 early and late T dwarfs are discoverable in the UKIDSS Large Area Survey (LAS) data, falling significantly short of published model projections and suggesting that initial mass functions and/or birth rates may be at the low end of possible models. Thus, deeper optical data have good potential to exploit the UKIDSS survey depth more fully, but may still find the potential Y dwarf sample to be extremely rare. © 2008 The Authors. Journal compilation © 2008 RAS.
• Cool, R. J., Eisenstein, D. J., Fan, X., Fukugita, M., Jiang, L., Maraston, C., Meiksin, A., Schneider, D. P., & Wake, D. A. (2008). Luminosity function constraints on the evolution of massive red galaxies since z ∼ 0.9. Astrophysical Journal, 682(2), 919-936.
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  Abstract: We measure the evolution of the luminous red galaxy (LRG) luminosity function in the redshift range 0.1 < z < 0.9 using samples of galaxies from the Sloan Digital Sky Survey as well as new spectroscopy of high-redshift massive red galaxies. Our high-redshift sample of galaxies is largest spectroscopic sample of massive red galaxies at z ∼ 0.9 collected to date and covers 7 deg2, minimizing the impact of large-scale structure on our results. We find that the LRG population has evolved little beyond the passive fading of its stellar populations since z ∼ 0.9. Based on our luminosity function measurements and assuming a nonevolving Salpeter stellar initial mass function, we find that the most massive (L > 3L*) red galaxies have grown by less than 50% (at 99% confidence), since z = 0.9, in stark contrast to the factor of 2-4 growth observed in the L* red galaxy population over the same epoch. We also investigate the evolution of the average LRG spectrum since z ∼ 0.9 and find the high-redshift composite to be well described as a passively evolving example of the composite galaxy observed at low redshift. From spectral fits to the composite spectra, we find at most 5% of the stellar mass in massive red galaxies may have formed within 1 Gyr of z = 0.9. While L* red galaxies are clearly assembled at z < 1, 3L* galaxies appear to be largely in place and evolve little beyond the passive evolution of their stellar populations over the last half of cosmic history. © 2008. The American Astronomical Society. All rights reserved.
• Fan, X. (2008). Observational constraints of the end of cosmic reionization. AIP Conference Proceedings, 990, 437-441.
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  Abstract: The epoch of reionization is the last unexplored phase of cosmic evolution, corresponding to a crucial era in cosmic structure formation - the formation of the first luminous objects. I review some recent constraints on the end of reionization using observations of the highest redshift quasars and galaxies. Spectroscopic studies of Gunn-Peterson (GP) absorption, and related phenomena, suggest a qualitative change in the state of the intergalactic medium (IGM) at z∼6, indicating a rapid increase in the neutral fraction of the IGM, from xHI10 -3, and perhaps as high as 0.3, at z≥6. On the other hand, transmission spikes in the GP trough, and observations of the evolution of the Lyα galaxy luminosity function indicate xHI
• Gallerani, S., Ferrara, A., Fan, X., & Choudhury, T. R. (2008). Glimpsing through the high-redshift neutral hydrogen fog. Monthly Notices of the Royal Astronomical Society, 386(1), 359-369.
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  Abstract: We analyse the transmitted flux in a sample of 17 QSOs spectra at 5.74 ≤ zem ≤ 6.42 to obtain tighter constraints on the volume-averaged neutral hydrogen fraction, xH i, at z ≈ 6. We study separately the narrow transmission windows (peaks) and the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with a semi-analytical model of the Lyα forest, we conclude that xH i evolves smoothly from 10-4.4 at z = 5.3 to 10 -4.2 at z = 5.6, with a robust upper limit xH i < 0.36 at z = 6.3. The frequency and physical sizes of the peaks imply an origin in cosmic underdense regions and/or in H ii regions around faint quasars or galaxies. In one case (the intervening H ii region of the faint quasar RD J1148+5253 at z = 5.70 along the line of sight of SDSS J1148+5251 at z = 6.42) the increase of the peak spectral density is explained by the first-ever detected transverse proximity effect in the H i Lyα forest; this indicates that at least some peaks result from a locally enhanced radiation field. We then obtain a strong lower limit on the foreground QSO lifetime of tQ > 11 Myr. The observed widths of the peaks are found to be systematically larger than the simulated ones. Reasons for such discrepancy might reside either in the photoionization equilibrium assumption or in radiative transfer effects. © 2008 RAS.
• Jiang, L., Fan, X., & Vestergaard, M. (2008). A sample of quasars with strong nitrogen emission lines from the sloan digital sky survey. Astrophysical Journal, 679(2), 962-966.
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  Abstract: We report on 293 quasars with strong N IV] λ1486 or N III λ1750 emission lines (rest-frame equivalent width >3 Å) at 1.7 < z < 4.0 selected from the Sloan Digital Sky Survey (SDSS) Fifth Data Release. These nitrogen-rich (N-rich) objects comprise ∼ 1.1 % of the SDSS quasars. The comparison between the N-rich quasars and other quasars shows that the two quasar subsets share many common properties. We also confirm previous results that N-rich quasars have much stronger Lya and N v λ1240 emission lines. Strong nitrogen emission in all ionization states indicates high overall nitrogen abundances in these objects. We find evidence that the nitrogen abundance is closely related to quasar radio properties. The radio-loud fraction in the N m]-rich quasars is 26% and in the N IV]-rich quasars is 69%, significantly higher than ∼8% measured in other quasars with similar redshift and luminosity. Therefore, the high nitrogen abundance in N-rich quasars could be an indicator of a special quasar evolution stage, in which the radio activity is also strong. © 2008. The American Astronomical Society. All rights reserved.
• Jiang, L., Fan, X., Annis, J., Becker, R. H., White, R. L., Chiu, K., Lin, H., Lupton, R. H., Richards, G. T., Strauss, M. A., Jester, S., & Schneider, D. P. (2008). A survey of z ∼ 6 Quasars in the sloan digital sky survey deep stripe. I. A flux-limited sample at z_AB < 21. Astronomical Journal, 135(3), 1057-1066.
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  Abstract: We present the discovery of five quasars at z 6 selected from 260 deg 2 of the Sloan Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by repeatedly scanning a stripe along the celestial equator. The five quasars with 20 < zAB < 21 are 1-2 magnitudes fainter than the luminous z 6 quasars discovered in the SDSS main survey. One of them was independently discovered by the UKIRT Infrared Deep Sky Survey. These quasars, combined with another z 6 quasar known in this region, make a complete flux-limited quasar sample at zAB < 21. The sample spans the redshift range 5.85 ≤ z ≤ 6.12 and the luminosity range -26.5 ≤ M 1450 ≤ -25.4 (H 0 = 70 km s-1 Mpc -1, Ωm = 0.3, and ΩΛ = 0.7). We use the 1/Va method to determine that the comoving quasar spatial density at 〈z〉 = 6.0 and 〈M 1450〉 = -25.8 is (5.0 2.1) × 10-9 Mpc-3 mag-1. We model the bright-end quasar luminosity function (QLF) at z 6 as a power law Φ(L 1450) L β1450. The slope β calculated from a combination of our sample and the luminous SDSS quasar sample is -3.1 0.4, significantly steeper than the slope of the QLF at z 4. Based on the derived QLF, we find that the quasar/active galactic nucleus (AGN) population cannot provide enough photons to ionize the intergalactic medium (IGM) at z 6 unless the IGM is very homogeneous and the luminosity (L*1450) at which the QLF power law breaks is very low. © 2008. The American Astronomical Society. All rights reserved.
• Kelly, B. C., Fan, X., & Vestergaard, M. (2008). A flexible method of estimating luminosity functions. Astrophysical Journal, 682(2), 874-895.
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  Abstract: We describe a Bayesian approach to estimating luminosity functions. We derive the likelihood function and posterior probability distribution for the luminosity function, given the observed data, and we compare the Bayesian approach with maximum likelihood by simulating sources from a Schechter function. For our simulations confidence intervals derived from bootstrapping the maximum likelihood estimate can be too narrow, while confidence intervals derived from the Bayesian approach are valid. We develop our statistical approach for a flexible model where the luminosity function is modeled as a mixture of Gaussian functions. Statistical inference is performed using Markov chain Monte Carlo ( MCMC) methods, and we describe a Metropolis-Hastings algorithm to perform the MCMC. The MCMC simulates random draws from the probability distribution of the luminosity function parameters, given the data, and we use a simulated data set to show how these random draws may be used to estimate the probability distribution for the luminosity function. In addition, we show how the MCMC output may be used to estimate the probability distribution of any quantities derived from the luminosity function, such as the peak in the space density of quasars. The Bayesian method we develop has the advantage that it is able to place accurate constraints on the luminosity function even beyond the survey detection limits, and that it provides a natural way of estimating the probability distribution of any quantities derived from the luminosity function, including those that rely on information beyond the survey detection limits. © 2008. The American Astronomical Society. All rights reserved.
• Vestergaard, M., Fan, X., Tremonti, C. A., Osmer, P. S., & Richards, G. T. (2008). Mass functions of the active black holes in distant quasars from the sloan digital sky survey data release 3. Astrophysical Journal, 674(1 PART 2), L1-L4.
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  Abstract: We present the mass functions of actively accreting supermassive black holes over the redshift range 0.3 ≤ z ≤ 5 for a well-defined, homogeneous sample of 15,180 quasars from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3) within an effective area of 1644 deg2. This sample is the most uniform statistically significant subset available for the DR3 quasar sample. It was used for the DR3 quasar luminosity function, presented by Richards et al., and is the only sample suitable for the determination of the SDSS quasar black hole mass function. The sample extends from i = 15 to i = 19.1 at z ≤ 3 and to i = 20.2 for z ≳. The mass functions display a rise and fall in the space density distribution of active black holes at all epochs. Within the uncertainties the high-mass decline is consistent with a constant slope of β ≈ -3.3 at all epochs. This slope is similar to the bright-end slope of the luminosity function for epochs below z = 4. Our tests suggest that the downturn toward lower mass values is due to incompleteness of the quasar sample with respect to black hole mass. Further details and analysis of these mass functions will be presented in forthcoming papers. © 2008. The American Astronomical Society. All rights reserved.
• Wang, R., Carilli, C. L., Wagg, J., Bertoldi, F., Walter, F., Menten, K. M., Omont, A., Cox, P., Strauss, M. A., Fan, X., Jiang, L., & Schneider, D. P. (2008). Thermal emission from warm dust in the most distant quasars. Astrophysical Journal, 687(2), 848-858.
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  Abstract: We report new continuum observations of 14 z ∼ 6 quasars at 250 GHz and 14 quasars at 1.4 GHz. We summarize all recent millimeter and radio observations of the sample of the 33 quasars known with 5.71 ≤ z ≤ 6.43 and present a study of the rest-frame far-infrared (FIR) properties of this sample. These quasars were observed with the Max Planck Millimeter Bolometer Array (MAMBO) at 250 GHz with mJy sensitivity, and 30% of them were detected. We also recover the average 250 GHz flux density of the MAMBO undetected sources at 4 σ by stacking the on-source measurements. The derived mean radio-to-UV spectral energy distributions (SEDs) of the full sample and the 250 GHz nondetections show no significant differences from lower redshift optical quasars. Obvious FIR excesses are seen in the individual SEDs of the strong 250 GHz detections, with FIR-to-radio emission ratios consistent with those of typical star-forming galaxies. Most 250 GHz-detected sources follow the L FIR-Lbol relationship derived from a sample of local IR-luminous quasars (LIR > 1012 l⊙), while the average L-FIR/Lbol ratio of the nondetections is consistent with that of the optically selected PG quasars. The MAMBO detections also tend to have weaker Lya emission than the nondetected sources. We discuss possible FIR dust-heating sources and critically assess the possibility of active star formation in the host galaxies of the z ∼ 6 quasars. The average star formation rate of the MAMBO nondetections is likely to be less than a few hundred M⊙ yr-1, but in the strong detections, the host galaxy star formation is probably at a rate of ≳103 M ⊙ yr-1, which dominates the FIR dust heating. © 2008. The American Astronomical Society. All rights reserved.
• Wang, R., Wagg, J., Carilli, C. L., Benford, D. J., Dowell, C. D., Bertoldi, F., Walter, F., Menten, K. M., Omont, A., Cox, P., Strauss, M. A., Fan, X., & Jiang, L. (2008). SHARC-II 350 μm observations of thermal emission from warm dust in z ≥ 5 quasars. Astronomical Journal, 135(4), 1201-1206.
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  Abstract: We present observations of four z ≥ 5 SDSS quasars at 350 μm with the SHARC-II bolometer camera on the Caltech Submillimeter Observatory. These are among the deepest observations that have been made by SHARC-II at 350 μm, and three quasars are detected at ≥3σ significance, greatly increasing the sample of 350 μm (corresponds to rest frame wavelengths of
• Yuexing, L. i., Hopkins, P. F., Hernquist, L., Finkbeiner, D. P., Cox, T. J., Springel, V., Jiang, L., Fan, X., & Yoshida, N. (2008). Modeling the dust properties of z ∼ 6 quasars with art² - All-wavelength radiative transfer with adaptive refinement tree. Astrophysical Journal, 678(1), 41-63.
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  Abstract: The detection of large quantities of dust in z ∼ 6 quasars by infrared and radio surveys presents puzzles for the formation and evolution of dust in these early systems. Previously, Li et al. showed that luminous quasars at z ≲ 6 can form through hierarchical mergers of gas-rich galaxies, and that these systems are expected to evolve from starburst through quasar phases. Here, we calculate the dust properties of simulated quasars and their progenitors using a three-dimensional Monte Carlo radiative transfer code, ART2 (All-wavelength Radiative Transfer with Adaptive Refinement Tree). ART 2 incorporates a radiative equilibrium algorithm which treats dust emission self-consistently, an adaptive grid method which can efficiently cover a large dynamic range in both spatial and density scales, a multiphase model of the interstellar medium which accounts for the observed scaling relations of molecular clouds, and a supernova-origin model for dust which can explain the existence of dust in cosmologically young objects. By applying ART2 to the hydrodynamic simulations of Li et al., we reproduce the observed spectral energy distribution (SED) and inferred dust properties of SDSS J1148+5251, the most distant Sloan quasar. We find that the dust and infrared emission are closely associated with the formation and evolution of the quasar host. The system evolves from a cold to a warm ultraluminous infrared galaxy (ULIRG) owing to heating and feedback from stars and the active galactic nucleus (AGN). Furthermore, the AGN activity has significant implications for the interpretation of observation of the hosts. Our results suggest that vigorous star formation in merging progenitors is necessary to reproduce the observed dust properties of z ∼ 6 quasars, supporting a merger-driven origin for luminous quasars at high redshifts and the starburst-to-quasar evolutionary hypothesis. © 2008. The American Astronomical Society. All rights reserved.
• Adelman-McCarthy, J. K., Agüeros, M. A., Allam, S. S., Anderson, K. S., Anderson, S. F., Annes, J., Bahcall, N. A., A., C., Baldry, I. K., Barentine, J. C., Beers, T. C., Belokurov, V., Berlind, A., Bernardi, M., Blanton, M. R., Bochanski, J. J., Boroski, W. N., Bramich, D. M., Brewington, H. J., , Brenchmann, J., et al. (2007). The fifth data release of the sloan digital sky survey. Astrophysical Journal, Supplement Series, 172(2), 634-644.
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  Abstract: This paper describes the Fifth Data Release (DR5) of the Sloan Digital Sky Survey (SDSS). DR5 includes all survey quality data taken through 2005 June and represents the completion of the SDSS-I project (whose successor, SDSS-II, will continue through mid-2008). It includes five-band photometric data for 217 million objects selected over 8000 deg 2 and 1,048,960 spectra of galaxies, quasars, and stars selected from 5713 deg 2 ofthat imaging data. These numbers represent a roughly 20% increment over those of the Fourth Data Release; all the data from previous data releases are included in the present release. In addition to "standard" SDSS observations, DR5 includes repeat scans of the southern equatorial stripe, imaging scans across M31 and the core of the Perseus Cluster of galaxies, and the first spectroscopic data from SEGUE, a survey to explore the kinematics and chemical evolution of the Galaxy. The catalog database incorporates several new features, including photometric redshifts of galaxies, tables of matched objects in overlap regions of the imaging survey, and tools that allow precise computations of survey geometry for statistical investigations. © 2007. The American Astronomical Society. All rights reserved.
• Carilli, C. L., Neri, R., Wang, R., Cox, P., Bertoldi, F., Walter, F., Fan, X., Menten, K., Wagg, J., Maiolino, R., Omont, A., Strauss, M. A., Riechers, D., Lo, K. Y., Bolatto, A., & Scoville, N. (2007). Detection of 1.6 × 10¹⁰ M_⊙ of molecular gas in the host galaxy of the z = 5.77 SDSS quasar J0927+2001. Astrophysical Journal, 666(1 PART 2), L9-L12.
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  Abstract: We have detected emission by the CO(5-4) and (6-5) rotational transitions atfromthez p 5.7722 ± 0.0006 from the host galaxy of the SDSS quasar J0927+2001 using the Plateau de Bure Interferometer. The peak line flux density for the CO(5-4) line is 0.72 ± 0.09 mJy, with a line FWHM = 610 ± 110 km s-1. The implied molecular gas mass is (1.6 ± 0.3) × 100.1210 M⊙. We also detect the 90 GHz continuum at 0.12 ± 0.03 mJy, consistent with a 47 K dust spectrum extrapolated from higher frequencies. J0927+2001 is the second example of a huge molecular gas reservoir within the host galaxy of a quasar within 1 Gyr of the big bang. Observations of J0927+2001 are consistent with a massive starburst coeval with a bright quasar phase in the galaxy, suggesting the rapid formation of both a supermassive black hole through accretion, and the stellar host spheroid, at a time close to the end of cosmic reionization. © 2007. The American Astronomical Society.
• Fan, X. (2007). Observational constraints of cosmic reionization. Nuovo Cimento della Societa Italiana di Fisica B, 122(9-11), 947-958.
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  Abstract: The epoch of reionization is the last unexplored phase of cosmic evolution, corresponding to a crucial era in cosmic structure formation-the formation of the first luminous objects. I review some recent constraints on the end of reionization using observations of the highest-redshift quasars and galaxies. Spectroscopic studies of Gunn-Peterson (GP) absorption, and related phenomena, suggest a qualitative change in the state of the intergalactic medium (IGM) at z ∼ 6, indicating a rapid increase in the neutral fraction of the IGM, from im < 10-4 at 2 ≤ 5.5, to xHI > 10-3, and perhaps as high as 0.3, at z ≥ 6. On the other hand, transmission spikes in the GP trough, and observations of the evolution of the Lya galaxy luminosity function indicate xHI < 50% at z ∼ 6.5. The IGM characteristics at this epoch are consistent with the end of the overlapping stage of reionization. Current data are consistent with star-forming galaxies, in particular low-luminosity galaxies, rather than quasars and AGNs, as being the dominant sources of reionizing photons, although large uncertainties still exist in estimating the total contribution from the galaxy population at high redshift. © Società Italiana di Fisica.
• Fan, X., Krupinski, E. A., Roehrig, H., & Fan, X. -. (2007). Does the age of liquid crystal displays influence observer performance?. Academic radiology, 14(4).
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  As liquid crystal displays (LCDs) age, the whitepoint shifts toward a yellow hue, changing the appearance of the displayed images. This study examined whether this shift in whitepoint influences observer performance and visual search efficiency of radiologists interpreting clinical images.
• Gallerani, S., Ferrara, A., Fan, X., Choudhury, T. R., & Salvaterra, R. (2007). Was the Universe neutral beyond redshift six?. Nuovo Cimento della Societa Italiana di Fisica B, 122(9-11), 977-983.
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  Abstract: We provide measurements of the neutral hydrogen fraction xHI at z ≈ 6, by comparing semi-analytical models of the Lyα forest with observations of high-z quasars and gamma-ray bursts absorption spectra. We analyze the transmitted flux in a sample of 17 QSOs spectra at 5.74 ≤ z em ≤ 6.42 studying separately the narrow transmission windows (peaks) and the wide dark portions (gaps) in the observed absorption spectra. By comparing the statistics of these spectral features with our models, we conclude that xHI evolves smoothly from 10-4.4 at z = 5.3 to 10-4.2 at z = 5.6, with a robust upper limit xHI < 0.36 at z = 6.3. We show the results of the first-ever detected transverse proximity effect in the HI Lyα forest, produced by the HII region of the faint quasar RD J1148+5253 at z = 5.70 intervening along the LOS of SDSS J1148+5251 at z = 6.42. Moreover, we propose a novel method to study cosmic reionization using absorption line spectra of high-redshift GRBs afterglows. We show that the time evolution and the statistics of gaps in the observed spectra represent exquisite tools to discriminate among different reionization models. By applying our methods to GRB 050904 detected at z = 6.29, we show that the observation of this burst provides strong indications of a highly ionized intergalactic medium at z ∼ 6, with an estimated mean neutral hydrogen fraction xHI = 6.4 ± 0.3 × 10-5 along that line of sight. © Società Italiana di Fisica.
• Gardner, J. P., Fan, X., Wilson, G., & Stiavelli, M. (2007). A spitzer warm mission ultra-wide survey as a target finder for the james webb space telescope. AIP Conference Proceedings, 943, 229-241.
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  Abstract: The James Webb Space Telescope (JWST) is the successor to the Hubble and Spitzer Space Telescopes. It has a broad scientific mission which includes spectroscopic studies of the epoch of reionization through observations of z>8 quasars. The Spitzer warm mission provides a unique opportunity to conduct an infrared survey of several hundred square degrees to a depth of several micro-Janskys, capable of finding quasars out to z=10. Deep JWST continuum spectroscopy of these quasars will establish the epoch and history of the Universe through detection of the Gunn-Peterson trough and/or Lyman-α damping wings. The statistics and luminosity function of high-z quasars will reveal the early history of accretion in the most extreme systems, providing insights in the role of black holes in galaxy evolution. Data obtained from an ultra-wide warm Spitzer survey will also be useful for other science, including studies of high-redshift galaxy clusters. © 2007 American Institute of Physics.
• Jiang, L., Fan, X., Ivezić, Ž., Richards, G. T., Schneider, D. P., Strauss, M. A., & Kelly, B. C. (2007). The radio-loud fraction of quasars is a strong function of redshift and optical luminosity. Astrophysical Journal, 656(2 I), 680-690.
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  Abstract: Using a sample of optically selected quasars from the Sloan Digital Sky Survey, we have determined the radio-loud fraction (RLF) of quasars as a function of redshift and optical luminosity. The sample contains more than 30,000 objects and spans a redshift range of 0 < z ≤ 5 and a luminosity range of -30 ≤ Mi < -22. We use both the radio-to-optical flux ratio (R parameter) and the radio luminosity to define radio-loud quasars. After breaking the correlation between redshift and luminosity due to the flux-limited nature of the sample, we find that the RLF of quasars decreases with increasing redshift and decreasing luminosity. The relation can be described in the form of log [RLF/( 1 - RLF)] = b0 + bz log (1 + z) + bM(M2500 + 26), where M2500 is the absolute magnitude at rest-frame 2500 Å, and bz, b M < 0. When using R > 10 to define radio-loud quasars, we find that b0 = -0.132 ±0.116, bz = -2.052 ± 0.261, and bM = -0.183 ± 0.025. The RLF at z = 0.5 declines from 24.3% to 5.6% as luminosity decreases from M2500 = -26 to -22, and the RLF at M2500 = -26 declines from 24.3% to 4.1% as redshift increases from 0.5 to 3, suggesting that the RLF is a strong function of both redshift and luminosity. We also examine the impact of flux-related selection effects on the RLF determination using a series of tests and find that the dependence of the RLF on redshift and luminosity is highly likely to be physical, and that the selection effects we considered are not responsible for the dependence. © 2007. The American Astronomical Society. All rights reserved.
• Jiang, L., Fan, X., Vestergaard, M., Kurk, J. D., Walter, F., Kelly, B. C., & Strauss, M. A. (2007). Gemini near-infrared spectroscopy of luminous z ∼ 6 quasars: Chemical abundances, black hole masses, and Mg II absorption. Astronomical Journal, 134(3), 1150-1161.
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  Abstract: We present Gemini near-infrared spectroscopic observations of six luminous quasars at z = 5.8 to ∼6.3. Five of them were observed using Gemini South GNIRS, which provides a simultaneous wavelength coverage of 0.9-2.5 μm in cross-dispersion mode. The other source was observed in the K band with Gemini North NIRI. We calculate line strengths for all detected emission lines and use their ratios to estimate gas metallicity in the broad-line regions of the quasars. The metallicity is found to be supersolar, with a typical value of ∼4 Z⊙, and a comparison with low-redshift observations shows no strong evolution in metallicity up to z ∼ 6. The Fe II/Mg II ratio of the quasars is 4.9 ± 1.4, consistent with low-redshift measurements. We estimate central black hole masses of 109-1010 M ⊙ and Eddington luminosity ratios of order unity. We identify two Mg II λλ2796, 2803 absorbers with rest equivalent width W 0λ2796 > 1 Å at 2.2 < z < 3 and three Mg II absorbers with W0λ2796 > 1.5 Å at z > 3 in the spectra, with the two most distant absorbers at z = 4.8668 and 4.8823, respectively. The redshift number densities (dN/dz) of Mg II absorbers with W0λ2796 > 1.5 Å are consistent with no cosmic evolution up to z > 4. © 2007. The American Astronomical Society. All rights reserved.
• Kurk, J. D., Walter, F., Fan, X., Jiang, L., Riechers, D. A., Rix, H., Pentericci, L., Strauss, M. A., Carilli, C., & Wagner, S. (2007). Black hole masses and enrichment of z ∼ 6 SPSS quasars. Astrophysical Journal, 669(1), 32-44.
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  Abstract: We present sensitive near-infrared spectroscopic observations for a sample of five z ∼ 6 quasars. These quasars are among the most distant, currently known quasars in the universe. The spectra have been obtained using ISAAC at the VLT and include the C IV, Mg II, and Fe II lines. We measure the Fe II/Mg II line ratio, as an observational proxy for the Fe/α-element ratio. We derive a ratio of 2.7 ±0.8 for our sample, which is similar to that found for lower redshift quasars; i.e., we provide additional evidence for the lack of evolution in the Fe II/Mg II line ratio of quasars up to the highest redshifts. This result demonstrates that star formation must have commenced at z ≥ 8 in the quasar hosts. The line widths of the Mg II and C IV lines give two estimates for the black hole masses. A third estimate is given by assuming that the quasars emit at their Eddington luminosity. The derived masses using these three methods agree well, implying that the quasars are not likely to be strongly lensed. We derive central black hole masses of (0.3-5.2) × 109 M⊙. We use the difference between the redshift of Mg II (a proxy for the systemic redshift of the quasar) and the onset of the Gunn-Peterson trough to derive the extent of the ionized Strömgren spheres around our target quasars. The derived physical radii are about 5 Mpc. Using a simple ionization model, the emission of the central quasars would need of order 106-108 yr to create these cavities. As the e-folding timescale for the central accreting black hole is on the order of a few times 107 yr, it can grow by one e-folding or less within this time span. © 2007. The American Astronomical Society. All rights reserved.
• Leggett, S. K., Saumon, D., Marley, M. S., Geballe, T. R., Golimowski, D. A., Stephens, D., & Fan, X. (2007). 3.6-7.9 μm photometry of L and T dwarfs and the prevalence of vertical mixing in their atmospheres. Astrophysical Journal, 655(2 I), 1079-1094.
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  Abstract: We present new L′ (3.75 μm) photometry of six L and T dwarfs, and M′ (4.70 μm) photometry of 10 L and T dwarfs, observed at the Gemini (North) Observatory, and new 3.55, 4.49, 5.73, and 7.87 μm photometry of nine L and T dwarfs, obtained with the Spitzer Space Telescope. The sample includes unusually blue and red dwarfs selected from our near-infrared studies. The data are combined with previously published L′, M′, and Spitzer photometry of L and T dwarfs, and trends of colors with spectral type and other colors are examined. Recent model atmospheres by Marley and Saumon are used to generate synthetic colors for ranges of effective temperature, gravity, grain sedimentation efficiency, metallicity, and vertical mixing efficiency. We explore how these parameters affect the mid-infrared colors of L and T dwarfs and find that the data are modeled satisfactorily only if substantial vertical mixing occurs in both L and T dwarf atmospheres. The location and range of the L and T dwarf sequences in IRAC color-color and color-magnitude diagrams is also only reproduced if this mixing occurs, with a range of efficiency described by Kzz ∼ 102-106 cm2 s -1. The colors of the unusually red dwarfs are best reproduced by nonequilibrium models with low sedimentation efficiency, i.e., thick cloud decks, and the colors of the unusually blue dwarfs by nonequilibrium models with high sedimentation efficiency, i.e., thin cloud decks. The K - L′ and Spitzer [3.55] - [4.49] colors can be used as indicators of effective temperature for L and T dwarfs, but care must be taken to include gravity and metallicity effects for late-T dwarfs and vertical mixing for both late-L and T dwarfs. © 2007. The American Astronomical Society. All rights reserved.
• Saumon, D., Marley, M. S., Leggett, S. K., Geballe, T. R., Stephens, D., Golimowski, D. A., Cushing, M. C., Fan, X., Rayner, J. T., Lodders, K., & Freedman, R. S. (2007). Physical parameters of two very cool T dwarfs. Astrophysical Journal, 656(2 I), 1136-1149.
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  Abstract: We present new infrared spectra of the T8 brown dwarf 2MASS J04151954-0935066: 2.9-4.1 μm spectra obtained with the Infrared Camera and Spectrograph on the Subaru Telescope, and 5.2-14.5 μm spectra obtained with the Infrared Spectrograph on the Spitzer Space Telescope. We use these data and models to determine an accurate bolometric luminosity of log L bol/L⊙ = -5.67 and to constrain the effective temperature, gravity, mass, and age to 725-775 K, log g = 5.00-5.37, M = 33-58 MJup, and age = 3-10 Gyr. We perform the same analysis using published 0.6-15 μm spectra for the T7.5 dwarf 2MASSJ12171110-0311131, for which we find a metal-rich composition ([Fe/H] ∼ 0.3), and log L bol/L⊙ = -5.31, Teff = 850-950 K, log g = 4.80-5.42, M = 25-66 MJup, and age = 1-10 Gyr. These luminosities and effective temperatures straddle those determined with the same method and models for Gl 570D by Saumon et al. and make 2MASS J04151954-0935066 the coolest and least luminous T dwarf with well-determined properties. We find that synthetic spectra generated by the models reproduce the observed red through mid-infrared spectra of 2MASS J04151954-0935066 and 2MASS J12171110-0311131 very well, except for known discrepancies that are most likely due to the incomplete CH4 opacities. Both objects show evidence of departures from strict chemical equilibrium, and we discuss this result in the context of other late T dwarfs in which disequilibrium phenomena have been observed. © 2007. The American Astronomical Soeiety. All rights reserved.
• Schneider, D. P., Hall, P. B., Richards, G. T., Strauss, M. A., E., D., Anderson, S. F., Brandt, W. N., Xiaohui, F. A., Jester, S., Gray, J. I., Gunn, J. E., SubbaRao, M. U., Thakar, A. R., Stoughton, C., Szalay, A. S., Yanny, B., York, D. G., Bahcall, N. A., Barentine, J., , Blanton, M. R., et al. (2007). The sloan digital sky survey quasar catalog. IV. Fifth data release. Astronomical Journal, 134(1), 102-117.
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  Abstract: We present the fourth edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog contains 77,429 objects; this is an increase of over 30,000 entries since the previous edition. The catalog consists of the objects in the SDSS Fifth Data Release that have luminosities larger than Mi = -22.0 (in a cosmology with H0 = 70 km s-1 Mpc -1, ΩM = 0.3, and ΩΛ = 0.7), have at least one emission line with FWHM larger than 1000 km s-1 or have interesting/complex absorption features, are fainter than i ≈15.0, and have highly reliable redshifts. The area covered by the catalog is ≈5740 deg2. The quasar redshifts range from 0.08 to 5.41, with a median value of 1.48; the catalog includes 891 quasars at redshifts greater than 4, of which 36 are at redshifts greater than 5. Approximately half of the catalog quasars have i < 19; nearly all have i < 21. For each object the catalog presents positions accurate to better than 0.2" rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains basic radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3800-9200 Å at a spectral resolution of ≃2000; the spectra can be retrieved from the public database using the information provided in the catalog. The average SDSS colors of quasars as a function of red-shift, derived from the catalog entries, are presented in tabular form. Approximately 96% of the objects in the catalog were discovered by the SDSS. © 2007. The American Astronomical Society. All rights reserved.
• Shen, Y., Strauss, M. A., Oguri, M., Hennawi, J. F., Xiaohui, F., Richards, G. T., Hall, P. B., Gunn, J. E., Schneider, D. P., Szalay, A. S., Thakar, A. R., E., D., Anderson, S. F., Bahcall, N. A., Connolly, A. J., & Knapp, G. R. (2007). Clustering of high-redshift (z ≥ 2.9) quasars from the sloan digital sky survey. Astronomical Journal, 133(5), 2222-2241.
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  Abstract: We study the two-point correlation function of a uniformly selected sample of 4426 luminous optical quasars with redshift 2.9 ≤ z ≤ 5.4 selected over 4041 deg2 from the Fifth Data Release of the Sloan Digital Sky Survey. We fit a power-law to the projected correlation function w p(rp) to marginalize over redshift-space distortions and redshift errors. For a real-space correlation function of the form ξ(r) = (r/r0)-γ, the fitted parameters in comoving coordinates are r0 = 15.2 ± 2.7 h-1 Mpc and γ = 2.0 ± 0.3, over a scale range 4 h-1 Mpc ≤ rp ≤ 150 h-1 Mpc. Thus high-redshift quasars are appreciably more strongly clustered than their z ≈ 1.5 counterparts, which have a comoving clustering length r0 ≈ 6.5 h-1 Mpc. Dividing our sample into two redshift bins, 2.9 ≤ z ≤ 3.5 and z ≥ 3.5, and assuming a power-law index γ = 2.0, we find a correlation length of r0 = 16.9 ± 1.7 h-1 Mpc for the former and r0 = 24.3 ± 2.4 h-1 Mpc for the latter. Strong clustering at high redshift indicates that quasars are found in very massive, and therefore highly biased, halos. Following Martini & Weinberg, we relate the clustering strength and quasar number density to the quasar lifetimes and duty cycle. Using the Sheth & Tonnen halo mass function, the quasar lifetime is estimated to lie in the range ∼4-50Myr for quasars with 2.9 ≤ z ≤ 3.5, and ∼30-600 Myr for quasars with z ≥ 3.5. The corresponding duty cycles are ∼0.004-0.05 for the lower redshift bin and ∼0.03-0.6 for the higher redshift bin. The minimum mass of halos in which these quasars reside is (2-3) × 1012 h-1 M⊙ for quasars with 2.9 ≤ z ≤ 3.5 and (4-6) × 1012 h-1 M ⊙ for quasars with z ≥ 3.5; the effective bias factor b eff increases with redshift, e.g., beff ∼ 8 at z = 3.0 and beff ∼ 16 at z = 4.5. © 2007. The American Astronomical Society. All rights reserved.
• Wang, R., Carilli, C. L., Beelen, A., Bertoldi, F., Fan, X., Walter, F., Menten, K. M., Omont, A., Cox, P., Strauss, M. A., & Jiang, L. (2007). Millimeter and radio observations of z ∼ 6 quasars. Astronomical Journal, 134(2), 617-627.
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  Abstract: We present millimeter and radio observations of 13 SDSS quasars at redshifts z ∼ 6. We observed 11 of them with the Max Planck Millimeter Bolometer Array (MAMBO-2) at the IRAM 30 m telescope at 250 GHz and all of them with the Very Large Array (VLA) at 1.4 GHz. Four sources are detected by MAMBO-2 and six are detected by the VLA at the ≳ 3 σ level. These sources, together with another six published in previous papers, yield a submillimeter/millimeter- and radio-observed SDSS quasar sample at z ∼ 6. We use this sample to investigate the far-infrared (FIR) and radio properties of optically bright quasars in the early universe. We compare this sample to lower redshift samples of quasars observed in the submillimeter and millimeter wavelengths [(sub)mm] and find that the distribution of the FIR - to - B-band optical luminosity ratio (LFIRLB) is similar from z ∼ 2 to 6. We find a weak correlation between the FIR luminosity (LFIR) and B-band optical luminosity (LB) by including the (sub)mm observed samples at all redshifts. Some strong (sub)mm detections in the z ∼ 6 sample have radio-to-FIR ratios within the range defined by star-forming galaxies, which suggests possible coeval star-forming activity with the powerful AGN in these sources. We calculate the rest-frame radio-to-optical ratios (R 1.4* = Lv,1.4GHz/Lv,4400 Å) for all of the VLA-observed sources in the z ∼ 6 quasar sample. Only one radio detection in this sample, J083643.85+005453.3, has R1.4* ∼ 40 and can be considered radio-loud. There are no strong radio sources (R 1.4* ≥ 100) among these SDSS quasars at z ∼ 6. These data are consistent with, although do not set strong constraints on, a decreasing radio-loud quasar fraction with increasing redshift. © 2007. The American Astronomical Society. All rights reserved.
• Adelman-McCarthy, J. K., Agüeros, M. A., Allam, S. S., Anderson, K. S., Anderson, S. F., Annis, J., Bahcall, N. A., Baldry, I. K., Barentine, J. C., Berlind, A., Bernardi, M., Blanton, M. R., Boroski, W. N., Brewington, H. J., Brinchmann, J., Brinkmann, J., Brunner, R. J., Budavári, T., Carey, L. N., , Carr, M. A., et al. (2006). The fourth data release of the Sloan Digital Sky Survey. Astrophysical Journal, Supplement Series, 162(1), 38-48.
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  Abstract: This paper describes the Fourth Data Release of the Sloan Digital Sky Survey (SDSS), including all survey-quality data taken through 2004 June. The data release includes five-band photometric data for 180 million objects selected over 6670 deg 2 and 673,280 spectra of galaxies, quasars, and stars selected from 4783 deg 2 of those imaging data using the standard SDSS target selection algorithms. These numbers represent a roughly 27% increment over those of the Third Data Release; all the data from previous data releases are included in the present release. The Fourth Data Release also includes an additional 131,840 spectra of objects selected using a variety of alternative algorithms, to address scientific issues ranging from the kinematics of stars in the Milky Way thick disk to populations of faint galaxies and quasars. © 2006. The American Astronomical Society. All rights reserved.
• Chiu, K., Fan, X., Leggett, S. K., Golimowski, D. A., Zheng, W., Geballe, T. R., Schneider, D. P., & Brinkmann, J. (2006). Seventy-one new L and T dwarfs from the sloan digital sky survey. Astronomical Journal, 131(5), 2722-2736.
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  Abstract: We present near-infrared observations of 71 newly discovered L and T dwarfs, selected from imaging data of the Sloan Digital Sky Survey (SDSS) using the i-dropout technique. Sixty-five of these dwarfs have been classified spectroscopically according to the near-infrared L dwarf classification scheme of Geballe et al. and the unified T dwarf classification scheme of Burgasser et al. The spectral types of these dwarfs range from L3 to T7 and include the latest types yet found in the SDSS. Six of the newly identified dwarfs are classified as early to mid-L dwarfs according to their photometric near-infrared colors, and two others are classified photometrically as M dwarfs. We also present new near-infrared spectra for five previously published SDSS L and T dwarfs, and one L dwarf and one T dwarf discovered by Burgasser et al. from the Two Micron All Sky Survey. The new SDSS sample includes 27 T dwarfs and 30 dwarfs with spectral types spanning the complex L-T transition (L7-T3). We continue to see a large (∼0.5 mag) spread in J - H for L3-T1 types and a similar spread in H - K for all dwarfs later than L3. This color dispersion is probably due to a range of grain sedimentation properties, metallicity, and gravity. We also find L and T dwarfs with unusual colors and spectral properties that may eventually help to disentangle these effects. © 2006. The American Astronomical Society. All rights reserved.
• Fan, X. (2006). Evolution of high-redshift quasars. New Astronomy Reviews, 50(9-10), 665-671.
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  Abstract: The discovery of luminous quasars at z > 6 indicates the existence billion-solar-mass black holes at the end of reionization epoch. These quasars provide the best probes of the early growth of supermassive black holes in the universe and the relation between the formation of early galaxies and black holes. With the advent of the Sloan Digital Sky Survey and other large area surveys, about 1000 quasars have been discovered at z > 4, including >50 at z > 5 and nine at z > 6. I here review the recent observational results studies of the highest redshift quasars, including the evolution of quasar density and luminosity function, the evolution of spectral properties and chemical enrichment history, distribution of black hole masses and properties of quasar environment and host galaxies at high redshift. © 2006 Elsevier B.V. All rights reserved.
• Fan, X., Carilli, C. L., & Keating, B. (2006). Observational constraints on cosmic reionization. Annual Review of Astronomy and Astrophysics, 44, 415-462.
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  Abstract: Observations have set the first constraints on the epoch of reionization (EoR), corresponding to the formation epoch of the first luminous objects. Studies of Gunn-Peterson (GP) absorption indicate a rapid increase in the neutral fraction of the intergalactic medium (IGM) from x HI < 10 -4 at z ≤ 5.5, to x HI > 10 -3, perhaps up to 0.1, at z ∼ 6, while the large scale polarization of the cosmic microwave background (CMB) implies a significant ionization fraction extending to higher redshifts, z ∼ 11 ± 3. These results, as well as observations of galaxy populations, suggest that reionization is a process that begins as early as z ∼ 14, and ends with the "percolation" phase at z ∼ 6 to 8. Low luminosity star-forming galaxies are likely the dominant sources of reionizing photons. Future low-frequency radio telescopes will make direct measurements of HI 21-cm emission from the neutral IGM during the EoR, and measurements of secondary CMB temperature anisorropy will provide details of the dynamics of the reionized IGM. Copyright © 2006 by Annual Reviews. All rights reserved.
• Fan, X., Strauss, M. A., Becker, R. H., White, R. L., Gunn, J. E., Knapp, G. R., Richards, G. T., Schneider, D. P., Brinkmann, J., & Fukugita, M. (2006). Constraining the evolution of the ionizing background and the epoch of reionization with z ∼ 6 quasars. II. A sample of 19 quasars. Astronomical Journal, 132(1), 117-136.
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  Abstract: We study the evolution of the ionization state of the intergalactic medium (IGM) at the end of the reionization epoch using moderate-resolution spectra of a sample of 19 quasars at 5.74 < zem < 6.42 discovered in the Sloan Digital Sky Survey. Three methods are used to trace IGM properties: (1) the evolution of the Gunn-Peterson (GP) optical depth in the Lyα, Lyβ, and Lyγ transitions; (2) the distribution of lengths of dark absorption gaps; and (3) the size of H II regions around luminous quasars. Using this large sample, we find that the evolution of the ionization state of the IGM accelerated at z > 5.7: the GP optical depth evolution changes from τ GPeff ∼ (1 + z) 4.3 to (1 + z) ≳11, and the average length of dark gaps with τ > 3.5 increases from 80 comoving Mpc. The dispersion of IGM properties along different lines of sight also increases rapidly, implying fluctuations by a factor of gsim;4 in the UV background at z > 6, when the mean free path of UV photons is comparable to the correlation length of the starforming galaxies that are thought to have caused reionization. The mean length of dark gaps shows the most dramatic increase at z ∼ 6, as well as the largest line-of-sight variations. We suggest using dark gap statistics as a powerful probe of the ionization state of the IGM at yet higher redshift. The sizes of H II regions around luminous quasars decrease rapidly toward higher redshift, suggesting that the neutral fraction of the IGM has increased by a factor of gsim;10 from z = 5.7 to 6.4, consistent with the value derived from the GP optical depth. The mass-averaged neutral fraction is 1%-4% at z ∼ 6.2 based on the GP optical depth and H II region size measurements. The observations suggest that z ∼ 6 is the end of the overlapping stage of reionization and are inconsistent with a mostly neutral IGM at z ∼ 6, as indicated by the finite length of the dark absorption gaps. © 2006. The American Astronomical Society. All rights reserved.
• Fan, X., Strauss, M. A., Richards, G. T., Hennawi, J. F., Becker, R. H., White, R. L., Diamond-Stanic, A. M., Donley, J. L., Jiang, L., Kim, J. S., Vestergaard, M., Young, J. E., Gunn, J. E., Lupton, R. H., Knapp, G. R., Schneider, D. P., Brandt, W. N., Bahcall, N. A., Barentine, J. C., , Brinkmann, J., et al. (2006). A survey of z> 5.7 quasars in the sloan digital sky survey. IV. Discovery of seven additional quasars. Astronomical Journal, 131(3), 1203-1209.
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  Abstract: We present the discovery of seven quasars at z > 5.7, selected from ∼2000 deg 2 of multicolor imaging data of the Sloan Digital Sky Survey. The new quasars have redshifts z from 5.79 to 6.13. Five are selected as part of a complete flux-limited sample in the SDSS northern Galactic cap; two have larger photometric errors and are not part of the complete sample. One of the new quasars, SDSS J1335+3533 (z = 5.93), exhibits no emission lines; the 3 σ limit on the rest-frame equivalent width of the Lyα+N v line is 5 Å. It is the highest redshift lineless quasar known and could be a gravitational lensed galaxy, a BL Lac object, or a new type of quasar. Two new z > 6 quasars, SDSS 1250+3130 (z = 6.13) and SDSS J1137+3549(z = 6.01), show deep Gunn-Peterson absorption gaps in Lyα. These gaps are narrower than the complete Gunn-Peterson absorption troughs observed among quasars at z > 6.2 and do not have complete Lyß absorption. © 2006. The American Astronomical Society. All rights reserved.
• Gnedin, N. Y., & Fan, X. (2006). Cosmic reionization redux. Astrophysical Journal Letters, 648(1 I), 1-6.
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  Abstract: We show that numerical simulations of reionization that resolve the Lyman limit systems (and, thus, correctly count absorptions of ionizing photons) have converged to about a 10% level for 5 < z < 6.2 and are in reasonable agreement (within 10%) with the SDSS data in this redshift interval. The SDSS data thus constrain the redshift of overlap of cosmic H II regions to z OVL = 6.1 ± 0.15. At higher redshifts, the simulations are far from convergence on the mean Gunn-Peterson optical depth but achieve good convergence for the mean neutral hydrogen fraction. The simulations that fit the SDSS data, however, do not have nearly enough resolution to resolve the earliest episodes of star formation and are very far from converging on the precise value of the optical depth to Thomson scattering - any value between 6% and 10% is possible, depending on the convergence rate of the simulations and the fractional contribution of Population III stars. This is generally consistent with the third-year WMAP results, but much higher resolution simulations are required to come up with the sufficiently precise value for the Thomson optical depth that can be statistically compared with the WMAP data. © 2006. The American Astronomical Society. All rights reserved.
• Hennawi, J. F., Prochaska, J. X., Burles, S., Strauss, M. A., Richards, G. T., Schlegel, D. J., Fan, X., Schneider, D. P., Zakamska, N. L., Oguri, M., Gunn, J. E., Lupton, R. H., & Brinkmann, J. (2006). Quasars probing quasars. I. Optically thick absorbers near luminous quasars. Astrophysical Journal Letters, 651(1 I), 61-83.
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  Abstract: With close pairs of quasars at different redshifts, a background quasar sight line can be used to study a foreground quasar's environment in absorption. We search 149 moderate-resolution background quasar spectra from Gemini, Keck, the MMT, and the SDSS to survey Lyman limit systems (LLSs) and damped Lyα systems (DLAs) in the vicinity of 1.8 < z < 4.0 luminous foreground quasars. A sample of 27 new quasar-absorber pairs is uncovered with column densities 1017.2 cm-2 < NH I < 10 20.9 cm-2 and transverse (proper) distances of 22 h -1 kpc < R < 1.7 h-1 Mpc from the foreground quasars. If they emit isotropically, the implied ionizing photon fluxes are a factor of ∼5-8000 times larger than the ambient extragalactic UV background over this range of distances. The observed probability of intercepting an absorber is very high for small separations: six out of eight projected sight lines with transverse separations R < 150 h-1 kpc have an absorber coincident with the foreground quasar, of which four have NH I > 1019 cm-2. The covering factor of NH I > 1019 cm-2 absorbers is thus ∼50% (4/8) on these small scales, whereas ≲2% would have been expected at random. There are many cosmological applications of these new sight lines: they provide laboratories for studying fluorescent Lyα recombination radiation from LLSs; they constrain the environments, emission geometry, and radiative histories of quasars; and they shed light on the physical nature of LLSs and DLAs. © 2006. The American Astronomical Society. All rights reserved.
• Hennawi, J. F., Strauss, M. A., Oguri, M., Inada, N., Richards, G. T., Pindor, B., Schneider, D. P., Becker, R. H., Gregg, M. D., Hall, P. B., Johnston, D. E., Fan, X., Burles, S., Schlegel, D. J., Gunn, J. E., Lupton, R. H., Bahcall, N. A., Brunner, R. J., & Brinkmann, J. (2006). Binary quasars in the sloan digital sky survey: Evidence for excess clustering on small scales. Astronomical Journal, 131(1), 1-23.
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  Abstract: We present a sample of 221 new quasar pairs with proper transverse separations R prop < 1 h -1 Mpc over the redshift range 0.5 < z < 3.0, discovered from an extensive follow-up campaign to find companions around the Sloan Digital Sky Survey and 2dF QSO Redshift Survey quasars. This sample includes 26 new binary quasars with separations R prop < 50 h -1 kpc (θ < 10″), more than doubling the number of such systems known. We define a statistical sample of binaries selected with homogeneous criteria and compute its selection function, taking into account sources of incompleteness. The first measurement of the quasar correlation function on scales 10 h -1 kpc < R prop < 400 h -1 kpc is presented. For R prop ≲ 40 h -1 kpc, we detect an order of magnitude excess clustering over the expectation from the large-scale (R prop ≳ 3 h -1 Mpc) quasar correlation function, extrapolated down as a power law (γ = 1.53) to the separations probed by our binaries. The excess grows to ∼30 at R prop 10 h -1 kpc and provides compelling evidence that the quasar autocorrelation function gets progressively steeper on submegaparsec scales. This small-scale excess can likely be attributed to dissipative interaction events that trigger quasar activity in rich environments. Recent small-scale measurements of galaxy clustering and quasar-galaxy clustering are reviewed and discussed in relation to our measurement of small-scale quasar clustering. © 2006. The American Astronomical Society. All rights reserved.
• Jiang, L., Fan, X., Cool, R. J., Eisenstein, D. J., Zehavi, I., Richards, G. T., Scranton, R., Johnston, D., Strauss, M. A., Schneider, D. P., & Brinkmann, J. (2006). A spectroscopic survey of faint quasars in the SDSS deep stripe. I. Preliminary results from the co-added catalog. Astronomical Journal, 131(6), 2788-2800.
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  Abstract: In this paper we present the first results of a deep spectroscopic survey of faint quasars in the Sloan Digital Sky Survey (SDSS) Southern Survey, a deep survey carried out by repeatedly imaging a 270 deg 2 area. Quasar candidates were selected from the deep data with good completeness over 0 < z < 5 and 2-3 mag fainter than the SDSS main survey. Spectroscopic follow-up was carried out on the 6.5 m MMT with Hectospec. The preliminary sample of this SDSS faint quasar survey (SFQS) covers ∼3.9 deg 2, contains 414 quasars, and reaches g = 22.5. The overall selection efficiency is ∼66% (∼80% at g < 21.5); the efficiency in the most difficult redshift range (2 < z < 3) is better than 40%. We use the 1/V a method to derive a binned estimate of the quasar luminosity function (QLF) and model the QLF using maximum likelihood analysis. The best model fits confirm previous results showing that the QLF has steep slopes at the bright end and much flatter slopes (-1.25 at z ≲ 2.0 and -1.55 at z ≳ 2.0) at the faint end, indicating a break in the QLF slope. Using a luminosity-dependent density evolution model, we find that the quasar density at M g < -22.5 peaks at z ∼ 2, which is later in cosmic time than the peak of z ∼ 2.5 found from surveys of more luminous objects. The SFQS QLF is consistent with the results of the 2dF QSO Redshift Survey, the SDSS, and the 2dF-SDSS LRG and QSO Survey, but probes fainter quasars. We plan to obtain more quasars from future observations and establish a complete faint quasar sample with more than 1000 objects over 10 deg 2. © 2006. The American Astronomical Society. All rights reserved.
• Kurk, J. D., Walter, F., Riechers, D., Rix, H., Wagner, S., Pentericci, L., & Fan, X. (2006). Metallicity and black hole masses of z≃6 quasars. Proceedings of the International Astronomical Union, 2(14), 257-.
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  Abstract: We present NIR spectroscopy of emission lines of a sample of five z6 quasars, including fainter objects than observed before. The measured FeII/MgII ratios are around solar and consistent with a lack of evolution of the metallicity of quasar BLRs up to z6, suggesting that stars in their hosts formed at z6. The BH masses, measured from both MgII and CIV line widths are within the range 2-16108M, the smallest found in such distant objects. © 2007 International Astronomical Union.
• Liu, M. C., Leggett, S. K., Golimowski, D. A., Chiu, K., Fan, X., Geballe, T. R., Schneider, D. P., & Brinkmann, J. (2006). SDSS J1534+1615AB: A novel T dwarf binary found with Keck laser guide star adaptive optics and the potential role of binarity in the L/T transition. Astrophysical Journal Letters, 647(2 I), 1393-1404.
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  Abstract: We have resolved the newly discovered T dwarf SDSS J153417.05+161546.1 into a 0′11 binary using the Keck sodium laser guide star adaptive optics system. With an integrated-light near-IR spectral type of T3.5 ±0.5, this binary provides a new benchmark for studying the distinctive ./-band brightening previously noted among early and mid-T dwarfs, using two brown dwarfs with different spectral types but having a common metallicity and age and very similar surface gravities. We estimate spectral types of T1.5 ±0.5 and T5.5 ±0.5 for the two components based on their near-IR colors, consistent with modeling the integrated-light spectrum as the blend of two components. The observed near-IR flux ratios of SDSS J1534+1615 are unique compared to all previously known substellar binaries: the component that is fainter at H and K' is brighter at J. This inversion of the near-IR fluxes is a manifestation of the J-band brightening within this individual binary system. Therefore, SDSS J1534+1615 demonstrates that the brightening can be intrinsic to ultracool photospheres (e.g., arising from cloud disruption and/or rapid increase in cloud sedimentation) and does not necessarily result from physical variations among the observed ensemble of T dwarfs (e.g., a range in masses, ages, and/or metallicities). We suggest that the apparently large amplitude of the J-band brightening may be due to a high incidence of unresolved binaries and that the true amplitude of the brightening phenomenon could be more modest. This scenario would imply that truly single objects in these spectral subclasses are relatively rare, in agreement with the small effective temperature range inferred for the L/T transition. © 2006. The American Astronomical Society. All rights reserved.
• Richards, G. T., Haiman, Z., Pindor, B., Strauss, M. A., Fan, X., Eisenstein, D., Schneider, D. P., Bahcall, N. A., Brinkmann, J., & Fukugita, M. (2006). A snapshot survey for gravitational lenses among z ≥ 4.0 quasars. II. Constraints on the 4.0 < z < 5.4 quasar population. Astronomical Journal, 131(1), 49-54.
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  Abstract: We report on i-band snapshot observations of 157 Sloan Digital Sky Survey quasars at 4.0 < z < 5.4 using the Advanced Camera for Surveys on the Hubble Space Telescope (HST) to search for evidence of gravitational lensing of these sources. None of the quasars appear to be strongly lensed and multiply imaged at the angular resolution (∼0″1) and sensitivity of HST. The nondetection of strong lensing in these systems constrains the z = 4-5 luminosity function to an intrinsic slope of β > -3.8(3 σ), assuming a break in the quasar luminosity function at M* 1450 ∼ -24.5. This constraint is considerably stronger than the limit of β > -4.63 obtained from the absence of lensing in four z > 5.7 quasars. Such constraints are important to our understanding of the true space density of high-redshift quasars and the ionization state of the early universe. © 2006. The American Astronomical Society. All rights reserved.
• Richards, G. T., Haiman, Z., Pindor, B., Strauss, M. A., Xiaohui, F. A., Eisenstein, D., Schneider, D. P., Bahcall, N. A., Brinkmann, J., & Fukugita, M. (2006). Erratum: A snapshot survey for gravitational lenses among z ≥ 4.0 quasars. II. Constraints on the 4.0 < z < 5.4 quasar population (Astronomical Journal (2006) 131 (49)). Astronomical Journal, 132(2), 967-.
• Richards, G. T., Lacy, M., Storrie-Lombardi, L. J., Hall, P. B., Gallagher, S. C., Hines, D. C., Xiaohui, F., Papovich, C., E., D., Trammell, G. B., Schneider, D. P., Vestergaard, M., York, D. G., Jester, S., Anderson, S. F., Budavári, T., & Szalay, A. S. (2006). Spectral energy distributions and multiwavelength selection of type 1 quasars. Astrophysical Journal, Supplement Series, 166(2), 470-497.
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  Abstract: We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ∼ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGNs) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA, and ROSAT date, where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as 50% for individual quasars when inferring a bolometric luminosity from an optical luminosity. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened / obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio. © 2006. The American Astronomical Society. All rights reserved.
• Richards, G. T., Strauss, M. A., Fan, X., Hall, P. B., Jester, S., Schneider, D. P., E., D., Stoughton, C., Anderson, S. F., Brunner, R. J., Gray, J., Gunn, J. E., Ivezić, Ž., Kirkland, M. K., Knapp, G. R., Loveday, J., Meiksin, A., Pope, A., Szalay, A. S., , Thakar, A. R., et al. (2006). The sloan digital sky survey quasar survey: Quasar luminosity function from data release 3. Astronomical Journal, 131(6), 2766-2787.
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  Abstract: We determine the number counts and z = 0-5 luminosity function for a well-defined, homogeneous sample of quasars from the Sloan Digital Sky Survey (SDSS). We conservatively define the most uniform statistical sample possible, consisting of 15,343 quasars within an effective area of 1622 deg 2 that was derived from a parent sample of 46,420 spectroscopically confirmed broad-line quasars in the 5282 deg 2 of imaging data from SDSS Data Release 3. The sample extends from i = 15 to 19.1 at z ≲ 3 and to i = 20.2 for z ≳ 3. The number counts and luminosity function agree well with the results of the Two Degree Field QSO Redshift Survey (2QZ) at redshifts and luminosities at which the SDSS and 2QZ quasar samples overlap, but the SDSS data probe to much higher redshifts than does the 2QZ sample. The number density of luminous quasars peaks between redshifts 2 and 3, although uncertainties in the selection function in this range do not allow us to determine the peak redshift more precisely. Our best-fit model has a flatter bright-end slope at high redshift than at low redshift. For z < 2.4 the data are best fit by a redshift-independent slope of β = -3.1 [Φ(L) ∝ L β]. Above z = 2.4 the slope flattens with redshift to β ≳ - 2.37 at z = 5. This slope change, which is significant at the ≳ 5 σ level, must be accounted for in models of the evolution of accretion onto supermassive black holes. © 2006. The American Astronomical Society. All rights reserved.
• Shemmer, O., Brandt, W. N., Schneider, D. P., Fan, X., Strauss, M. A., Diamond-Stanic, A. M., Richards, G. T., Anderson, S. F., Gunn, J. E., & Brinkmanns, J. (2006). Chandra observations of the highest redshift quasars from the sloan digital sky survey. Astrophysical Journal Letters, 644(1), 86-99.
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  Abstract: We present new Chandra observations of 21 z > 4 quasars, including 11 sources at z > 5. These observations double the number of X-ray-detected quasars at z > 5, allowing investigation of the X-ray spectral properties of a substantial sample of quasars at the dawn of the modern universe. By jointly fitting the spectra of 15 z > 5 radio-quiet quasars (RQQs), including sources from the Chandra archive, with a total of 185 photons, we find a mean X-ray power-law photon index of F = 1.95-0.26+0.30, and a mean neutral intrinsic absorption column density of NH; 6 × 1022 cm-2. These results show that quasar X-ray spectral properties have not evolved up to the highest observable redshifts. We also find that the mean optical-X-ray spectral slope (αox) of optically selected z > 5 RQQs, excluding broad absorption line quasars, is αox = -1.69 ± 0.03, which is consistent with the value predicted from the observed relationship between αox and ultraviolet luminosity. Four of the sources in our sample are members of the rare class of weak emission-line quasars, and we detect two of them in X-rays. We discuss the implications our X-ray observations have for the nature of these mysterious sources and, in particular, whether their weak-line spectra are a consequence of continuum boosting or a deficit of high-ionization line-emitting gas. © 2006. The American Astronomical Society. All rights reserved.
• Trump, J. R., Hall, P. B., Reichard, T. A., Richards, G. T., Schneider, D. P., E., D., Knapp, G. R., Anderson, S. F., Fan, X., Brinkman, J., Kleinman, S. J., & Nitta, A. (2006). A catalog of broad absorption line quasars from the Sloan Digital Sky Survey Third Data Release. Astrophysical Journal, Supplement Series, 165(1), 1-18.
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  Abstract: We present a total of 4784 unique broad absorption line quasars from the Sloan Digital Sky Survey Third Data Release. An automated algorithm was used to match a continuum to each quasar and to identify regions of flux at least 10% below the continuum over a velocity range of at least 1000 km s -1 in the C IV and Mg n absorption regions. The model continuum was selected as the best-fit match from a set of template quasar spectra binned in luminosity, emission line width, and redshift, with the power-law spectral index and amount of dust reddening as additional free parameters. We characterize our sample through the traditional "balnicity" index and a revised absorption index, as well as through parameters such as the width, outflow velocity, fractional depth, and number of troughs. From a sample of 16,883 quasars at 1.7 ≤ z ≤ 4.38, we identify 4386 (26.0%) quasars with broad C IV absorption, of which 1756 (10.4%) satisfy traditional selection criteria. From a sample of 34,973 quasars at 0.5 ≤ z ≤ 2.15, we identify 457 (1.31%) quasars with broad Mg n absorption, 191 (0.55%) of which satisfy traditional selection criteria. We also provide a supplementary list of 39 visually identified z > 4.38 quasars with broad C IV absorption. We find that broad absorption line quasars may have broader emission lines on average than other quasars. © 2006. The American Astronomical Society. All rights reserved.
• Abazajian, K., Adelman-McCarthy, J. K., Agüeros, M. A., Allam, S. S., Anderson, K. S., Anderson, S. F., Annis, J., Bahcall, N. A., Baldry, I. K., Bastian, S., Berlind, A., Bernardi, M., Blanton, M. R., Bochanski Jr., J. J., Boroski, W. N., Brewington, H. J., Briggs, J. W., Brinkmann, J., Brunner, R. J., , Budavári, T., et al. (2005). The third data release of the sloan digital sky survey. Astronomical Journal, 129(3), 1755-1759.
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  Abstract: This paper describes the Third Data Release of the Sloan Digital Sky Survey (SDSS), This release, containing data taken up through 2003 June, includes imaging data in five bands over 5282 deg2, photometric and astrometric catalogs of the 141 million objects detected in these imaging data, and spectra of 528,640 objects selected over 4188 deg2. The pipelines analyzing both images and spectroscopy are unchanged from those used in our Second Data Release. © 2005. The American Astronomical Society. All rights reserved.
• E., D., Schneider, D. P., Richards, G. T., Hall, P. B., Strauss, M. A., Brunner, R., Fan, X., Baldry, I. K., York, D. G., Gunn, J. E., Nichol, R. C., Meiksin, A., & Brinkmann, J. (2005). An empirical calibration of the completeness of the SDSS quasar survey. Astronomical Journal, 129(5), 2047-2061.
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  Abstract: Spectra of nearly 20,000 pointlike objects to a Galactic reddening-corrected magnitude of i = 19.1 have been obtained to test the completeness of the Sloan Digital Sky Survey (SDSS) quasar survey. We focus on spatially unresolved quasars, which comprise 94% of all SDSS quasars to the main survey magnitude limit. The objects were selected from all regions of color space, sparsely sampled from within a 278 deg 2 area (effective area 233 deg 2) of sky covered by this study. Only 10 quasars were identified that were not targeted as candidates by the SDSS quasar survey (including both color and radio source selection). The inferred density of unresolved quasars on the sky that are missed by the SDSS algorithm is 0.44 deg -2, compared with 8.28 deg -2 for the selected quasar density, giving a completeness of 94.9 -3.8+2.6% to the limiting magnitude. Omitting radio selection reduces the color-only selection completeness by about 1%. Of the 10 newly identified quasars, three have detected broad absorption line systems, six are significantly redder than other quasars at the same redshift, and four have redshifts between 2.7 and 3.0 (the redshift range where the SDSS colors of quasars intersect the stellar locus). The fraction of quasars (and other unresolved sources) missed because of image defects and blends, independent of the selection algorithm, is ≈4%, but this number varies by a few percent with magnitude. Quasars with extended images comprise about 6% of the SDSS sample, and the completeness of the selection algorithm for extended quasars is approximately 81%, based on the SDSS galaxy survey. The combined end-to-end completeness for the SDSS quasar survey is ≈89%. The total corrected density of quasars on the sky to i = 19.1 is estimated to be 10.2 deg -2. The SDSS completeness restricted to z ≳ 3 quasars is expected to be considerably lower and is a strong function of redshift. The determination of the global completeness is required for the statistical properties of quasars to i = 19.1 derived from the SDSS data set, such as the luminosity function and correlation function, to be accurately determined. © 2005. The American Astronomical Society. All rights reserved.
• Fan, X., Chawla, A. S., Roehrig, H., Rodriguez, J. J., & Fan, X. -. (2005). Determining the MTF of medical imaging displays using edge techniques. Journal of digital imaging, 18(4).
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  The modulation transfer function (MTF) of a medical imaging display is typically determined by measuring its response to square waves (bar patterns), white noise, and/or line stimuli. However, square waves and white noise methods involve capture and analysis of multiple images and are thus quite tedious. Measurement of the line-spread function (LSF) offers a good alternative. However, as previously reported, low-frequency response obtained from the LSF method is not as good as that obtained from measurement of edge-spread function (ESF). In this paper, we present two methods for evaluating the MTF of a medical imaging display from its ESF. High degree of accuracy in the higher frequency region (near the Nyquist frequency of the system) was achieved by reducing the noise. In the first method, which is a variant of the Gans' original method, the periodic raster noise is reduced by subtracting a shifted ESF from the ESF. The second method employs a low-pass differentiator (LPD). A novel near maximally flat LPD with the desired cut-off frequency was designed for this purpose. Noise reduction in both the methods was also achieved by averaging over large portions of the image data to form the ESF. Experimental results show that the MTF obtained by these methods is comparable to that obtained from the square wave response. Furthermore, the MTFs of rising and falling edges of a cathode ray tube (CRT) were measured. The results show that the rising and falling vertical MTFs are practically the same, whereas the rising horizontal MTF is poorer than the falling horizontal MTF in the midfrequency region.
• Fan, X., Krupinski, E. A., Roehrig, H., Dallas, W., & Fan, X. -. (2005). Differential use of image enhancement techniques by experienced and inexperienced observers. Journal of digital imaging, 18(4).
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  Full-field digital mammography (FFDM) systems are currently being used to acquire mammograms in digital format, but digital displays are less than ideal compared to traditional film-screen display. Certain physical properties of softcopy displays [e.g., modulation transfer function (MTF)] are less than optimal compared to film. We developed methods to compensate for some of these softcopy display deficiencies, based on careful physical characterization of the displays and image-processing software. A series of 100 FFDM and 60 digitized images was shown to six observers-half experienced (mammographers) and half inexperienced (radiology residents). The observers had to decide if a mass or microcalcification cluster was present and classify it as benign or malignant. A window could be activated that brought the image detail within the window to full resolution and corrected for the nonisotropic MTF of the Cathode Ray Tube (CRT) display. Experienced readers had better diagnostic performance and took less time to view the images. Experienced readers used window/level more than inexperienced readers, but inexperienced readers used magnification and the MTF compensation tool more often. Use of the magnification and the MTF tool increased reader decision confidence. Experienced and inexperienced readers use image-processing tools differently, with certain tools increasing reader confidence. Understanding how observers use image-processing tools may help in the development of better and more automated user interfaces.
• Golimowski, D. A., Leggett, S. K., Marley, M. S., Fan, X., Geballe, T. R., & Knapp, G. R. (2005). Near-infrared photometry and spectroscopy of L and T dwarfs: The effects of clouds, gravity, and effective temperature. European Space Agency, (Special Publication) ESA SP, 575-578.
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  Abstract: We present new JHKL′M′ photometry on the MKO system for a large sample of L and T dwarfs identified from SDSS and 2MASS and classified according to the scheme of Geballe et al. (2002). We have compiled a sample of 105 L and T dwarfs that are uniformly classified and measured on a single photometric system. The scattered JHK spectral indices and colors of L dwarfs are likely caused by variations in the altitudes, distributions, and thicknesses of condensate clouds. Scatter in the H-K colors of late T dwarfs probably reflects the sensitivity of the K-band flux to pressure induced H 2 opacity, which itself is sensitive to surface gravity. The M′ luminosities of late-T dwarfs are 1.5-2.5 times fainter than predicted under conditions of chemical equilibrium. We have computed L bol and T eff for 42 L and T dwarfs whose trigonometric parallaxes have been measured. We find that T eff ≈1450 K for types L7-T4, which supports recent models that attribute the changing JH K-band luminosities and spectral features across the L-T transition to rapid changes in the condensate clouds over a narrow range of T eff. We compute T eff = 600-750 K for 2MASS 0415-0935 (T9), which supplants Gl 570D as the coolest known brown dwarf.
• Hao, L., Strauss, A., Fan, X., Tremonti, C. A., Schlegel, D. J., Heckman, T. M., Kauffmann, G., Blanton, M. R., Gunn, J. E., Hall, P. B., Ivezić, Ž., Knapp, G. R., Krolik, J. H., Lupton, R. H., Richards, G. T., Schneider, D. P., Strateva, I. V., Zakamska, N. L., Brinkmann, J., & Szokoly, G. P. (2005). Active galactic nuclei in the sloan digital sky survey. II. Emission-line luminosity function. Astronomical Journal, 129(4), 1795-1808.
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  Abstract: The emission-line luminosity function of active galactic nuclei (AGNs) is measured from about 3000 AGNs included in the main galaxy sample of the Sloan Digital Sky Survey within a redshift range of 0 < z < 0.15. The Hα and [O III] λ5007 luminosity functions for Seyfert galaxies cover a luminosity range of 10 5-10 9 L ⊙ in Hα, and the shapes are well fitted by broken power laws, without a turnover at fainter nuclear luminosities. Assuming a universal conversion from emission-line strength to continuum luminosity, the inferred 5-band magnitude luminosity function is comparable to both the AGN luminosity function of previous studies and the low-redshift quasar luminosity function derived from the Two-Degree Field redshift survey. The inferred AGN number density is approximately one-fifth of all galaxies, and about 6 × 10 -3 of the total light of galaxies in the r band comes from nuclear activity. The numbers of Seyfert 1 and Seyfert 2 galaxies are comparable at low luminosity, while at high luminosity, Seyfert 1 galaxies outnumber Seyfert 2 galaxies by a factor of 2-4. In making the luminosity function measurements, we assume that the nuclear luminosity is independent of the host galaxy luminosity, an assumption we test a posteriori and show to be consistent with the data. Given the relationship between black hole mass and host galaxy bulge luminosity, the lack of correlation between nuclear and host luminosity suggests that the main variable that determines the AGN luminosity is the Eddington ratio, not the black hole mass. This appears to be different from luminous quasars, which are most likely to be shining near the Eddington limit. © 2005. The American Astronomical Society. All rights reserved.
• Hao, L., Strauss, M. A., Tremonti, C. A., Schlegel, D. J., Heckman, T. M., Kauffmann, G., Blanton, M. R., Fan, X., Gunn, J. E., Hall, P. B., Ivezić, Ž., Knapp, G. R., Krolik, J. H., Lupton, R. H., Richards, G. T., Schneider, D. P., Strateva, I. V., Zakamska, N. L., Brinkmann, J., , Brunner, R. J., et al. (2005). Active galactic nuclei in the sloan digital sky survey. I. Sample selection. Astronomical Journal, 129(4), 1783-1794.
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  Abstract: We have compiled a large sample of low-redshift active galactic nuclei (AGNs) identified via their emission-line characteristics from the spectroscopic data of the Sloan Digital Sky Survey. Since emission lines are often contaminated by stellar absorption lines, we developed an objective and efficient method of subtracting the stellar continuum from every galaxy spectrum before making emission-line measurements. The distribution of the measured Hα FWHM values of emission-line galaxies is strongly bimodal, with two populations separated at about 1200 km s -1. This feature provides a natural separation between narrow-line and broad-line AGNs. The narrow-line AGNs are identified using standard emission-line ratio diagnostic diagrams. There are 1317 broad-line and 3074 narrow-line AGNs identified from about 100,000 galaxy spectra selected over 1151 deg 2. This sample is used in a companion paper to determine the emission-line luminosity function of AGNs. © 2005. The American Astronomical Society, All rights reserved.
• Richards, G. T., Croom, S. M., Anderson, S. F., Bland-Hawthorn, J., Boyle, B. J., Propris, R. D., Drinkwater, M. J., Fan, X., Gunn, J. E., Ivezić, Ž., Jester, S., Loveday, J., Meiksin, A., Miller, L., Myers, A., Nichol, R. C., Outram, P. J., Pimbblet, K. A., Roseboom, I. G., , Ross, N., et al. (2005). The 2dF-SDSS LRG and QSO (2SLAQ) Survey: The z < 2.1 quasar luminosity function from 5645 quasars to g = 21.85. Monthly Notices of the Royal Astronomical Society, 360(3), 839-852.
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  Abstract: We have used the Two-Degree Field (2dF) instrument on the Anglo-Australian Telescope (AAT) to obtain redshifts of a sample of z < 3 and 18.0 < g < 21.85 quasars selected from Sloan Digital Sky Survey (SDSS) imaging. These data are part of a larger joint programme between the SDSS and 2dF communities to obtain spectra of faint quasars and luminous red galaxies, namely the 2dF-SDSS LRG and QSO (2SLAQ) Survey. We describe the quasar selection algorithm and present the resulting number counts and luminosity function of 5645 quasars in 105.7 deg 2. The bright-end number counts and luminosity functions agree well with determinations from the 2dF QSO Redshift Survey (2QZ) data to g ∼ 20.2. However, at the faint end, the 2SLAQ number counts and luminosity functions are steeper (i.e. require more faint quasars) than the final 2QZ results from Croom et al., but are consistent with the preliminary 2QZ results from Boyle et al. Using the functional form adopted for the 2QZ analysis (a double power law with pure luminosity evolution characterized by a second-order polynomial in redshift), we find a faint-end slope of ß= -1.78 ± 0.03 if we allow all of the parameters to vary, and = -1.45 ± 0.03 if we allow only the faint-end slope and normalization to vary (holding all other parameters equal to the final 2QZ values). Over the magnitude range covered by the 2SLAQ survey, our maximum-likelihood fit to the data yields 32 per cent more quasars than the final 2QZ parametrization, but is not inconsistent with other g > 21 deep surveys for quasars. The 2SLAQ data exhibit no well-defined 'break' in the number counts or luminosity function, but do clearly flatten with increasing magnitude. Finally, we find that the shape of the quasar luminosity function derived from 2SLAQ is in good agreement with that derived from Type I quasars found in hard X-ray surveys. © 2005 RAS,.
• Schneider, D. P., Hall, P. B., Richards, G. T., E., D., Anderson, S. F., Fan, X., Jester, S., Stoughton, C., Strauss, M. A., SubbaRao, M., Brandt, W. N., Gunn, J. E., Yanny, B., Bahcall, N. A., Barentine, J. C., Blanton, M. R., Boroski, W. N., Brewington, H. J., Brinkmann, J., , Brunner, R., et al. (2005). The sloan digital sky survey quasar catalog. III. Third data release. Astronomical Journal, 130(2), 367-380.
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  Abstract: We present the third edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog consists of the 46,420 objects in the SDSS Third Data Release that have luminosities larger than M i = -22 (in a cosmology with H 0 = 70 km s -1 Mpc -1, Ω M = 0.3, and Ω Λ = 0.7), have at least one emission line with FWHM larger than 1000 km s -1 or are unambiguously broad absorption line quasars, are fainter than i = 15.0, and have highly reliable redshifts. The area covered by the catalog is ≈4188 deg 2. The quasar redshifts range from 0.08 to 5.41, with a median value of 1.47; the high-redshift sample includes 520 quasars at redshifts greater than 4, of which 17 are at redshifts greater than 5. For each object the catalog presents positions accurate to better than 0″.2 rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3800-9200 Å at a spectral resolution of ≃2000; the spectra can be retrieved from the public database using the information provided in the catalog. A total of 44,221 objects in the catalog were discovered by the SDSS; 28,400 of the SDSS discoveries are reported here for the first time. © 2005. The American Astronomical Society. All rights reserved.
• Shemmer, O., Brandt, W. N., Vignali, C., Schneider, D. P., Fan, X., Richards, G. T., & Strauss, M. A. (2005). The X-ray spectral properties and variability of luminous high-redshift active galactic nuclei. Astrophysical Journal Letters, 630(2 I), 729-739.
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  Abstract: We perform a detailed investigation of moderate- to high-quality X-ray spectra of 10 of the most luminous active galactic nuclei (AGNs) known at z > 4 (up to z ∼ 6.28). This study includes five new XMM-Newton observations and five archived X-ray observations (four by XMM-Newton and one by Chandrd). We find that the X-ray power-law photon indices of our sample, composed of eight radio-quiet sources and two that are moderately radio-loud, are not significantly different from those of lower redshift AGNs. The upper limits obtained on intrinsic neutral hydrogen column densities, NH ≲ 1022 - 1023 cm-2, indicate that these AGNs are not significantly absorbed. A joint fit performed on our eight radio-quiet sources, with a total of ∼ 7000 photons, constrains the mean photon index of z > 4 radio-quiet AGNs to F = 1.97-0.04+0.06, with no detectable intrinsic dispersion from source to source. We also obtain a strong constraint on the mean intrinsic column density, NH ≲ 10 22 - 1021 cm-2, showing that optically selected radio-quiet AGNs at z > 4 are, on average, not more absorbed than their lower redshift counterparts. All this suggests that the X-ray production mechanism and the central environment in radio-quiet AGNs have not significantly evolved over cosmic time. The mean equivalent width of a putative neutral narrow Fe Kα line is constrained to be ≲ 190 eV, and similarly we place constraints on the mean Compton reflection component (R ≲ 1.2). None of the AGNs varied on short (∼ 1 hr) timescales, but on longer timescales (months to years) strong variability is observed in five of the sources. In particular, the X-ray flux of the z = 5.41 radio-quiet AGN SDSS 0231-0728 dropped by a factor of ∼4 over a rest-frame period of 73 days. This is the most extreme X-ray variation observed in a luminous z > 4 radioquiet AGN. © 2005. The American Astronomical Society. All rights reserved.
• White, R. L., Becker, R. H., Fan, X., & Strauss, M. A. (2005). Hubble space telescope advanced camera for surveys observations of the Z = 6.42 quasar sdss J1148+5251: A leak in the gunn-peterson trough. Astronomical Journal, 129(5), 2102-2107.
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  Abstract: The Hubble Space Telescope Advanced Camera for Surveys has been used to obtain a narrowband image of the weak emission peak seen at λ = 7205 Å in the Gunn-Peterson Lyβ absorption trough of the highest redshift quasar, SDSS J1148+5251. The emission looks perfectly pointlike; there is no evidence for the intervening galaxy that we previously suggested might be contaminating the quasar spectrum. We derive a more accurate astrometric position for the quasar in the two filters and see no indication of gravitational lensing. We conclude that the light in the Lyβ trough is leaking through two unusually transparent, overlapping windows in the intergalactic medium (IGM) absorption, one in the Lyβ forest at z ∼ 6 and one in the Lyα forest at z ∼ 5. If there are significant optical depth variations on velocity scales small compared with our spectral resolution (∼ 150 km s -1), the Lyα trough has the potential to become more transparent for a given Lyβ optical depth because the Lyα transmission is extremely sensitive to the most transparent sight lines in the IGM while Lyβ is sensitive to sight lines with a broader range of optical depths. Such variations can only strengthen our conclusion that the fraction of neutral hydrogen in the IGM increases dramatically at z > 6. We argue that the transmission in the Lyβ trough is not only a more sensitive measure of the neutral fraction than is Lyα but that it also provides a less biased estimator of the neutral hydrogen fraction than does the Lyα transmission. © 2005. The American Astronomical Society. All rights reserved.
• Yahata, K., Suto, Y., Kayo, I., Matsubara, T., Connolly, A. J., Berk, D. V., Sheth, R., Szapudi, I., Anderson, S. F., Bahcall, N., Brinkmann, J., Csabai, I., Fan, X., Loveday, J., Szalay, A. S., & York, D. (2005). Large-scale clustering of sloan digital sky survey quasars: Impact of the baryon density and the cosmological constant. Publications of the Astronomical Society of Japan, 57(4), 529-540.
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  Abstract: We report on the first result from the clustering analysis of SDSS quasars. We computed the two-point correlation function (2PCF) of SDSS quasars in redshift space at 8 h -1 Mpc < s < 500 h -1 Mpc, with particular attention to its baryonic signature. Our sample consisted of 19986 quasars extracted from the SDSS Data Release 4. The redshift range of the sample is 0.72 ≤ z ≤ 2.24 (the mean redshift is z̄ = 1.46) and the reddening-corrected i-band apparent magnitude range is 15.0 ≤ m i.rc ≤ 19.1. Due to the relatively low number density of the quasar sample, the bump in the power spectrum due to the baryon density, Ω b, is not clearly visible. The effect of the baryon density is, however, to distort the overall shape of the 2PCF. The degree of distortion makes it an interesting alternate measure of the baryonic signature. Assuming a scale-independent linear bias and a spatially flat universe, we combined the observed quasar 2PCF and the predicted matter 2PCF to put constraints on Ω b and Ω Λ (the cosmological constant). Our result was fitted as 0.80 -2.8Ω b < Ω Λ < 0.90 - 1.4Ω b at the 2σ confidence level. The "mean" bias parameter of our quasar sample is 1.59 σ 8-1 (for Ω b = 0.04 and Ω Λ = 0.7), where σ 8 is the top-hat mass fluctuation amplitude at 8 h -1 Mpc. We also estimated the corresponding bias parameter of quasars at z = 0, b QSO.Fry(0), assuming Fry's bias evolution model. For Ω b = 0.04, Ω Λ = 0.73, and Ω d = 0.23, we found b QSO.Fry(0) = 0.54 + 0.83 σ 8-1 which is valid for 0.6 < σ 8 < 1.0. © 2005. Astronomical Society of Japan.
• Abazajian, K., Adelman-McCarthy, J. K., Agüeros, M. A., Allam, S. S., J., S., Anderson, S. F., Annis, J., Bahcall, N. A., Baldry, I. K., Bastian, S., Berlind, A., Bernardi, M., Blanton, M. R., Bochanski Jr., J. J., Boroski, W. N., Briggs, J. W., Brinkmann, J., Brunner, R. J., Budavári, T., , Carey, L. N., et al. (2004). The second data release of the sloan digital sky survey. Astronomical Journal, 128(1 1783), 502-512.
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  Abstract: The Sloan Digital Sky Survey (SDSS) has validated and made publicly available its Second Data Release. This data release consists of 3324 deg 2 of five-band (ugriz) imaging data with photometry for over 88 million unique objects, 367,360 spectra of galaxies, quasars, stars, and calibrating blank sky patches selected over 2627 deg2 of this area, and tables of measured parameters from these data. The imaging data reach a depth of r ≈ 22.2 (95% completeness limit for point sources) and are photometrically and astrometrically calibrated to 2% rms and 100 mas rms per coordinate, respectively. The imaging data have all been processed through a new version of the SDSS imaging pipeline, in which the most important improvement since the last data release is fixing an error in the model fits to each object. The result is that model magnitudes are now a good proxy for point-spread function magnitudes for point sources, and Petrosian magnitudes for extended sources. The spectroscopy extends from 3800 to 9200 Å at a resolution of 2000. The spectroscopic software now repairs a systematic error in the radial velocities of certain types of stars and has substantially improved spectrophotometry. All data included in the SDSS Early Data Release and First Data Release are reprocessed with the improved pipelines and included in the Second Data Release. Further characteristics of the data are described, as are the data products themselves and the tools for accessing them.
• Blanc, G., Afonso, C., Alard, C., Albert, J. N., Aldering, G., Amadon, A., Andersen, J., Ansari, R., Aubourg, É., Balland, C., Bareyre, P., Beaulieu, J. P., Charlot, X., Conley, A., Coutures, C., Dahlén, T., Derue, F., Fan, X., Ferlet, R., , Folatelli, G., et al. (2004). Type la supernova rate at a redshift of ∼0.1. Astronomy and Astrophysics, 423(3), 881-894.
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  Abstract: We present the type la rate measurement based on two EROS supernova search campaigns (in 1999 and 2000). Sixteen supernovae identified as type la were discovered. The measurement of the detection efficiency, using a Monte Carlo simulation, provides the type la supernova explosion rate at a redshift ∼0.13. The result is 0.125 -0.034-0.028+0.044+0.028 h 702 SNu where 1 SNu= 1 SN / 10 10 L ⊙B / century. This value is compatible with the previous EROS measurement (Hardin et al. 2000), done with a much smaller sample, at a similar redshift. Comparison with other values at different redshifts suggests an evolution of the type la supernova rate.
• Brandt, W. N., Vignali, C., Schneider, D. P., Alexander, D. M., Anderson, S. F., Bauer, F. E., Fan, X., Garmire, G. P., Kaspi, S., & Richards, G. T. (2004). X-rays from the first massive black holes. Advances in Space Research, 34(12 SPEC.ISS.), 2478-2485.
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  Abstract: X-ray studies of high-redshift (z > 4) active galaxies have advanced substantially over the past few years, largely due to results from the new generation of X-ray observatories. As of this writing X-ray emission has been detected from nearly 60 high-redshift active galaxies. This paper reviews the observational results and their implications for models of the first massive black holes, and it discusses future prospects for the field. © 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.
• Carilli, C. L., Walter, F., Bertoldi, F., Menten, K. M., Fan, X., Lewis, G. F., Strauss, M. A., Cox, P., Beelen, A., Omont, A., & Mohan, N. (2004). Radio continuum imaging of far-infrared-luminous QSOs at z > 6. Astronomical Journal, 128(3 1785), 997-1001.
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  Abstract: We present sensitive imaging at 1.4 GHz of the two highest redshift far-infrared (FIR) luminous QSOs, SDSS J114816.65+525150.2 (z = 6.42) and SDSS J104845.05+463718.3 (z = 6.2). Radio continuum emission is detected from J1148+5251 with S1.4 = 55 ± 12 μJy, while J1048+4637 is marginally detected with S1.4 = 26 ± 12 μJy. Comparison of the radio and FIR luminosities shows that both sources follow the radio-FIR correlation for star-forming galaxies, with implied (massive) star formation rates ∼103 M⊙ yr-1, although we cannot rule out as much as 50% of the FIR luminosity being powered by the active galactic nucleus. Five bright (>22 mJy) radio sources are detected within 8′ of J1148+5251. This is a factor of 30 more than expected for a random field. Two sources have SDSS redshifts, including a z = 1.633 radio-loud quasar and a z = 0.05 radio galaxy. However, we do not find evidence for a galaxy cluster in the SDSS data, at least out to z = 0.2. Considering the faint SDSS magnitudes of the remaining radio sources, we conclude that the overdensity of radio sources could either be a statistical fluke or a very large scale structure (>8 Mpc comoving) at z ≥ 1. We also consider the possibility of gravitational lensing by the closest (in angle) bright galaxy in the SDSS data at z = 0.05 and conclude that the galaxy provides negligible magnification.
• Fan, X., Hennawi, J. F., Richards, G. T., Strauss, M. A., Schneider, D. P., Donley, J. L., Young, J. E., Annis, J., Lin, H., Lampeitl, H., Lupton, R. H., Gunn, J. E., Knapp, G. R., Brandt, W. N., Anderson, S., Bahcall, N. A., Brinkmann, J., Brunner, R. J., Fukugita, M., , Szalay, A. S., et al. (2004). A survey of z > 5.7 quasars in the sloan digital sky survey. III. Discovery of five additional quasars. Astronomical Journal, 128(2 1784), 515-522.
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  Abstract: We present the discovery of five new quasars at z > 5.7, selected from the multicolor imaging data of the Sloan Digital Sky Survey (SDSS). Three of them, at redshifts 5.93, 6.07, and 6.22, were selected from ∼1700 deg 2 of new SDSS Main Survey imaging in the northern Galactic cap. An additional quasar, at redshift 5.85, was discovered by co-adding the data obtained in the Fall Equatorial Stripe in the SDSS Southern Survey Region. The fifth object, at redshift 5.80, is selected from a nonstandard SDSS scan in the southern Galactic cap outside the Main Survey area. The spectrum of SDSS J162331.81+311200.5 (z = 6.22) shows a complete Gunn-Peterson trough at z abs > 5.95, similar to the troughs detected in the other three z ≳ 6.2 quasars known. We present a composite spectrum of the z > 5.7 quasars discovered in the SDSS to date. The average emission-line and continuum properties of z ∼ 6 quasars exhibit no significant evolution compared with those at low redshift. Using a complete sample of nine z > 5.7 quasars, we find that the density of quasars with M 1450 < -26.7 at z ∼ 6 is (6 ± 2) × 10 -10 Mpc -3 (H 0 = 65 km s -1 Mpc -1, Ω = 0.35, and Γ = 0.65), consistent with our previous estimates. The luminosity distribution of the sample is fitted with a power-law luminosity function ψ(L) ∝ L -3.2±0.7, somewhat steeper than but consistent with our previous estimates.
• Finkbeiner, D. P., Padmanabhan, N., Schlegel, D. J., Carr, M. A., Gunn, J. E., Rockosi, C. M., Sekiguchi, M., Lupton, R. H., Knapp, G. R., Ivezić, Ž., Blanton, M. R., Hogg, D. W., Adelman-McCarthy, J. K., Annis, J., Hayes, J., Kinney, E., Long, D. C., Seljak, U., Strauss, M. A., , Yanny, B., et al. (2004). Sloan digital sky survey imaging of low galactic latitude fields: Technical summary and data release. Astronomical Journal, 128(5), 2577-2592.
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  Abstract: The Sloan Digital Sky Survey (SDSS) mosaic camera and telescope have obtained five-band optical-wavelength imaging near the Galactic plane outside of the nominal survey boundaries. These additional data were obtained during commissioning and subsequent testing of the SDSS observing system, and they provide unique wide-area imaging data in regions of high obscuration and star formation, including numerous young stellar objects, Herbig-Haro objects, and young star clusters. Because these data are outside the survey regions in the Galactic caps, they are not part of the standard SDSS data releases. This paper presents imaging data for 832 square degrees of sky (including repeats), in the star-forming regions of Orion, Taurus, and Cygnus. About 470 deg 2 are now released to the public, with the remainder to follow at the time of SDSS Data Release 4. The public data in Orion include the star-forming region NGC 2068/NGC 2071/HH 24 and a large part of Barnard's loop.
• Golimowski, D. A., Leggett, S. K., Marley, M. S., Fan, X., Geballe, T. R., Knapp, G. R., Vrba, F. J., Henden, A. A., Luginbuhl, C. B., Guetter, H. H., Munn, J. A., Canzian, B., Zheng, W., Tsvetanov, Z. I., Chiu, K., Glazebrook, K., Hoversten, E. A., Schneider, D. P., & Brinkmann, J. (2004). L′ and M′ photometry of ultracool dwarfs. Astronomical Journal, 127(6 1782), 3516-3536.
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  Abstract: We have compiled L′ (3.4-4.1 μm) and M′ (4.6-4.8 μm) photometry of 63 single and binary M, L, and T dwarfs obtained at the United Kingdom Infrared Telescope using the Mauna Kea Observatory filter set. This compilation includes new L′ measurements of eight L dwarfs and 13 T dwarfs and new M′ measurements of seven L dwarfs, five T dwarfs, and the M1 dwarf G1 229A. These new data increase by factors of 0.6 and 1.6, respectively, the numbers of ultracool dwarfs (T eff ≲ 2400 K) for which L′ and M′ measurements have been reported. We compute L bol, BC K, and T eff for 42 dwarfs whose flux-calibrated JHK spectra, L′ photometry, and trigonometric parallaxes are available, and we estimate these quantities for nine other dwarfs whose parallaxes and flux-calibrated spectra have been obtained. BC K is a well-behaved function of near-infrared spectral type with a dispersion of ∼0.1 mag for types M6-T5; it is significantly more scattered for types T5-T9. T eff declines steeply and monotonically for types M6-L7 and T4-T9, but it is nearly constant at ∼1450 K for types L7-T4 with assumed ages of ∼3 Gyr. This constant T eff is evidenced by nearly unchanging values of L′-M′ between types L6 and T3. It also supports recent models that attribute the changing near-infrared luminosities and spectral features across the L-T transition to the rapid migration, disruption, and/or thinning of condensate clouds over a narrow range of J eff. The L′ and M′ luminosities of early-T dwarfs do not exhibit the pronounced humps or inflections previously noted in the I through K bands, but insufficient data exist for types L6-T5 to assert that M L′ and M M′ are strictly monotonic within this range of types. We compare the observed K, L′, and M′ luminosities of L and T dwarfs in our sample with those predicted by precipitating-cloud and cloud-free models for varying surface gravities and sedimentation efficiencies. The models indicate that the L3-T4.5 dwarfs generally have higher gravities (log g = 5.0-5.5) than the T6-T9 dwarfs (log g = 4.5-5.0). The predicted M′ luminosities of late-T dwarfs are 1.5-2.5 times larger than those derived empirically for the late-T dwarfs in our sample. This discrepancy is attributed to absorption at 4.5-4.9 μm by CO, which is not expected under the condition of thermochemical equilibrium assumed in the models. Our photometry and bolometric calculations indicate that the L3 dwarf Kelu-1 and the T0 dwarf SDSS J042348.57-041403.5 are probable binary systems. We compute log (L bol/L ⊙) = -5.73 ± 0.05 and T eff= 600-750 K for the T9 dwarf 2MASSI J0415195-093506, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.
• Knapp, G. R., Leggett, S. K., Fan, X., Marley, M. S., Geballe, T. R., Golimowski, D. A., Finkbeiner, D., Gunn, J. E., Hennawi, J., Ivezić, Z., Lupton, R. H., Schlegel, D. J., Strauss, M. A., Tsvetanov, Z. I., Chiu, K., Hoversten, E. A., Glazebrook, K., Zheng, W., Hendrickson, M., , Williams, C. C., et al. (2004). Near-infrared photometry and spectroscopy of L and T dwarfs: The effects of temperature, clouds, and gravity. Astronomical Journal, 127(6 1782), 3553-3578.
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  Abstract: We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs. Photometry has been obtained for 71 dwarfs, and spectroscopy for 56. The sample comprises newly identified very red objects from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four previously defined indices from Geballe et al. that measure the strengths of the near infrared water and methane bands. We identify nine new L8-9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest yet found by SDSS, the T7 dwarf SDSS J175805.46+463311.9. We classify 2MASS J04151954-0935066 as T9, the latest and coolest dwarf found to date. We combine the new results with our previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs with imaging data on a single photometric system and with uniform spectroscopic classification. We compare the near-infrared colors and absolute magnitudes of brown dwarfs near the L-T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is some scatter in the Geballe et al. spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere and are affected by the location of the condensate cloud layer. The near-infrared colors of the L dwarfs also show scatter within a given spectral type, which is likely due to variations in the altitudes, spatial distributions, and thicknesses of the clouds. We have identified a small group of late-L dwarfs that are relatively blue for their spectral type and that have enhanced FeH, H 2O, and K I absorption, possibly due to an unusually small amount of condensates. The scatter seen in the H-K color for late-T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H 2 opacity. The correlation of H-K color with gravity is supported by the observed strengths of the J-band K I doublet. Gravity is closely related to mass for field T dwarfs with ages greater than 10 8 yr and the gravities implied by the H-K colors indicate that the T dwarfs in our sample have masses in the range 15-75M Juptier. One of the SDSS dwarfs, SDSS J111010.01+011613.1, is possibly a very low mass object, with log g ∼ 4.2-4.5 and mass ∼ 10-15M Juptier.
• Richards, G. T., Nichol, R. C., Gray, A. G., Brunner, R. J., Lupton, R. H., E., D., Chong, S. S., Weinstein, M. A., Schneider, D. P., Anderson, S. F., Munn, J. A., Harris, H. C., Strauss, M. A., Fan, X., Gunn, J. E., Ivezić, Ž., York, D. G., Brinkmann, J., & Moore, A. W. (2004). Efficient photometric selection of quasars from the sloan digital sky survey: 100,000 z < 3 quasars from data release one. Astrophysical Journal, Supplement Series, 155(2), 257-269.
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  Abstract: We present a catalog of 100,563 unresolved, UV-excess (UVX) quasar candidates to g = 21 from 2099 deg 2 of the Sloan Digital Sky Survey (SDSS) Data Release One (DR1) imaging data. Existing spectra of 22,737 sources reveals that 22,191 (97.6%) are quasars; accounting for the magnitude dependence of this efficiency, we estimate that 95,502 (95.0%) of the objects in the catalog are quasars. Such a high efficiency is unprecedented in broadband surveys of quasars. This "proof-of-concept" sample is designed to be maximally efficient, but still has 94.7% completeness to unresolved, g ≲ 19.5, UVX quasars from the DR1 quasar catalog. This efficient and complete selection is the result of our application of a probability density type analysis to training sets that describe the four-dimensional color distribution of stars and spectroscopically confirmed quasars in the SDSS. Specifically, we use a nonparametric Bayesian classification, based on kernel density estimation, to parameterize the color distribution of astronomical sources-allowing for fast and robust classification. We further supplement the catalog by providing photometric redshifts and matches to FIRST/VLA, ROSAT, and USNO-B sources. Future work needed to extend this selection algorithm to larger redshifts, fainter magnitudes, and resolved sources is discussed. Finally, we examine some science applications of the catalog, particularly a tentative quasar number counts distribution covering the largest range in magnitude (14.2 < g < 21.0) ever made within the Jjamework of a single quasar survey.
• Richards, G. T., Strauss, M. A., Pindor, B., Haiman, Z., Fan, X., Eisenstein, D., Schneider, D. P., Bahcall, N. A., Brinkmann, J., & Brunner, R. (2004). A snapshot survey for gravitational lenses among z ≥ 4.0 quasars. I. The z > 5.7 sample. Astronomical Journal, 127(3 1779), 1305-1312.
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  Abstract: Over the last few years, the Sloan Digital Sky Survey (SDSS) has discovered several hundred quasars with redshift between 4.0 and 6.4. Including the effects of magnification bias, one expects a priori that an appreciable fraction of these objects are gravitationally lensed. We have used the Advanced Camera for Surveys on the Hubble Space Telescope to carry out a snapshot imaging survey of high-redshift SDSS quasars to search for gravitationally split lenses. This paper, the first in a series reporting the results of the survey, describes snapshot observations of four quasars at z = 5.74, 5.82, 5.99, and 6.30, respectively. We find that none of these objects has a lensed companion within 5 mag with a separation larger than 0″3; within 2.5 mag we can rule out companions within 0″.1. Based on the nondetection of strong lensing in these four systems, we constrain the z ~ 6 luminosity function to a slope of β > -4.63 (3 σ), assuming a break in the quasar luminosity function at M* 1450 = -24.1. We discuss the implications of this constraint on the ionizing background due to quasars in the early universe. Given that these quasars are not highly magnified, estimates of the masses of their central engines by the Eddington argument must be taken seriously, possibly challenging models of black hole formation.
• Smolčić, V., Ivezić, Ž., Knapp, G. R., Lupton, R. H., Pavlovski, K., Ilijić, S., Schlegel, D., Smith, J., McGehee, P. M., Silvestri, N. M., Hawley, S. L., Rockosi, C., Gunn, J. E., Strauss, M. A., Fan, X., Eisenstein, D., & Harris, H. (2004). A second stellar color locus: A bridge from white dwarfs to M stars. Astrophysical Journal Letters, 615(2 II), L141-L144.
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  Abstract: We report the discovery of a locus of binary stars in the Sloan Digital Sky Survey (SDSS) g-r versus u-g color-color diagram that connects the colors of white dwarfs and M dwarfs. While its contrast with respect to the main stellar locus is only ∼1 : 2300, this previously unrecognized feature includes 863 stars from the SDSS Data Release 1 (DR1). The position and shape of the feature are in good agreement with predictions of a simple binary star model that consists of a white dwarf and an M dwarf, with the components' luminosity ratio controlling the position along this binary system locus. SDSS DR1 spectra for 47 of these objects strongly support this model. The absolute magnitude-color distribution inferred for the white dwarf component is in good agreement with the models of Bergeron et al.
• Vrba, F. J., Henden, A. A., Luginbuhl, C. B., Guetter, H. H., Munn, J. A., Canzian, B., Burgasser, A. J., Kirkpatrick, J. D., Fan, X., Geballe, T. R., Golimowski, D. A., Knapp, G. R., Leggett, S. K., Schneider, D. P., & Brinkmann, J. (2004). Preliminary parallaxes of 40 L and T dwarfs from the US naval observatory infrared astrometry program. Astronomical Journal, 127(5 1781), 2948-2968.
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  Abstract: We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager on the US Naval Observatory (USNO) 1.55 m Strand Astrometric Reflector. The results presented here are based on observations obtained between 2000 September and 2002 November; about half of the objects have an observational time baseline of Δt = 1.3 yr and half Δt = 2.0 yr. Despite these short time baselines, the astrometric quality is sufficient to produce significant new results, especially for the nearer T dwarfs. Seven objects are in common with the USNO optical CCD parallax program for quality control and seven in common with the European Southern Observatory 3.5m New Technology Telescope parallax program. We compare astrometric quality with both of these programs. Relative to absolute parallax corrections are made by employing Two Micron All Sky Survey and/or Sloan Digital Sky Survey photometry for reference-frame stars. We combine USNO infrared and optical parallaxes with the best available California Institute of Technology (CIT) system photometry to determine M J, M H, and M K values for 37 L dwarfs between spectral types L0 and L8 and 19 T dwarfs between spectral types T0.5 and T8 and present selected absolute magnitude versus spectral type and color diagrams, based on these results. Luminosities and temperatures are estimated for these objects. Of special interest are the distances of several objects that are at or near the L-T dwarf boundary so that this important transition can be better understood. The previously reported early to mid T dwarf luminosity excess is clearly confirmed and found to be present at J, H, and K. The large number of objects that populate this luminosity-excess region indicate that it cannot be due entirely to selection effects. The T dwarf sequence is extended to M j ≈ 16.9 by 2MASS J041519-0935, which, at d = 5.74 pc, is found to be the least luminous [log(L/L ⊙) = -5.58] and coldest (T eff ≈ 760 K) brown dwarf known. Combining results from this paper with earlier USNO CCD results we find that, in contrast to the L dwarfs, there are no examples of lowvelocity (V tan < 20 km s -1) T dwarfs. This is consistent with the T dwarfs in this study being generally older than the L dwarfs. We briefly discuss future directions for the USNO infrared astrometry program.
• Walter, F., Carilli, C., Bertoldi, F., Menten, K., Cox, P., Lo, K. Y., Fan, X., & Strauss, M. A. (2004). Resolved molecular gas in a quasar host galaxy at redshift z = 6.42. Astrophysical Journal Letters, 615(1 II), L17-L20.
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  Abstract: We present high-resolution Very Large Array (VLA) observations of the molecular gas in the host galaxy of the highest redshift quasar currently known, SDSS J1148+5251 (z = 6.42). Our VLA data of the CO (3-2) emission have a maximum resolution of 0Prime;.17 × 0″.13 (≤1 kpc) and enable us to resolve the molecular gas emission both spatially and in terms of velocity. The molecular gas in J1148+5251 is extended to a radius of 2.5 kpc, and the central region shows two peaks separated by 0″.3 (1.7 kpc). These peaks account for about half of the total emission, while the remainder is more extended. Each of these unresolved peaks contains a molecular gas mass of ∼5 × 109 M⊙ (similar to the total mass found in nearby ultraluminous infrared galaxies) and has an intrinsic brightness temperature of ∼35 K (averaged over the 1 kpc-sized beam), comparable to what is found in nearby starburst centers. Assuming that the molecular gas is gravitationally bound, we estimate a dynamical mass of ∼4.5 × 1010 M⊙ within a radius of 2.5 kpc (∼5.5 × 1010 M⊙ if corrected for a derived inclination of i ∼ 65°). This dynamical mass estimate leaves little room for matter other than the detected molecular gas; in particular, the data are inconsistent with a ∼1012 M⊙ stellar bulge that would be predicted based on the MBH-bulgeσ relation. This finding may indicate that black holes form prior to the assembly of the stellar bulges and that the dark matter halos are less massive than those predicted on the basis of the black hole/bulge mass relationship.
• Abazajian, K., Adelman-McCarthy, J. K., Agüeros, M. A., Allam, S. S., Anderson, S. F., Annis, J., Bahcall, N. A., Baldry, I. K., Bastian, S., Berlind, A., Bernardi, M., Blanton, M. R., Blythe, N., Bochanski Jr., J. J., Boroski, W. N., Brewington, H., Briggs, J. W., Brinkmann, J., Brunner, R. J., , Budavári, T., et al. (2003). The first data release of the sloan digital sky survey. Astronomical Journal, 126(4 1774), 2081-2086.
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  Abstract: The Sloan Digital Sky Survey (SDSS) has validated and made publicly available its First Data Release. This consists of 2099 deg2 of five-band (u, g, r, i, z) imaging data, 186,240 spectra of galaxies, quasars, stars and calibrating blank sky patches selected over 1360 deg2 of this area, and tables of measured parameters from these data. The imaging data go to a depth of r ≈ 22.6 and are photometrically and astrometrically calibrated to 2% rms and 100 mas rms per coordinate, respectively. The spectra cover the range 3800-9200 Å, with a resolution of 1800-2100. This paper describes the characteristics of the data with emphasis on improvements since the release of commissioning data (the SDSS Early Data Release) and serves as a pointer to extensive published and on-line documentation of the survey.
• Anderson, S. F., Voges, W., Margon, B., Trümper, J., Agüeros, M. A., Boller, T., Collinge, M. J., Homer, L., Stinson, G., Strauss, M. A., Annis, J., Gómez, P., Hall, P. B., Nichol, R. C., Richards, G. T., Schneider, D. P., E., D., Fan, X., Ivezić, Ž., , Munn, J. A., et al. (2003). A large, uniform sample of X-ray-emitting AGNs: Selection approach and an initial catalog from the ROSAT all-sky and sloan digital sky surveys. Astronomical Journal, 126(5 1775), 2209-2229.
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  Abstract: Many open questions in X-ray astronomy are limited by the relatively small number of objects in uniform optically identified and observed samples, especially when rare subclasses are considered or when subsets are isolated to search for evolution or correlations between wavebands. We describe the initial results of a new program aimed to ultimately yield ∼10 4 fully characterized X-ray source identifications - a sample about an order of magnitude larger than earlier efforts. The technique is detailed and employs X-ray data from the ROSAT All-Sky Survey (RASS) and optical imaging and spectroscopic follow-up from the Sloan Digital Sky Survey (SDSS); these two surveys prove to be serendipitously very well matched in sensitivity. As part of the SDSS software pipelines, optical objects in the SDSS photometric catalogs are automatically positionally cross-correlated with RASS X-ray sources. Then priorities for follow-on SDSS optical spectra of candidate counterparts are automatically assigned using an algorithm based on the known ratios of f x/f opt for various classes of X-ray emitters at typical RASS fluxes of ∼10 -13 ergs cm -2 s -1. SDSS photometric parameters for optical morphology, magnitude, and colors, plus FIRST radio information, serve as proxies for object class. Initial application of this approach to RASS/SDSS data from 1400 deg 2 of sky provides a catalog of more than 1200 spectroscopically confirmed quasars and other AGNs that are probable RASS identifications. Most of these are new identifications, and only a few percent of the AGN counterparts are likely to be random superpositions. The magnitude and redshift ranges of the counterparts are very broad, extending over 15 < m < 21 and 0.03 < z < 3.6, respectively. Although most identifications are quasars and Seyfert 1 galaxies, a variety of other AGN subclasses are also sampled. Substantial numbers of rare AGN types are found, including more than 130 narrow-line Seyfert 1 galaxies and 45 BL Lac candidates. These early results already provide a very sizable set of source identifications, demonstrate the utility of the sample in multiwaveband investigations, and show the capability of the joint RASS/SDSS approach to efficiently proceed toward the largest homogeneously selected/observed sample of X-ray-emitting quasars and other kinds of AGNs.
• Bertoldi, F., Carilli, C. L., Cox, P., Fan, X., Strauss, M. A., Beelen, A., Omont, A., & Zylka, R. (2003). Dust emission from the most distant quasars. Astronomy and Astrophysics, 406(3), L55-L58.
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  Abstract: We report observations of three SDSS z > 6 QSOs at 250 GHz (1.2 mm) using the 117-channel Max-Planck Millimeter Bolometer (MAMBO-2) array at the IRAM 30-meter telescope. J1148+5251 (z = 6.42) and J1048+4637 (z = 6.23) were detected with 250 GHz flux densities of 5.0 ± 0.6 mJy and 3.0 ± 0.4 mJy, respectively. J1630+4012 (z = 6.05) was not detected with a 3 σ upper limit of 1.8 mJy. Upper flux density limits from VLA observations at 43 GHz for J1148+5251 and J1048+4637 imply steeply rising spectra, indicative of thermal infrared emission from warm dust. The far-infrared luminosities are estimated to be ≈ 1013 L⊙, and the dust masses ≈ 108 M⊙, assuming Galactic dust properties. The presence of large amounts of dust in the highest redshift QSOs indicates that dust formation must be rapid during the early evolution of QSO host galaxies. Dust absorption may hinder the escape of ionizing photons which reionize the intergalactic medium at this early epoch.
• Bertoldi, F., Cox, P., Neri, R., Carilli, C. L., Walter, F., Omont, A., Beelen, A., Henkel, C., Fan, X., Strauss, M. A., & Menten, K. M. (2003). High-excitation CO in a quasar host galaxy at z = 6.42. Astronomy and Astrophysics, 409(3), L47-L50.
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  Abstract: We report the detection of high excitation CO emission from the most distant quasar currently known, SDSS J114816.64+525150.3 (hereafter J1148+5251), at a redshift z = 6.419. The CO (J = 6 → 5) and (J = 7 → 6) lines were detected using the IRAM Plateau de Bure interferometer, showing a width of ≈280 km s-1. An upper flux limit for the CO (J = 1 → 0) line was obtained from observations with the Effelsberg 100-meter telescope. Assuming no gravitational magnification, we estimate a molecular gas mass of ≈2 × 1010 M⊙. Using the CO (3 → 2) observations by Walter et al. (2003), a comparison of the line flux ratios with predictions from a large velocity gradient model suggests that the gas is likely of high excitation, at densities ∼104.5 cm-3 and a temperature ∼100 K. Since in this case the CO lines appear to have moderate optical depths, the gas must be extended over a few kpc. The gas mass detected in J1148+5251 can fuel star formation at the rate implied by the far-infrared luminosity for less than 10 million years, a time comparable to the dynamical time scale of the region. The gas must therefore be replenished quickly, and metal and dust enrichment must occur fast. The strong dust emission and the massive, dense gas reservoir at z ∼ 6.4 provide further evidence that vigorous star formation is co-eval with the rapid growth of massive black holes at these early epochs of the Universe.
• Chiu, W. A., Fan, X., & Ostriker, J. P. (2003). Combining wilkinson microwave anisotropy probe and sloan digital sky survey quasar data on reionization constrains cosmological parameters and star formation efficiency. Astrophysical Journal Letters, 599(2 I), 759-772.
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  Abstract: We present constraints on cosmological and star formation parameters based on combining observations from the Wilkinson Microwave Anisotropy Probe (WMAP) and high-redshift quasars from the Sloan Digital Sky Survey (SDSS). We use a semianalytic model for reionization that takes into account a number of important physical processes both within collapsing halos (e.g., H2 cooling) and in the intergalactic medium (e.g., H2 cooling, Compton cooling, and photoionization heating). We find that the Gunn-Peterson absorption data provide tight constraints on the power spectrum at small scales in a manner analogous to that derived from the cluster mass function. Assuming that the efficiency of producing UV photons per baryon is constant, the constraint takes on the form σ8ω00.5 ≈ 0.33 in a flat, λ-dominated universe with h = 0.72, n = 0.99, and ωbh2 = 0.024. However, the calculated optical depth to electron scattering of τes ∼ 0.06 is well below the value found by WMAP of 0.17 ± (0.04 ∼ 0.07). Since the WMAP constraints on τes are somewhat degenerate with the value of the spectral index n, we then permit the primordial spectral index n to float and consider the 1 σ WMAP-only determination of ω 0h2 = 0.14 ± 0.02 (implying ω0 = 0.27 ± 0.04), while normalizing the power spectrum using WMAP. In addition, we allow the UV efficiency to be greater in the past. Combining the WMAP constraints with the quasar transmission data, our analysis then favors a model with τes = 0.11-0.03+0.02 (ω0/0.27)-1, n = 0.96-0.03+0.02(ω0/0.27)-0.57, implying a WMAP normalization of σ8 = 0.83-0.05+0.03(ω0/0.27)0.53 (all at 95% confidence) and an effective UV efficiency that was at least ∼10 times greater at z ≫ 6. The implied UV efficiency is not unreasonable for stars, spanning the range from 10-5.5 to 10-4. These results indicate that the quasar and WMAP observations are consistent. If future observations confirm an optical depth to electron scattering τes ∼ 0.1, then it would appear that no more "exotic" sources of UV photons, such as miniquasars or active galactic nuclei, are necessary. However, unless one considers more radical sources of UV photons or alternative forms for the power spectrum of density fluctuations, one cannot achieve a value of τes ≳ 0.17 without violating some combination of constraints from quasar transmission data from z = 4 to 6 and WMAP measurements at large scales.
• Fan, X. (2003). Astronomy: Black holes at the cosmic dawn. Science, 300(5620), 752-753.
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  PMID: 12730588;
• Fan, X., & Fan, X. -. (2003). Astronomy. Black holes at the cosmic dawn. Science (New York, N.Y.), 300(5620).
• Fan, X., Strauss, M. A., Schneider, D. P., Becker, R. H., White, R. L., Haiman, Z., Gregg, M., Pentericci, L., Grebel, E. K., Narayanan, V. K., Loh, Y., Richards, G. T., Gunn, J. E., Lupton, R. H., Knapp, G. R., Ivezić, Ž., Brandt, W. N., Collinge, M., Hao, L., , Harbeck, D., et al. (2003). A survey of z > 5.7 quasars in the Sloan Digital Sky Survey. II - Discovery of three additional quasars at z > 6. Astronomical Journal, 125(4 1768), 1649-1659.
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  Abstract: We present the discovery of three new quasars at z > 6 in ∼ 1300 deg2 of Sloan Digital Sky Survey imaging data, J114816.64+525150.3 (z = 6.43), J104845.05+463718.3 (z = 6.23), and J163033.90+401209.6 (z = 6.05). The first two objects have weak Lyα emission lines; their redshifts are determined from the positions of the Lyman break. They are only accurate to ∼0.05 and could be affected by the presence of broad absorption line systems. The last object has a Lyα strength more typical of lower redshift quasars. Based on a sample of six quasars at z > 5.7 that cover 2870 deg2 presented in this paper and in Paper I, we estimate the comoving density of luminous quasars at z ∼ 6 and M1450 < -26.8 to be (8 ± 3) × 10-10 Mpc-3 (for H 0 = 50 km s-1 Mpc-1, Ω = 1). Hubble Space Telescope imaging of two z > 5.7 quasars and high-resolution, ground-based images (seeing ∼0″.4) of three additional z > 5.7 quasars show that none of them is gravitationally lensed. The luminosity distribution of the high-redshift quasar sample suggests the bright-end slope of the quasar luminosity function at z ∼ 6 is shallower than ψ ∝ L-3.5 (2 σ), consistent with the absence of strongly lensed objects.
• Hall, P. B., Hutsemékers, D., Anderson, S. F., Brinkmann, J., Fan, X., Schneider, D. P., & York, D. G. (2003). VLT+UVES spectroscopy of the Ca II low-ionization broad absorption line quasar SDSS J030000.56+004828.0. Astrophysical Journal Letters, 593(1 I), 189-202.
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  Abstract: We study high-resolution spectra of the "overlapping-trough" low-ionization broad absorption line (LoBAL) quasar SDSS J030000.56+004828.0. The Ca II, Mg II, and Mg I column densities in this object are the largest reported to date for any BAL outflow. The broad Ca II absorption is mildly blended, but the blending can be disentangled to measure the Ca II column density, which is large enough that the outflow must include a strong hydrogen ionization front. The outflow begins at a blueshift of ∼1650 km s -1 from the systemic redshift. The lowest velocity BAL region produces strong Ca II absorption but does not produce significant excited Fe II absorption, while the higher velocity excited Fe II absorption region produces very little Ca II absorption. We have found that only a disk wind outflow can explain this segregation. Whether the outflow is smooth or clumpy, we conclude that the Ca II BAL region has a density high enough to populate excited levels of Fe II but a temperature low enough to prevent them from being significantly populated. This requirement means the Ca II BAL region has T ≲, 1100 K, and perhaps even T ≲ 550 K. This quasar also has an associated absorption line system (AAL) that exhibits partial covering and therefore is likely located near the central engine. Its association with the BAL outflow is unclear. Blending of the AAL with the BAL trough shows that the spatial region covered by the BAL outflow can vary over velocity differences of ∼1700 km s-1.
• Harris, H. C., Liebert, J., Kleinman, S. J., Nitta, A., Anderson, S. F., Knapp, G. R., Krzesiński, J., Schmidt, G., Strauss, M. A., Berk, D. V., Eisenstein, D., Hawley, S., Margon, B., Munn, J. A., Silvestri, N. M., Smith, J. A., Szkody, P., Collinge, M. J., Dahn, C. C., , Fan, X., et al. (2003). An initial survey of white dwarfs in the Sloan Digital Sky Survey. Astronomical Journal, 126(2 1772), 1023-1040.
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  Abstract: An initial assessment is made of white dwarf and hot subdwarf stars observed in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees), observed much like the full survey will be, 269 white dwarfs (WDs) and 56 hot subdwarfs are identified spectroscopically where only 44 white dwarfs and five hot subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and DB (helium) types. In addition, in the full survey to date, a number of WDs have been found with uncommon spectral types. Among these are blue DQ stars displaying lines of atomic carbon; red DQ stars showing molecular bands of C2 with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na, and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry alone allows identification of stars hotter than 12,000 K, and the density of these stars for 15 < g < 20 is found to be ∼2.2 deg-2 at Galactic latitudes of 29°-62°. Spectra are obtained for roughly half of these hot stars. The spectra show that for 15 < g < 17, 40% of hot stars are WDs, and the fraction of WDs rises to ∼90% at g = 20. The remainder are hot sdB and sdO stars.
• Liebert, J., Harris, H. C., Dahn, C. C., Schmidt, G. D., Kleinman, S. J., Nitta, A., Krzesiński, J., Eisenstein, D., Smith, J. A., Szkody, P., Hawley, S., Anderson, S. F., Brinkmann, J., Collinge, M. J., Fan, X., Hall, P. B., Knapp, G. R., Lamb, D. Q., Margon, B., , Schneider, D. P., et al. (2003). SDSS white dwarfs with spectra showing atomic oxygen and/or carbon lines. Astronomical Journal, 126(5 1775), 2521-2528.
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  Abstract: We discuss 18 white dwarfs, one of which (G227-5) was previously known, whose SDSS spectra show lines of neutral and/or singly ionized carbon. At least two and perhaps four show lines of neutral or singly ionized oxygen. Apart from the extremely hot "PG 1159" stars, these are the first white dwarfs with photospheric oxygen detected in their optical spectra. The photometry strongly suggests that these stars lie in the 11,000-30,000 K temperature range of the helium-atmosphere DB white dwarfs, though only one of them shows weak neutral helium lines in the spectrum. Trigonometric parallaxes are known for G227-5 and another, previously known white dwarf (G35-26) showing atomic carbon lines, and they indicate that both are massive stars. Theoretical arguments suggest that all members of this class of rare white dwarfs are massive (∼1 M ⊙), and this finding could explain the paucity of massive DB white dwarfs.
• Pentericci, L., Rix, H. -., Prada, F., Fan, X., Strauss, M. A., Schneider, D. P., Grebel, E. K., Harbeck, D., Brinkmann, J., & Narayanan, V. K. (2003). The near-IR properties and continuum shapes of high redshift quasars from the sloan digital sky survey. Astronomy and Astrophysics, 410(1), 75-82.
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  Abstract: We present J - H - K′ photometry for a sample of 45 high redshift quasars found by the Sloan Digital Sky Survey. The sample was originally selected on the basis of optical colors and spans a redshift range from 3.6 to 5.03. Our photometry reflects the rest-frame SED longward of Lyα for all redshifts. The results show that the near-IR colors of high redshift quasars are quite uniform. We have modelled the continuum shape of the quasars (from just beyond Lyα to ∼4000 Å) with a power law of the form fv α vα, and find = -0.57 with a scatter of 0.33. This value is similar to what is found for lower redshift quasars over the same restframe wavelength range, and we conclude that there is hardly any evolution in the continuum properties of over the same restframe wavelength range, and we conclude that there is hardly any evolution in the continuum properties of optically selected quasars up to redshift 5. The spectral indices found by combining near-IR with optical photometry are in general consistent but slightly flatter than what is found for the same quasars using the optical spectra and photometry alone, showing that the continuum region used to determine the spectral indices can somewhat influence the results.
• Petric, A. O., Carilli, C. L., Bertoldi, F., Fan, X., Cox, P., Strauss, M. A., Omont, A., & Schneider, D. P. (2003). Sensitive observations at 1.4 and 250 GHz of z > 5 QSOs. Astronomical Journal, 126(1 1771), 15-23.
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  Abstract: We present 1.4 and 5 GHz observations taken with the Very Large Array, and observations at 250 GHz obtained with the Max Planck Millimeter Bolometer Array at the IRAM 30 m telescope, of 10 optically selected quasi-stellar objects (QSOs) at 5.0 ≤ z ≤ 6.28. Four sources are detected at 1.4 GHz. Two of the sources have rest-frame 1.4 GHz luminosity densities greater than 5.0 × 1026 W Hz-1, placing them in the regime of radio-loud QSOs. Both of these sources are also detected at 5 GHz. These results are roughly consistent with there being no evolution of the radio-loud QSO fraction out to z ∼ 6. Three sources have been detected at 250 or 350 GHz by these, and previous, observations. The (sub-) millimeter flux densities for these three sources are much larger than their 1.4 GHz flux densities. The rapidly rising spectra into the (rest-frame) far-IR (FIR) argue that the observed millimeter emission is likely thermal emission from warm dust, although more exotic possibilities cannot be precluded. The implied IR luminosities are between 1012 and 1013 L ⊙. For J0301+0020, the radio continuum emission is clearly above that expected for a star-forming galaxy based on the radio-FIR correlation. In this case, it seems likely that the radio emission relates to the active galactic nucleus. For J0756+4104, the radio emission is within the range expected for a star-forming galaxy, while for J1044-0125 the radio upper limit is at least consistent with a star-forming galaxy. If the dust is heated by star formation, the implied massive star formation rates are between 200 and 1000 M⊙ yr-1. We do not detect radio emission from the reported X-ray jet associated with J1306+0356. The lack of radio emission implies that the magnetic field is well below typical equipartition values in powerful radio jets or that particle acceleration ceased between 106 and 107 yr ago or that the X-ray emission is not inverse Compton emission from a jet related to J1306+0356. The highest redshift source in our sample (J1030+0524 at z = 6.28) is not detected at 1.4 or 250 GHz, but four fairly bright radio sources (S1.4GHz > 0.2 mJy) are detected in a 2′ field centered on the QSO, including an edge-brightened ("FR II") double radio source with an extent of about 1′. A similar overdensity of radio sources is seen in the field of the highest redshift QSO J1148+5251. We speculate that these overdensities of radio sources may indicate clusters along the lines of sight, in which case gravitational lensing by the cluster could magnify the QSO emission by a factor 2 or so without giving rise to arcsecond-scale distortions in the optical images of the QSOs.
• Pettini, M., Madau, P., Bolte, M., Prochaska, J. X., Ellison, S. L., & Fan, X. (2003). The C IV mass density of the universe at redshift 5. Astrophysical Journal Letters, 594(2 I), 695-703.
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  Abstract: In order to search for metals in the Lyα forest at redshifts z abs > 4, we have obtained spectra of high signal-to-noise ratio and moderately high resolution of three QSOs at zem > 5.4 discovered by the Sloan Digital Sky Survey. These data allow us to probe to metal enrichment of the intergalactic medium at early times with higher sensitivity than previous studies. We find 16 C IV absorption systems with column densities log N(C IV) = 12.50-13.98 over a total redshift path ΔX = 3.29. In the redshift interval z = 4.5-5.0, where our statistics are most reliable, we deduce a comoving mass density of C3+ ions Ω C IV = (4.3 ± 2.5) × 10-8 (90% confidence limits) for absorption systems with log N(C IV) ≥ 13.0 (for an Einstein-de Sitter cosmology with h = 0.65). This value of ΩC IV is entirely consistent with those measured at z < 4; we confirm the earlier finding by Songaila that neither the column density distribution of C IV absorbers nor its integral show significant redshift evolution over a period of time that stretches from ∼1.25 to ∼4.5 Gyr after the big bang. This somewhat surprising conclusion may be an indication that the intergalactic medium was enriched in metals at z ≫ 5, perhaps by the sources responsible for its reionization. Alternatively, the C IV systems we see may be associated with outflows from massive star-forming galaxies at later times, while the truly intergalactic metals may reside in regions of the Lyα forest of lower density than those probed up to now.
• Reichard, T. A., Richards, G. T., Hall, P. B., Schneider, D. P., E., D., Xiaohui, F. A., York, D. G., Knapp, G. R., & Brinkmann, J. (2003). Continuum and emission-line properties of broad absorption line quasars. Astronomical Journal, 126(6 1776), 2594-2607.
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  Abstract: We investigate the continuum and emission-line properties of 224 broad absorption line quasars (BALQSOs) with 0.9 ≲ z ≲ 4.4 drawn from the Sloan Digital Sky Survey Early Data Release, which contains 3814 bona fide quasars. We find that low-ionization BALQSOs (LoBALs) are significantly reddened as compared with normal quasars, in agreement with previous work. High-ionization BALQSOs (HiBALs) are also more reddened than the average non-BALQSO. Assuming SMC-like dust reddening at the quasar redshift, the amount of reddening needed to explain HiBALs is E(B-V) ∼ 0.023 and LoBALs is E(B-V) ∼ 0.077 (compared with the ensemble average of the entire quasar sample). We find that there are differences in the emission-line properties between the average HiBAL, LoBAL, and non-BAL quasar. These differences, along with differences in the absorption-line troughs, may be related to intrinsic quasar properties such as the slope of the intrinsic (unreddened) continuum; more extreme absorption properties are correlated with bluer intrinsic continua. Despite the differences among BALQSO subtypes and non-BALQSOs, BALQSOs appear to be drawn from the same parent population as non-BALQSOs when both are selected by their UV/optical properties. We find that the overall fraction of traditionally defined BALQSOs, after correcting for color-dependent selection effects due to different SEDs of BALQSOs and non-BALQSOs, is 13.4% ± 1. 2% and shows no significant redshift dependence for 1.7 ≤ z ≤ 3.45. After a rough completeness correction for the effects of dust extinction, we find that approximately one in every six quasars is a BALQSO.
• Richards, G. T., Hall, P. B., E., D., Strauss, M. A., Schneider, D. P., Weinstein, M. A., Reichard, T. A., York, D. G., Knapp, G. R., Fan, X., Ivezić, Ž., Brinkmann, J., Budavári, T., Csabai, I., & Nichol, R. C. (2003). Red and reddened quasars in the sloan digital sky survey. Astronomical Journal, 126(3 1773), 1131-1147.
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  Abstract: We investigate the overall continuum and emission-line properties of quasars as a function of their optical/UV spectral energy distributions. Our sample consists of 4576 quasars from the Sloan Digital Sky Survey (SDSS) that were chosen using homogeneous selection criteria. Expanding on our previous work, which demonstrated that the optical/UV color distribution of quasars is roughly Gaussian but with a red tail, here we distinguish between (1) quasars that have intrinsically blue (optically flat) power-law continua, (2) quasars that have intrinsically red (optically steep) power-law continua, and (3) quasars whose colors are inconsistent with a single power-law continuum. We find that 273 (6.0%) of the quasars in our sample fall into the latter category and appear to be redder because of SMC-like dust extinction and reddening rather than because of synchrotron emission. Even though the SDSS quasar survey is optically selected and flux-limited, we demonstrate that it is sensitive to dust reddened quasars with E(B- V) ≲ 0.5, assuming a classical SMC extinction curve. The color distribution of our SDSS quasar sample suggests that the population of moderately dust reddened, but otherwise normal (i.e., type 1) quasars is smaller than the population of unobscured quasars: we estimate that a further 10% of the quasar population with Mi < -25.61 is missing from the SDSS sample because of extinction, bringing the total fraction of dust-reddened quasars to 15% of broad-line quasars. We also investigate the emission- and absorption-line properties of these quasars as a function of color and comment on how some of these results relate to Boroson-Green-type eigenvectors. Quasars with intrinsically red (optically steep) power-law continua tend to have narrower Balmer lines and weaker C IV, C III], He II, and 3000 A bump emission as compared with bluer (optically flatter) quasars. The change in strength of the 3000 Åbump appears to be dominated by the Balmer continuum and not by Fe II emission. The dust-reddened quasars have even narrower Balmer lines and weaker 3000 Å bumps, in addition to having considerably larger equivalent widths of [O II] and [O III] emission. The fraction of broad absorption line quasars (BALQSOs) increases from ∼3.4% for the bluest quasars to perhaps as large as 20% for the dust-reddened quasars, but the intrinsic color distribution will be much bluer if all BALQSOs are affected by dust reddening.
• Schneider, D. P., Fan, X., Hall, P. B., Jester, S., Richards, G. T., Stoughton, C., Strauss, M. A., Subbarao, M., E., D., Anderson, S. F., Brandt, W. N., Gunn, J. E., Gray, J., Trump, J. R., Voges, W., Yanny, B., Bahcall, N. A., Blanton, M. R., Boroski, W. N., , Brinkmann, J., et al. (2003). The sloan digital sky survey quasar catalog. II. First data release. Astronomical Journal, 126(6 1776), 2579-2593.
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  Abstract: We present the second edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog consists of the 16,713 objects in the SDSS First Data Release that have luminosities larger than M i = -22 (in a cosmology with H 0 = 70 km s -1 Mpc -1, Ω M= 0.3, and Ω ∇ = 0.7), have at least one emission line with FWHM larger than 1000 km s -1, and have highly reliable redshifts. The area covered by the catalog is ≈ 1360 deg 2. The quasar redshifts range from 0.08 to 5.41, with a median value of 1.43. For each object, the catalog presents positions accurate to better than 0.′2 rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains some radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. Calibrated digital spectra of all objects in the catalog, covering the wavelength region 3800-9200 Å at a spectral resolution of 1800-2100, are available. This publication supersedes the first SDSS Quasar Catalog, which was based on material from the SDSS Early Data Release. A summary of corrections to current quasar databases is also provided. The majority of the objects were found in SDSS commissioning data using a multicolor selection technique. Since the quasar selection algorithm was undergoing testing during the entire observational period covered by this catalog, care must be taken when assembling samples from the catalog for use in statistical studies. A total of 15,786 objects (94%) in the catalog were discovered by the SDSS; 12,173 of the SDSS discoveries are reported here for the first time. Included in the new discoveries are five quasars brighter than i= 16.0 and 17 quasars with redshifts larger than 4.5.
• Szkody, P., Anderson, S. F., Schmidt, G., Hall, P. B., Margon, B., Miceli, A., Subbarao, M., Frith, J., Harris, H., Hawley, S., Lawton, B., Covarrubias, R., Covey, K., Fan, X., Murphy, T., Narayanan, V., Raymond, S., Rest, A., Strauss, M. A., , Stubbs, C., et al. (2003). Two rare magnetic cataclysmic variables with extreme cyclotron features identified in the Sloan Digital Sky Survey. Astrophysical Journal Letters, 583(2 I), 902-906.
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  Abstract: Two newly identified magnetic cataclysmic variables discovered in the Sloan Digital Sky Survey (SDSS), SDSS J155331.12+551614.5 and SDSS J132411.57+032050.5, have spectra showing highly prominent, narrow, strongly polarized cyclotron humps with amplitudes that vary on orbital periods of 4.39 and 2.6 hr, respectively. In the former, the spacing of the humps indicates the third and fourth harmonics in a magnetic field of ∼60 MG. The narrowness of the cyclotron features and the lack of strong emission lines imply very low temperature plasmas and very low accretion rates, so that the accreting area is heated by particle collisions rather than accretion shocks. The detection of rare systems like these exemplifies the ability of the SDSS to find the lowest accretion rate close binaries.
• Vignali, C., Brandt, W. N., Schneider, D. P., Anderson, S. F., Fan, X., Gunn, J. E., Kaspi, S., Richards, G. T., & Strauss, M. A. (2003). Chandra and XMM-Newton observations of the first quasars: X-rays from the age of cosmic enlightenment. Astronomical Journal, 125(6 1770), 2876-2890.
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  Abstract: We report on Chandra and XMM-Newton observations of a sample of 13 quasars at z ≈ 4.7-5.4 mostly taken from the Sloan Digital Sky Survey (SDSS). The present sample complements previous X-ray studies of z ≥ 4 quasars, in which the majority of the objects are optically more luminous and at lower redshifts. All but two of our quasars have been detected in the X-ray band, thus doubling the number of z ≥ 4.8 X-ray-detected quasars. The two nondetections are likely to be due to a short exposure time (SDSSp J033829.31+002156.3) and to the presence of intrinsic absorption (SDSSp J173744.87+582829.5). We confirm and extend to the highest redshifts the presence of a correlation between AB1450(1+z) magnitude and soft X-ray flux for z ≥ 4 quasars and the presence of a steeper optical-to-X-ray spectral energy distribution (parameterized by αox) for high-luminosity, high-redshift quasars than for lower luminosity, lower redshift quasars. The second effect is likely due to the known anticorrelation between αox and rest-frame 2500 Å luminosity, whose significance is confirmed via partial correlation analysis. The joint ≈2.5-36 keV rest-frame spectrum of the z > 4.8 SDSS quasars observed thus far by Chandra is well parameterized by a power law with photon index Γ = 1.84-0.30+0.31; this photon index is consistent with those of z ≈ 0-3 quasars and that obtained from joint spectral fitting of z ≈ 4.1-4.5 optically luminous Palomar Digital Sky Survey quasars. No evidence for widespread intrinsic X-ray absorption has been found (NH ≲ 4.0 × 1022 cm-2 on average at 90% confidence). We also obtained Hobby-Eberly Telescope (HET) photometric observations for eight of our target quasars. None of these shows significant (greater than 30%) optical variability over the time interval of a few years (in the observed frame) between the SDSS and HET observations.
• Walter, F., Bertoidl, F., Carilli, C., Cox, P., Lo, K. Y., Neri, R., Fan, X., Omont, A., Strauss, M. A., & Menten, K. M. (2003). Molecular gas in the host galaxy of a quasar at redshift z = 6.42. Nature, 424(6947), 406-408.
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  PMID: 12879063;Abstract: Observations of molecular hydrogen in quasar host galaxies at high redshifts provide fundamental constraints on galaxy evolution, because it is out of this molecular gas that stars form. Molecular hydrogen is traced by emission from the carbon monoxide molecule, CO; cold H2 itself is generally not observable. Carbon monoxide has been detected in about ten quasar host galaxies with redshifts z > 2; the record-holder is at z = 4.69 (refs 1-3). Here we report CO emission from the quasar SDSS J114816.64 + 525150.3 (refs 5, 6) at z = 6.42. At that redshift, the Universe was only 1/16 of its present age, and the era of cosmic reionization was just ending. The presence of about 2 × 1010 M⊙ of H2 in an object at this time demonstrates that molecular gas enriched with heavy elements can be generated rapidly in the youngest galaxies.
• White, R. L., Becker, R. H., Fan, X., & Strauss, M. A. (2003). Probing the ionization state of the universe at z > 6. Astronomical Journal, 126(1 1771), 1-14.
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  Abstract: We present high signal-to-noise ratio Keck ESI spectra of the two quasars known to have Gunn-Peterson absorption troughs, SDSS J1030+0524 (z = 6.28) and SDSS J1148+5251 (z = 6.37). The Lyα and Lyβ troughs for SDSS J1030+0524 are very black and show no evidence of any emission over a redshift interval of ∼0.2, starting at z = 6. On the other hand, SDSS J1148+5251 shows a number of emission peaks in the Lyβ Gunn-Peterson trough along with a single weak peak in the Lyα trough. The Lyα emission has corresponding Lyβ emission, suggesting that it is indeed a region of lower optical depth in the intergalactic medium at z = 6.08. The stronger Lyβ peaks in the spectrum of SDSS J1148+5251 could also conceivably be the result of "leaks" in the intergalactic medium (IGM), but we suggest instead that they are Lyα emission from an intervening galaxy at z = 4.9. This hypothesis gains credence from a strong complex of C IV absorption at the same redshift and from the detection of continuum emission in the Lyα trough at the expected brightness. If this proposal is correct, the quasar light has probably been magnified through gravitational lensing by the intervening galaxy. The Strömgren sphere observed in the absorption spectrum of SDSS J1148+5251 is significantly smaller than expected on the basis of its brightness, which is consistent with the hypothesis that the quasar is lensed. If our argument for lensing is correct, the optical depths derived from the troughs of SDSS J1148+5251 are only lower limits [albeit still quite strong, with τ(Lyα) > 16 inferred from the Lyβ trough]. The Lyβ absorption trough of SDSS J1030+0524 gives the single best measurement of the IGM transmission at z > 6, with an inferred optical depth of τ(Lyα) > 22.
• Brandt, W. N., Schneider, D. P., Fan, X., Strauss, M. A., Gunn, J. E., Richards, G. T., Anderson, S. F., E., D., Bahcall, N. A., Brinkmann, J., Brunner, R., Chen, B., Hennessy, G. S., Lamb, D. Q., Voges, W., & York, D. G. (2002). Exploratory Chandra observations of the three highest redshift quasars known. Astrophysical Journal Letters, 569(1 II), L5-L9.
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  Abstract: We report on exploratory Chandra observations of the three highest redshift quasars known (z = 5.82, 5.99, and 6.28), all found in the Sloan Digital Sky Survey. These data, combined with a previous XMM-Newton observation of a z = 5.74 quasar, form a complete set of color-selected, z>5.7 quasars. X-ray emission is detected from all of the quasars at levels that indicate that the X-ray-to-optical flux ratios of z ≈ 6, optically selected quasars are similar to those of lower redshift quasars. The observations demonstrate that it will be feasible to obtain quality X-ray spectra of z ≈ 6 quasars with current and future X-ray missions.
• Fan, X., Narayanan, V. K., Strauss, M. A., White, R. L., Becker, R. H., Pentericci, L., & Hans-Walter, R. (2002). Evolution of the ionizing background and the epoch of reionization from the spectra of z ∼ 6 quasars. Astronomical Journal, 123(3 1755), 1247-1257.
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  Abstract: We study the process of cosmic reionization and estimate the ionizing background in the intergalactic medium (IGM) using the Lyman series absorption in the spectra of the four quasars at 5.7 < z < 6.3 discovered by the Sloan Digital Sky Survey. We derive the redshift evolution of the ionizing background at high redshifts, using both semianalytic techniques and cosmological simulations to model the density fluctuations in the IGM. The existence of the complete Lyα Gunn-Peterson (GP) trough in the spectrum of the z = 6.28 quasar SDSS 1030+0524 indicates a photoionization rate (Γ-12 in units of 10-12 s-1) at z ∼ 6 lower than 0.08, at least a factor of 6 smaller than the value at z ∼ 3. The Lyβ and Lyγ GP troughs give an even stronger limit Γ -12 ≲ 0.02 due to their smaller oscillator strengths, indicating that the ionizing background in the IGM at z ∼ 6 is more than 20 times lower than that at z ∼ 3. Meanwhile, the volume-averaged neutral hydrogen fraction increases from 10-5 at z ∼ 3 to greater than 10-3 at z ∼ 6. At this redshift, the mass-averaged neutral hydrogen fraction is larger than 1%; the mildly overdense regions (δ > 3) are still mostly neutral, and the comoving mean free path of ionizing photons is shorter than 8 Mpc. Comparison with simulations of cosmological reionization shows that the observed properties of the IGM at z ∼ 6 are typical of those in the era at the end of the overlap stage of reionization when the individual H II regions merge. Thus z ∼ 6 marks the end of the reionization epoch. The redshift of reionization constrains the small-scale power of the mass-density fluctuations and the star-forming efficiency of the first generation of objects.
• Geballe, T. R., Knapp, G. R., Leggett, S. K., Fan, X., Golimowski, D. A., Anderson, S., Brinkmann, J., Csabai, I., Gunn, J. E., Hawley, S. L., Hennessy, G., Henry, T. J., Hill, G. J., Hindsley, R. B., Ivezić, Ž., Lupton, R. H., McDaniel, A., Munn, J. A., Narayanan, V. K., , Peng, E., et al. (2002). Toward spectral classification of L and T Dwarfs: Infrared and optical spectroscopy and analysis. Astrophysical Journal Letters, 564(1 I), 466-481.
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  Abstract: We present 0.6-2.5 μm, R ≳ 400 spectra of 27 cool, low-luminosity stars and substellar objects. Based on these and previously published spectra, we develop a preliminary spectral classification system for L and T dwarfs. For late L and T types the classification system is based entirely on four spectral indices in the 1-2.5 μm interval. Two of these indices are derived from water absorption bands at 1.15 and 1.4 μm, the latter of which shows a smooth increase in depth through the L and T sequences and can be used to classify both spectral types. The other two indices make use of methane absorption features in the H and K bands, with the X-band index also applicable to mid-to-late L dwarfs. Continuum indices shortward of l μm used by previous authors to classify L dwarfs are found to be useful only through mid-L subclasses. We employ the 1.5 μm water index and the 2.2 μm methane index to complete the L classification through L9.5 and to link the new system with a modified version of the 2MASS "color-d" index. By correlating the depths of the methane and water absorption features, we establish a T spectral sequence from T0 to T8, based on all four indices, that is a smooth continuation of the L sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9. 5 and identify one SDSS object as L9. In the proposed system methane absorption appears in the K band approximately at L8, two subclasses earlier than its appearance in the H band. The L and T spectral classes are distinguished by the absence and presence, respectively, of H-band methane absorption.
• Gregg, M. D., Becker, R. H., White, R. L., Richards, G. T., Chaffee, F. H., & Fan, X. (2002). An FeLoBAL binary quasar. Astrophysical Journal Letters, 573(2 II), L85-L89.
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  Abstract: In an ongoing infrared imaging survey of quasars at the Keck Observatory, we have discovered that the z = 1.285 quasar SDSS J233646.2-010732.6 comprises two point sources with a separation of 1″.67. Resolved spectra show that one component is a standard quasar with a blue continuum and broad emission lines; the other is a broad absorption line (BAL) quasar, specifically, a BAL QSO with prominent absorption from Mg II and metastable Fe II, making it a member of the "FeLoBAL" class. The number of known FeLoBALs has recently grown dramatically from a single example to more than a dozen, including a gravitationally lensed example and the binary member presented here, suggesting that this formerly rare object may be fairly common. Additionally, the presence of this BAL quasar in a relatively small separation binary adds to the growing evidence that the BAL phenomenon is not due to viewing a normal quasar at a specific orientation but rather that it is an evolutionary phase in the life of many, if not all, quasars and is particularly associated with conditions found in interacting systems.
• Hall, P. B., Anderson, S. F., Strauss, M. A., York, D. G., Richards, G. T., Fan, X., Knapp, G. R., Schneider, D. P., E., D., Geballe, T. R., Bauer, A. E., Becker, R. H., Davis, M., Rix, H., Nichol, R. C., Bahcall, N. A., Brinkmann, J., Brunner, R., Connolly, A. J., , Csabai, I., et al. (2002). Unusual broad absorption line quasars from the Sloan Digital Sky Survey. Astrophysical Journal, Supplement Series, 141(2), 267-309.
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  Abstract: The Sloan Digital Sky Survey has confirmed the existence of populations of broad absorption line (BAL) quasars with various unusual properties. We present and discuss 23 such objects and consider the implications of their wide range of properties for models of BAL outflows and quasars in general. We have discovered one BAL quasar with a record number of absorption lines. Two other similarly complex objects with many narrow troughs show broad Mg II absorption extending longward of their systemic host galaxy redshifts. This can be explained as absorption of an extended continuum source by the rotation-dominated base of a disk wind. Five other objects have absorption that removes an unprecedented ∼90% of all flux shortward of Mg II. The absorption in one of them has varied across the ultraviolet with an amplitude and rate of change as great as ever seen. This same object may also show broad Hβ absorption. Numerous reddened BAL quasars have been found, including at least one reddened mini-BAL quasar with very strong Fe II emission. The five reddest objects have continuum reddenings of E(B-V) ≃ 0.5, and in two of them we find strong evidence that the reddening curve is even steeper than that of the SMC. We have found at least one object with absorption from Fe III but not Fe II. This may be due to a high column density of moderately high ionization gas, but the Fe III level populations must also be affected by some sort of resonance. Finally, we have found two luminous, probably reddened high-redshift objects that may be BAL quasars whose troughs partially cover different regions of the continuum source as a function of velocity.
• Hawley, S. L., Covey, K. R., Knapp, G. R., Golmowski, D. A., Fan, X., Anderson, S. F., Gunn, J. E., Harris, H. C., Ivezić, Z., Long, G. M., Lupton, R. H., McGehee, P. M., Narayanan, V., Peng, E., Schlegel, D., Schneider, D. P., Spahn, E. Y., Strauss, M. A., Szkody, P., , Tsvetanov, Z., et al. (2002). Characterization of M, L, and T dwarfs in the sloan digital sky survey. Astronomical Journal, 123(6 1758), 3409-3427.
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  Abstract: An extensive sample of M, L, and T dwarfs identified in the Sloan Digital Sky Survey (SDSS) has been compiled. The sample of 718 dwarfs includes 677 new objects (629 M dwarfs and 48 L dwarfs), together with 41 that have been previously published. All new objects and some of the previously published ones have new optical spectra obtained either with the SDSS spectrographs or with the Apache Point Observatory 3.5 m ARC telescope. Spectral types and SDSS colors are available for all objects; approximately 35% also have near-infrared magnitudes measured by 2MASS (Two Micron All Sky Survey) or on the Mauna Kea system. We use this sample to characterize the color-spectral type and color-color relations of late-type dwarfs in the SDSS filters and to derive spectroscopic and photometric parallax relations for use in future studies of the luminosity and mass functions based on SDSS data. We find that the i*-z* and i*-J colors provide good spectral type and absolute magnitude (Mi*) estimates for M and L dwarfs. Our distance estimates for the current sample indicate that SDSS is finding early M dwarfs out to ∼1.5 kpc, L dwarfs to ∼100 pc, and T dwarfs to ∼20 pc. The T dwarf photometric data show large scatter and are therefore less reliable for spectral type and distance estimation.
• Hong, W. u., Burstein, D., Deng, Z., Zhou, X., Shang, Z., Zheng, Z., Chen, J., Hongjun, S. u., Windhorst, R. A., Chen, W., Zou, Z., Xia, X., Jiang, Z., Jun, M. a., Xue, S., Zhu, J., Cheng, F., Byun, Y., Chen, R., , Deng, L., et al. (2002). Intermediate-band surface photometry of the edge-on galaxy NGC 4565. Astronomical Journal, 123(3 1755), 1364-1380.
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  Abstract: We present a deep, 42.79 hr image of the nearby, edge-on galaxy NGC 4565 in the Beijing-Arizona-Taipei-Connecticut 6660 Å band using the large-format CCD system on the 0.6 m Schmidt telescope at the Xinglong Station of the National Astronomical Observatories of China. Following the procedures previously developed by our team for the analysis of deep images of galaxies, we obtain a final image that is calibrated to an accuracy of 0.02 mag in zero point and for which we can measure galaxy surface brightness to an accuracy of 0.25 mag at a surface brightness of 27.5 mag arcsec-2 at 6660 Å, corresponding to a distance of 22 kpc from the center of the disk. The integrated magnitude of NGC 4565 in our filter is m6660 = 8.99 (=R magnitude of 9.1) to a surface brightness of 28 mag arcsec-2. We analyze the faint outer parts of this galaxy using a two-dimensional model comprised of three components: an exponential thin disk, an exponential thick disk, and a power-law halo. Combined with a need to provide a cutoff radius for the disk, a total of 12 parameters are included in our model. We determine the best values of our model parameters via 10,000 random initial values, 3700 of which converge to final, values. We then plot the x2 for each converged fit versus parameter value for each of the 12 parameters. The thin-disk and thick-disk parameters that we determine here are consistent with those of previous studies of this galaxy. However, our very deep image permits a better determination of the power-law fit to the halo, constraining this power law to be between r-3.2 and r-4.0, with a best-fit value of r-3.88 We findthe axis ratio of the halo to be 0.44 and its core radius to be 14.4 kpc (for an adopted distance of 14.5 Mpc). We also agree with others that the bulge of NGC 4565 is fitted well by an exponential luminosity distribution with a scale height similar to that found for the thin disk.
• Ivezíc, Ž., Menou, K., Knapp, G. R., Strauss, M. A., Lupton, R. H., E., D., Richards, G. T., Tremonti, C., Weinstein, M. A., Anderson, S., Bahcall, N. A., Becker, R. H., Bernardi, M., Blanton, M., Eisenstein, D., Fan, X., Finkbeiner, D., Finlator, K., Frieman, J., , Gunn, J. E., et al. (2002). Optical and radio properties of extragalactic sources observed by the first survey and the sloan digital sky survey. Astronomical Journal, 124(5 1763), 2364-2400.
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  Abstract: We discuss the optical and radio properties of ∼30,000 FIRST (radio, 20 cm, sensitive to 1 mJy) sources positionally associated within 1.″5 with a Sloan Digital Sky Survey (SDSS) (optical, sensitive to r* ∼ 22.2) source in 1230 deg2 of sky. The matched sample represents ∼ 30% of the 108,000 FIRST sources and 0.1% of the 2.5 × 10 7 SDSS sources in the studied region. SDSS spectra are available for 4300 galaxies and 1154 quasars from the matched sample and for a control sample of 140,000 galaxies and 20,000 quasars in 1030 deg2 of sky. Here we analyze only core sources, which dominate the sample; the fraction of SDSS-FIRST sources with complex radio morphology is determined to be less than 10%. This large and unbiased catalog of optical identifications provides much firmer statistical footing for existing results and allows several new findings. The majority (83%) of the FIRST sources identified with an SDSS source brighter than r* = 21 are optically resolved; the fraction of resolved objects among the matched sources is a function of the radio flux, increasing from ∼50% at the bright end to ∼90% at the FIRST faint limit. Nearly all optically unresolved radio sources have nonstellar colors indicative of quasars. We estimate an upper limit of ∼5% for the fraction of quasars with broadband optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux-optical flux plane suggests the existence of the "quasar radio dichotomy"; 8% ± 1% of all quasars with i* < 18.5 are radio-loud, and this fraction seems independent of redshift and optical luminosity. The radio-loud quasars have a redder median color by 0.08 ± 0.02 mag, and show a 3 times larger fraction of objects with extremely red colors. FIRST galaxies represent 5% of all SDSS galaxies with r* < 17.5, and 1% for r* < 20, and are dominated by red (u* - r* > 2.22) galaxies, especially those with r* > 17.5. Magnitude-and redshift-limited samples show that radio galaxies have a different optical luminosity distribution than nonradio galaxies selected by the same criteria; when galaxies are further separated by their colors, this result remains valid for both blue and red galaxies. For a given optical luminosity and redshift, the observed optical colors of radio galaxies are indistinguishable from those of all SDSS galaxies selected by identical criteria. The distributions of radio-to-optical flux ratio are similar for blue and red galaxies in redshift-limited samples; this similarity implies that the difference in their luminosity functions and resulting selection effects are the dominant cause for the preponderance of red radio galaxies in flux-limited samples. The fraction of radio galaxies whose emission-line ratios indicate an AGN (30%), rather than starburst, origin is 6 times larger than the corresponding fraction for all SDSS galaxies (r* < 17.5). We confirm that the AGN-to-starburst galaxy number ratio increases with radio flux and find that radio emission from AGNs is more concentrated than radio emission from starburst galaxies.
• Jurić, M., Ivezić, Ž., Lupton, R. H., Quinn, T., Tabachnik, S., Fan, X., Gunn, J. E., Hennessy, G. S., Knapp, G. R., Munn, J. A., Pier, J. R., Rockosi, C. M., Schneider, D. P., Brinkmann, J., Csabai, I., & Fukugita, M. (2002). Comparison of positions and magnitudes of asteroids observed in the sloan digital sky survey with those predicted for known asteroids. Astronomical Journal, 124(3 1761), 1776-1787.
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  Abstract: We positionally correlate known asteroids with a sample of ∼18,000 asteroids detected by the Sloan Digital Sky Survey (SDSS). We find 2641 unique matches, which represent the largest sample of asteroids with both accurate multicolor photometry and known orbital parameters. The matched objects are predominantly bright and demonstrate that the SDSS photometric pipeline recovers ∼90% of the known asteroids in the observed region. For the recovered asteroids, we find a large offset (∼0.4 mag) between Johnson V magnitudes derived from SDSS photometry and the predicted magnitudes. This offset varies with the asteroid color, from 0.34 mag for blue asteroids to 0.44 mag for red asteroids, and is probably caused by the use of unfiltered CCD observations in the majority of recent asteroid surveys. This systematic photometric error leads to an overestimate of the number of asteroids brighter than a given absolute magnitude limit by a factor of ∼1.7. The distribution of the matched asteroids in orbital parameter space indicates strong color segregation. We confirm that some families are dominated by a single asteroid type (e.g., the Koronis family by red asteroids and the Themis family by blue asteroids), while others appear to be a mixture of blue and red objects (e.g., the Nysa-Polana family). Asteroids with the bluest i*-z* colors, which can be associated with the Vesta family, show particularly striking localization in orbital parameter space.
• Leggett, S. K., Golimowski, D. A., Fan, X., Geballe, T. R., Knapp, G. R., Brinkmann, J., Csabai, I., Gunn, J. E., Hawley, S. L., Henry, T. J., Hindsley, R., Ivezić, Ž., Lupton, R. H., Pier, J. R., Schneider, D. P., Smith, J. A., Strauss, M. A., Uomoto, A., & York, D. G. (2002). Infrared photometry of late-M, L, and T dwarfs. Astrophysical Journal Letters, 564(1 I), 452-465.
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  Abstract: We present ZJHKL′M′ photometry of a sample of 58 late M, L, and T dwarfs, most of which are identified from the Sloan Digital Sky Survey and the Two Micron All-Sky Survey. Near-infrared spectra and spectral classifications for most of this sample are presented in a companion paper by Geballe et al. We derive the luminosities of 18 dwarfs in the sample with known parallaxes, and the results imply that the effective temperature range for the L dwarfs in our sample is approximately 2200-1300 K and for the T dwarfs 1300-800 K. We obtained new photometric data at the United Kingdom Infrared Telescope for: 42 dwarfs at Z, 34 dwarfs at JHK, 21 dwarfs at L′, as well as M′ data for two L dwarfs and two T dwarfs. The M′ data provide the first accurate photometry for L and T dwarfs in this bandpass - for a T2 and a T5 dwarf, we find K - M′ = 1.2 and 1.6, respectively. These colors are much bluer than predicted by published models, suggesting that CO may be more abundant in these objects than expected, as has been found for the T6 dwarf Gl 229B. We also find that K - L′ increases monotonically through most of the M, L, and T subclasses, but it is approximately constant between types L6 and T5, restricting its usefulness as a temperature indicator. The degeneracy is probably due to the onset of CH4 absorption at the blue edge of the L′ bandpass. The JHK colors of L dwarfs show significant scatter, suggesting that the fluxes in these bandpasses are sensitive to variations in photospheric dust properties. The H - K colors of the later T dwarfs also show some scatter, which we suggest is due to variations in pressure-induced H2 opacity, which is sensitive to gravity and metallicity.
• Margon, B., Anderson, S. F., Harris, H. C., Strauss, M. A., Knapp, G. R., Fan, X., Schneider, D. P., E., D., Schlegel, D. J., Deutsch, E. W., Ivezić, Ž., Hall, P. B., Williams, B. F., Davidsen, A. F., Brinkmann, J., Csabai, I., Hayes, J. J., Hennessy, G., Kinney, E. K., , Kleinman, S. J., et al. (2002). Faint high-latitude carbon stars discovered by the sloan digital sky survey: Methods and initial results. Astronomical Journal, 124(3 1761), 1651-1669.
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  Abstract: We report the discovery of 39 faint high-latitude carbon stars (FHLCs) from Sloan Digital Sky Survey (SDSS) commissioning data. The objects, each selected photometrically and verified spectroscopically, range over 16.6 < r* < 20.0 and show a diversity of temperatures as judged by both colors and NaD line strengths. Although a handful of these stars were previously known, these objects are, in general, too faint and too warm to be effectively identified in other modern surveys such as the Two Micron All Sky Survey, nor are their red/near-IR colors particularly distinctive. The implied surface density of FHLCs in this magnitude range is uncertain at this preliminary stage of the survey because of completeness corrections but is clearly greater than 0.05 deg-2. At the completion of the Sloan survey, there will be many hundred homogeneously selected and observed FHLCs in this sample. We present proper-motion measures for each object, indicating that the sample is a mixture of extremely distant (greater than 100 kpc) halo giant stars, useful for constraining halo dynamics, and members of the recently recognized exotic class of very nearby dwarf carbon (dC) stars. The broadband colors of the two populations are indistinguishable. Motions, and thus dC classification, are inferred for 40%-50% of the sample, depending on the level of statistical significance invoked. The new list of dC stars presented here, although selected from only a small fraction of the final SDSS, doubles the number of such objects found by all previous methods. The observed kinematics suggest that the dwarfs occupy distinct halo and disk populations. The coolest FHLCs with detectable proper motions in our sample also display multiple CaH bands in their spectra. It may be that CaH is another long-sought, low-resolution, spectroscopic luminosity discriminant between dC's and distant faint giants, at least for the cooler stars.
• Marley, M. S., Seager, S., Saumon, D., Lodders, K., Ackerman, A. S., Freedman, R. S., & Fan, X. (2002). Clouds and chemistry: Ultracool dwarf atmospheric properties from optical and infrared colors. Astrophysical Journal Letters, 568(1 I), 335-342.
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  Abstract: The optical and infrared colors of L and T dwarfs are sensitive to cloud sedimentation and chemical equilibrium processes in their atmospheres. The i′-z′ versus J-K color-color diagram provides a window into diverse atmospheric processes mainly because different chemical processes govern each color, and cloud opacity largely affects J-K but not i′-z′. Using theoretical atmosphere models that include for the first time a self-consistent treatment of cloud formation, we present an interpretation of the i′-z′ versus J-K color trends of known L and T dwarfs. We find that the i′-z′ color is extremely sensitive to chemical equilibrium assumptions; chemical equilibrium models accounting for cloud sedimentation predict redder i′-z′ colors - by up to 2 mag - than models that neglect sedimentation. We explore the previously known J-K color trends in which objects first become redder, then bluer with decreasing effective temperature. Only models that include sedimentation of condensates are able to reproduce these trends. We find that the exact track of a cooling brown dwarf in J-K (and i′-z′) is very sensitive to the details of clouds, in particular to the efficiency of sedimentation of condensates in its atmosphere. We also find that clouds still affect the strength of the J-, H-, and K-band fluxes of even the coolest T dwarfs. In addition, we predict the locus in the i′-z′ versus J-K color-color diagram of brown dwarfs cooler than yet discovered.
• Pentericci, L., Fan, X., Rix, H., Strauss, M. A., Narayanan, V. K., Richards, G. T., Schneider, D. P., Krolik, J., Heckman, T., Brinkmann, J., Lamb, D. Q., & Szokoly, G. P. (2002). VLT optical and near-infrared observations of the z = 6.28 quasar SDSS J1030+0524. Astronomical Journal, 123(5 1757), 2151-2158.
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  Abstract: We present new VLT spectroscopic observations of the most distant quasar known, SDSS J1030+0524 at z = 6.28, which was recently discovered by the Sloan Digital Sky Survey. We confirm the presence of a complete Gunn-Peterson trough caused by neutral hydrogen in the intergalactic medium. There is no detectable flux over the wavelength range from 8450 to 8710 Å. We set a stronger limit on the drop of the flux level blue-ward of the Lyα line: a factor of more than 200. Below 8450 Å the spectrum shows a rise in flux, with a large fraction (>60%) of the total emission produced by a few narrow features of transmitted flux. We discuss the proximity effect around this quasar, with the presence of transmitted flux with many absorption features in a region of about 23 h-1 comoving Mpc. If we assume that the surrounding medium is completely neutral, the size of this region would imply a quasar lifetime of ∼1.3 × 107 yr. We also present near-IR spectroscopy of both SDSS J1030+0524 and SDSS J1306+05, the second most distant quasar known, at redshift 6.0. We combine measurements of the C IV line and limits on the He II emission from the near-IR spectra with the N v line measurements from the optical spectra to derive the metal abundances of these early quasar environments. The results are indistinguishable from those of lower redshift quasars and indicate little or no evolution in the metal abundances from z ∼ 6 to 2. The line ratios suggest supersolar metallicities, implying that the first stars around the quasars must have formed at least a few hundreds of megayears prior to the observation, i.e., at redshifts higher than 8.
• Richards, G. T., Fan, X., Newberg, H. J., Strauss, M. A., E., D., Schneider, D. P., Yanny, B., Boucher, A., Burles, S., Frieman, J. A., Gunn, J. E., Hall, P. B., Ivezić, Ž., Kent, S., Loveday, J., Lupton, R. H., Rockosi, C. M., Schlegel, D. J., Stoughton, C., , SubbaRao, M., et al. (2002). Spectroscopic target selection in the sloan digital sky survey: The quasar sample. Astronomical Journal, 123(6 1758), 2945-2975.
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  Abstract: We describe the algorithm for selecting quasar candidates for optical spectroscopy in the Sloan Digital Sky Survey. Quasar candidates are selected via their nonstellar colors in ugriz broadband photometry and by matching unresolved sources to the FIRST radio catalogs. The automated algorithm is sensitive to quasars at all redshifts lower than z ∼ 5.8. Extended sources are also targeted as low-redshift quasar candidates in order to investigate the evolution of active galactic nuclei (AGNs) at the faint end of the luminosity function. Nearly 95% of previously known quasars are recovered (based on 1540 quasars in 446 deg2). The overall completeness, estimated from simulated quasars, is expected to be over 90%, whereas the overall efficiency (quasars/quasar candidates) is better than 65%. The selection algorithm targets ultraviolet excess quasars to i* = 19.1 and higher redshift (z ≳ 3) quasars to i* = 20.2, yielding approximately 18 candidates deg -2. In addition to selecting "normal" quasars, the design of the algorithm makes it sensitive to atypical AGNs such, as broad absorption line quasars and heavily reddened quasars.
• Schneider, D. P., Knapp, G. R., Hawley, S. L., Covey, K. R., Fan, X., Ramsey, L. W., Richards, G. T., Strauss, M. A., Gunn, J. E., Hill, G. J., MacQueen, P. J., Adams, M. T., Hill, G. M., Ivezić, Ž., Lupton, R. H., Pier, J. R., Saxe, D. H., Shetrone, M., Tufts, J. R., , Wolf, M. J., et al. (2002). L dwarfs found in sloan digital sky survey commissioning data. II. Hobby-Eberly Telescope observations. Astronomical Journal, 123(1 1753), 458-465.
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  Abstract: Low-dispersion optical spectra have been obtained with the Hobby-Eberly Telescope of 22 very red objects found in early imaging data from the Sloan Digital Sky Survey (SDSS). The objects are assigned spectral types on the Two Micron All Sky Survey (2MASS) system and are found to range from late M to late L. The red and near-infrared colors from SDSS and 2MASS correlate closely with each other, and most of the colors are closely related to spectral type in this range; the exception is the i*-z* color, which appears to be independent of spectral type between about M7 and L4. The spectra suggest that this independence is due to the disappearance of the TiO and VO absorption in the i band for later spectral types, the presence of strong Na I and K I absorption in the i band, and the gradual disappearance of the 8400 Å absorption of TiO and FeH in the z band.
• Schneider, D. P., Richards, G. T., Fan, X., Hall, P. B., Strauss, M. A., E., D., Gunn, J. E., Newberg, H. J., Reichard, T. A., Stoughton, C., Voges, W., Yanny, B., Anderson, S. F., Annis, J., Bahcall, N. A., Bauer, A., Bernardi, M., Blanton, M. R., Boroski, W. N., , Brinkmann, J., et al. (2002). The sloan digital sky survey Quasar Catalog. I. Early data release. Astronomical Journal, 123(2 1754), 567-577.
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  Abstract: We present the first edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog consists of the 3814 objects (3000 discovered by the SDSS) in the initial SDSS public data release that have at least one emission line with a full width at half-maximum larger than 1000 km s-1, luminosities brighter than Mi* = -23, and highly reliable redshifts. The area covered by the catalog is 494 deg2; the majority of the objects were found in SDSS commissioning data using a multicolor selection technique. The quasar redshifts range from 0.15 to 5.03. For each object the catalog presents positions accurate to better than 0″.2 rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.05 mag, radio and X-ray emission properties, and information on the morphology and selection method. Calibrated spectra of all objects in the catalog, covering the wavelength region 3800-9200 Å at a spectral resolution of 1800-2100, are also available. Since the quasars were selected during the commissioning period, a time when the quasar selection algorithm was undergoing frequent revisions, the sample is not homogeneous and is not intended for statistical analysis.
• Stoughton, C., Lupton, R. H., Bernardi, M., Blanton, M. R., Burles, S., Castander, F. J., Connolly, A. J., Eisenstein, D. J., Frieman, J. A., Hennessy, G. S., Hindsley, R. B., Ivezíc, Ž., Kent, S., Kunszt, P. Z., Lee, B. C., Meiksin, A., Munn, J. A., Newberg, H. J., Nichol, R. C., , Nicinski, T., et al. (2002). Sloan digital sky survey: Early data release. Astronomical Journal, 123(1 1753), 485-548.
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  Abstract: The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of ≈ 106 galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg 2 of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux- and wavelength-calibrated, with 4096 pixels from 3800 to 9200 Å at R ≈ 1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.
• Anderson, S. F., Fan, X., Richards, G. T., Schneider, D. P., Strauss, M. A., E., D., Gunn, J. E., Knapp, G. R., Schlegel, D., Voges, W., Yanny, B., Bahcall, N. A., Bernardi, M., Brinkmann, J., Brunner, R., Csabái, I., Doi, M., Fukugita, M., Hennessy, G. S., , Ivezić, Ž., et al. (2001). High-redshift quasars found in sloan digital sky survey commissioning data. VI. Sloan digital sky survey spectrograph observations. Astronomical Journal, 122(2), 503-517.
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  Abstract: We present data for over 100 high-redshift quasars found in ≈700 deg2 by the Sloan Digital Sky Survey (SDSS), using automated selection algorithms applied to SDSS imaging data and with spectroscopic confirmation obtained during routine spectroscopic observations by the Sloan 2.5 m telescope. The SDSS spectra cover the wavelength range 3900-9200 Å at a spectral resolution of 1800 and have been obtained for 116 quasars with redshifts greater than 3.94; 92 of these objects were previously uncataloged, significantly increasing the current tally of published z > 4 quasars. The paper also reports observations of seven additional new z > 4 quasars; all were selected from the SDSS imaging survey and spectroscopically confirmed with observations by the Apache Point Observatory's 3.5 m telescope or the Hobby-Eberly Telescope. The i′ magnitudes of the quasars range from 18.03 to 20.68. Of the 99 new objects reported in this paper, 13 appear to be broad absorption line quasars. Five quasars, including one object at a redshift of 5.11, have 20 cm peak flux densities greater than 1 mJy. Two of the quasars, both at z ≈ 4.5, have very weak emission lines; one of these objects is a radio source. Nineteen of the newly discovered objects have redshifts above 4.6, and the maximum redshift is z = 5.41; among objects reported to date, the latter is the third highest redshift AGN and is penultimate in redshift among luminous quasars.
• Becker, R. H., Fan, X., White, R. L., Strauss, M. A., Narayanan, V. K., Lupton, R. H., Gunn, J. E., Annis, J., Bahcall, N. A., Brinkmann, J., Connolly, A. J., Csabai, I., Czarapata, P. C., Doi, M., Heckman, T. M., Hennessy, G. S., Ivezić, Ž., Knapp, G. R., Lamb, D. Q., , Mckay, T. A., et al. (2001). Evidence for reionization at z ∼ 6: Detection of a gunn-peterson trough in a z = 6.28 quasar. Astronomical Journal, 122(6), 2850-2857.
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  Abstract: We present moderate-resolution Keck spectroscopy of quasars at z = 5.82, 5.99, and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Lyα absorption in the spectra of these quasars evolves strongly with redshift. To z ∼ 5.7, the Lyα absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest-redshift object, SDSSp J103027.10+052455.0 (z = 6.28), the average transmitted flux is 0.0038 ± 0.0026 times that of the continuum level over 8450 Å < λ < 8710 Å (5.95 < zabs < 6.16), consistent with zero flux. Thus the flux level drops by a factor of greater than 150 and is consistent with zero flux in the Lyα forest region immediately blueward of the Lyα emission line, compared with a drop by a factor of ∼10 at zabs ∼ 5.3. A similar break is seen at Lyβ; because of the decreased oscillator strength of this transition, this allows us to put a considerably stronger limit, τeff > 20, on the optical depth to Lyα absorption at z = 6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Lyα forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z ∼ 5 to 6, and the universe is approaching the reionization epoch at z ∼ 6.
• Brandt, W. N., Guainazzi, M., Kaspi, S., Fan, X., Schneider, D. P., Strauss, M. A., Clavel, J., & Gunn, J. E. (2001). An XMM-Newton detection of the z = 5.80 X-ray-weak quasar SDSSp J104433.04012502.2. Astronomical Journal, 121(2), 591-597.
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  Abstract: We report on an XMM-Newton observation of the most distant known quasar, SDSSp J104433.04-012502.2, at z = 5.80. We have detected this quasar with high significance in the rest-frame 3.4-13.6 keV band, making it the most distant cosmic object detected in X-rays; 32 ± 9 counts were collected. SDSSp J104433.04-012502.2 is notably X-ray-weak relative to other luminous, optically selected quasars, with αox = -1.91 ± 0.05 and a 3.4-13.6 keV luminosity of ≈ 1.8 × 1044 ergs s-1. The most likely reason for its X-ray weakness is heavy absorption with NH ≳ 1024 cm-2, as is seen in some broad absorption line quasars and related objects; we discuss this and other possibilities. High-quality spectroscopy from 0.95-1.10 μm to search for blueshifted C IV absorption may elucidate the origin of the X-ray weakness.
• Carilli, C. L., Bertoldi, F., Rupen, M. P., Fan, X., Strauss, M. A., Menten, K. M., Kreysa, E., Schneider, D. P., Bertarini, A., Yun, M. S., & Zylka, R. (2001). A 250 GHz survey of high-redshift quasars from the sloan digital sky survey. Astrophysical Journal Letters, 555(2 PART 1), 625-632.
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  Abstract: We present observations at 250 GHz (1.2 mm), 43 GHz, and 1.4 GHz of a sample of 41 QSOs at z > 3.7 found in the Sloan Digital Sky Survey. We detect 16 sources with a 250 GHz flux density greater than 1.4 mJy. The combination of centimeter and millimeter wavelength observations indicates that the 250 GHz emission is most likely thermal dust emission. Assuming a dust temperature of 50 K, the implied dust masses for the 16 detected sources are in the range 1.5-5.9 × 108 M⊙, and the dust emitting regions are likely to be larger than 1 kpc in extent. The radio-through-optical spectral energy distributions for these sources are within the broad range defined by lower redshift, lower optical luminosity QSOs. We consider possible dust heating mechanisms, including UV emission from the active galactic nucleus (AGN) and a starburst concurrent with the AGN, with implied star formation rates between 500 and 2000 M⊙ yr-1.
• E., D., Richards, G. T., Bauer, A., Strauss, M. A., Schneider, D. P., Heckman, T. M., York, D. G., Hall, P. B., Fan, X., Knapp, G. R., Anderson, S. F., Annis, J., Bahcall, N. A., Bernardi, M., Briggs, J. W., Brinkmann, J., Brunner, R., Burles, S., Carey, L., , Castander, F. J., et al. (2001). Composite quasar spectra from the sloan digital sky survey. Astronomical Journal, 122(2), 549-564.
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  Abstract: We have created a variety of composite quasar spectra using a homogeneous data set of over 2200 spectra from the Sloan Digital Sky Survey (SDSS). The quasar sample spans a redshift range of 0.044 ≤ z ≤ 4.789 and an absolute r′ magnitude range of -18.0 to -26.5. The input spectra cover an observed wavelength range of 3800-9200 Å at a resolution of 1800. The median composite covers a rest-wavelength range from 800 to 8555 Å and reaches a peak signal-to-noise ratio of over 300 per 1 Å resolution element in the rest frame. We have identified over 80 emission-line features in the spectrum. Emission-line shifts relative to nominal laboratory wavelengths are seen for many of the ionic species. Peak shifts of the broad permitted and semiforbidden lines are strongly correlated with ionization energy, as previously suggested, but we find that the narrow forbidden lines are also shifted by amounts that are strongly correlated with ionization energy. The magnitude of the forbidden line shifts is ≲100 km s-1, compared with shifts of up to 550 km s-1 for some of the permitted and semiforbidden lines. At wavelengths longer than the Lyα emission, the continuum of the geometric mean composite is well fitted by two power laws, with a break at ≈5000 Å. The frequency power-law index, αv, is -0.44 from ≈1300 to 5000 Å and -2.45 redward of ≈5000 Å. The abrupt change in slope can be accounted for partly by host-galaxy contamination at low redshift. Stellar absorption lines, including higher order Balmer lines, seen in the composites suggest that young or intermediate-age stars make a significant contribution to the light of the host galaxies. Most of the spectrum is populated by blended emission lines, especially in the range 1500-3500 Å, which can make the estimation of quasar continua highly uncertain unless large ranges in wavelength are observed. An electronic table of the median quasar template is available.
• Fan, X., Narayanan, V. K., Lupton, R. H., Strauss, M. A., Knapp, G. R., Becker, R. H., White, R. L., Pentericci, L., Leggett, S. K., Haiman, Z., Gunn, J. E., Ivezić, Ž., Schneider, D. P., Anderson, S. F., Brinkmann, J., Bahcall, N. A., Connolly, A. J., Csabai, I., Mamoru, D., , Fukugita, M., et al. (2001). A survey of z > 5.8 quasars in the sloan digital sky survey. I. Discovery of three new quasars and the spatial density of luminous quasars at z ∼ 6. Astronomical Journal, 122(6), 2833-2849.
• Fan, X., Strauss, M. A., Richards, G. T., Newman, J. A., Becker, R. H., Schneider, D. P., Gunn, J. E., Davis, M., White, R. L., Lupton, R. H., Anderson Jr., J. E., Annis, J., Bahcall, N. A., Brunner, R. J., Csabai, I., Doi, M., Fukugita, M., Hennessy, G. S., Hindsley, R. B., , Ivezić, Ž., et al. (2001). High-redshift quasars found in sloan digital sky survey commissioning data. III. A color-selected sample at i* < 20 in the fall equatorial stripe. Astronomical Journal, 121(1), 31-53.
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  Abstract: This is the third paper in a series aimed at finding high-redshift quasars from five-color (u′g′r′i′z′) imaging data taken along the celestial equator by the Sloan Digital Sky Survey (SDSS) during its commissioning phase. In this paper, we first present the observations of 14 bright, high-redshift quasars (3.66 ≤ z ≤ 4.77, i* ≲ 20) discovered in the SDSS fall equatorial stripe, and the SDSS photometry of two previously known high-redshift quasars in the same region of the sky. Combined with the quasars presented in Paper I and by Schneider et al., we define a color-selected flux-limited sample of 39 quasars at 3.6 < z < 5.0 and i* ≲ 20, covering a total effective area of 182 deg2. From this sample, we estimate the average spectral power-law slope in the rest-frame UV for quasars at z ∼ 4 to be -0.79 with a standard deviation of 0.34, and the average rest-frame equivalent width of the Lyα + N v emission line to be 69 Å with a standard deviation of 18 Å. The selection completeness of this multicolor sample is determined from the model colors of high-redshift quasars, taking into account the distributions of emission-line strengths, intrinsic continuum slope, the line and continuum absorption from intervening material, and the effects of photometric errors. The average completeness of this sample is about 75%. The selection function calculated in this paper will be used to correct the incompleteness of this color-selected sample and to derive the high-redshift quasar luminosity function in a companion paper. In an appendix, we present the observations of an additional 18 faint quasars (3.57 ≤ z ≤ 4.80, 20.1 ≤ i* ≤ 20.8) discovered in the region on the sky that has been imaged twice. Several quasars presented in this paper exhibit interesting properties, including a radio-loud quasar at z = 4.77 and a narrow-line quasar (FWHM = 1500 km s-1) at z = 3.57.
• Fan, X., Strauss, M. A., Schneider, D. P., Gunn, J. E., Lupton, R. H., Becker, R. H., Davis, M., Newman, J. A., Richards, G. T., White, R. L., Anderson Jr., J. E., Annis, J., Bahcall, N. A., Brunner, R. J., Csabai, I., Hennessy, G. S., Hindsley, R. B., Fukugita, M., Kunszt, P. Z., , Ivezić, Ž., et al. (2001). High-redshift quasars found in sloan digital sky survey commissioning data. IV. Luminosity function from the fall equatorial stripe sample. Astronomical Journal, 121(1), 54-65.
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  Abstract: This is the fourth paper in a series aimed at finding high-redshift quasars from five-color (u′g′r′i′z′) imaging data taken along the celestial equator by the Sloan Digital Sky Survey during its commissioning phase. In this paper, we use the color-selected sample of 39 luminous high-redshift quasars presented in Paper III to derive the evolution of the quasar luminosity function over the range 3.6 < z < 5.0 and -27.5 < M1450 < -25.5 (Ω = 1, H0 = 50 km s-1 Mpc-1). We use the selection function derived in Paper III to correct for sample incompleteness. The luminosity function is estimated using three different methods: (1) the 1/Va estimator; (2) a maximum likelihood solution, assuming that the density of quasars depends exponentially on redshift and as a power law in luminosity; and (3) Lynden-Bell's nonparametric C- estimator. All three methods yield consistent results. The luminous quasar density decreases by a factor of ∼6 from z = 3.5 to z = 5.0, consistent with the decline seen from several previous optical surveys at z < 4.5. The luminosity function follows ψ(L) ∝ L-2.5 for z ∼ 4 at the bright end, significantly flatter than the bright-end luminosity function ψ(L) ∝ L-3.5 found in previous studies for z < 3, suggesting that the shape of the quasar luminosity function evolves with redshift as well, and that the quasar evolution from z = 2 to z = 5 cannot be described as pure luminosity evolution. Possible selection biases and the effect of dust extinction on the redshift evolution of the quasar density are also discussed.
• Goodrich, R. W., Campbell, R., Chaffee, F. H., Hill, G. M., Sprayberry, D., Brandt, W. N., Schneider, D. P., Kaspi, S., Fan, X., Gunn, J. E., & Strauss, M. A. (2001). J-band spectroscopy of the z = 5.74 broad absorption line quasar SDSSP J104433.04-012502.2. Astrophysical Journal Letters, 561(1 PART 2), L23-L25.
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  Abstract: We use NIRSPEC, a near-IR spectrometer on Keck II, to obtain moderate-resolution (R = 1540) spectroscopy that shows conclusively that the C IV emission line in the z = 5.74 quasar SDSSp J104433.04-012502.2 is accompanied by broad, blueshifted C IV absorption. The line has a "balnicity index" of 900 km s-1 and a rest-frame equivalent width of 13.1 ± 1.3 Å relative to the continuum. This confirms its membership in the class of objects called "broad absorption line" (BAL) quasi-stellar objects (QSOs). SDSSp J1044-0125 was previously suggested to be a BAL QSO based on its UV-to-X-ray flux ratio, which is larger than most non-BAL QSOs. The C IV emission is of normal strength, implying a metallicity similar to that found in other, lower redshift, QSOs.
• Harris, H. C., M., B., Liebert, J., E., D., Anderson, S. F., Knapp, G. R., Fan, X., Margon, B., Munn, J. A., Nichol, R. C., Pier, J. R., Schneider, D. P., Smith, J. A., Winget, D. E., York, D. G., Anderson Jr., J. E., Brinkmann, J., Burles, S., Chen, B., , Connolly, A. J., et al. (2001). A new very cool white dwarf discovered by the Sloan Digital Sky Survey. Astrophysical Journal Letters, 549(1 PART 2), L109-L113.
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  Abstract: Early data taken during commissioning of the Sloan Digital Sky Survey (SDSS) have resulted in the discovery of a very cool white dwarf. It appears to have stronger collision-induced absorption from molecular hydrogen than any other known white dwarf, suggesting it has a cooler temperature than any other. While its distance is presently unknown, it has a surprisingly small proper motion, making it unlikely to be a halo star. An analysis of white dwarf cooling times suggests that this object may be a low-mass star with a helium core. The SDSS imaging and spectroscopy also recovered LHS 3250, the coolest previously known white dwarf, indicating that the SDSS will be an effective tool for identifying these extreme objects.
• Ivezić, Ž., Tabachnik, S., Rafikov, R., Lupton, R. H., Quinn, T., Hammergren, M., Eyer, L., Chu, J., Armstrong, J. C., Fan, X., Finlator, K., Geballe, T. R., Gunn, J. E., Hennessy, G. S., Knapp, G. R., Leggett, S. K., Munn, J. A., Pier, J. R., Rockosi, C. M., , Schneider, D. P., et al. (2001). Solar system objects observed in the Sloan Digital Sky Survey commissioning data. Astronomical Journal, 122(5), 2749-2784.
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  Abstract: We discuss measurements of the properties of ∼ 13,000 asteroids detected in 500 deg2 of sky in the Sloan Digital Sky Survey (SDSS) commissioning data. The moving objects are detected in the magnitude range 14 < r* < 21.5, with a baseline of ∼ 5 minutes, resulting in typical velocity errors of ∼ 3%. Extensive tests show that the sample is at least 98% complete, with a contamination rate of less than 3%. We find that the size distribution of asteroids resembles a broken power law, independent of the heliocentric distance: D-2.3 for 0.4 km ≲ D ≲ 5 km, and D-4 for 5 km ≲ D ≲ 40 km. As a consequence of this break, the number of asteroids with r* < 21.5 is 10 times smaller than predicted by extrapolating the power-law relation observed for brighter asteroids (r* ≲ 18). The observed counts imply that there are about 670,000 objects with D > 1 km in the asteroid belt, or up to 3 times less than previous estimates. The revised best estimate for the impact rate of the so-called "killer" asteroids (D > 1 km) is about 1 every 500,000 yr, uncertain to within a factor of 2. We predict that by its completion SDSS will obtain about 100,000 near simultaneous five-band measurements for a subset drawn from 340,000 asteroids brighter than r* < 21.5 at opposition. Only about a third of these asteroids have been previously observed, and usually in just one band. The distribution of main-belt asteroids in the four-dimensional SDSS color space is bimodal, and the two groups can be associated with S-(rocky) and C-(carbonaceous) type asteroids. A strong bimodality is also seen in the heliocentric distribution of asteroids: the inner belt is dominated by S-type asteroids centered at R ∼ 2.8 AU, while C-type asteroids, centered at R ∼ 3.2 AU, dominate the outer belt. The median color of each class becomes bluer by about 0.03 mag AU-1 as the heliocentric distance increases. The observed number ratio of S and C asteroids in a sample with r* < 21.5 is 1.5:1, while in a sample limited by absolute magnitude it changes from 4:1 at 2 AU, to 1:3 at 3.5 AU. In a size-limited sample with D > 1 km, the number ratio of S and C asteroids in the entire main belt is 1:2.3. The colors of Hungarias, Mars crossers, and near-Earth objects, selected by their velocity vectors, are more similar to the C-type than to S-type asteroids. In about 100 deg2 of sky along the celestial equator observed twice 2 days apart, we find one plausible Kuiper belt object (KBO) candidate, in agreement with the expected KBO surface density. The colors of the KBO candidate are significantly redder than the asteroid colors, in agreement with colors of known KBOs. We explore the possibility that SDSS data can be used to search for very red, previously uncataloged asteroids observed by 2MASS, by extracting objects without SDSS counterparts. We do not find evidence for a significant population of such objects; their contribution is no more than 10% of the asteroid population.
• Menou, K., E., D., Ivezić, Ž., S., R., Knapp, G. R., Richards, G. T., Strateva, I., Fan, X., Gunn, J. E., Hall, P. B., Heckman, T., Krolik, J., Lupton, R. H., Schneider, D. P., York, D. G., Anderson, S. F., Bahcall, N. A., Brinkmann, J., Brunner, R., , Csabai, I., et al. (2001). Broad absorption line quasars in the sloan digital sky survey with VLA first radio detections. Astrophysical Journal Letters, 561(2 PART 1), 645-652.
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  Abstract: We present 13 broad absorption line (BAL) quasars, including 12 new objects, identified in the Sloan Digital Sky Survey (SDSS) and matched within 2″ to sources in the FIRST radio survey catalog. The surface density of this sample of radio-detected BAL quasars is 4.5 ± 1.2 per 100 deg2, i.e., approximately 4 times as high as previously found by the shallower FIRST Bright Quasar Survey (FBQS). A majority of these radio-detected BAL quasars are moderately radio-loud objects. The fraction of BAL quasars in the entire radio quasar sample, 4.8% ± 1.3%, is comparable to the fraction of BAL quasars among the SDSS optical quasar sample (ignoring selection effects). We estimate that the true fraction of BAL quasars (mostly "HiBALs") in the radio sample is 9.2% ± 2.6%, once selection effects are accounted for. We caution that the absorption troughs of four of the 13 radio-detected quasars considered do not strictly satisfy the standard BALnicity criteria. One or possibly two of the new radio-detected BAL quasars are of the rare "FeLoBAL" type. BAL quasars are generally redder than the median SDSS quasar at the same redshift.
• Richards, G. T., Fan, X., Schneider, D. P., E., D., Strauss, M. A., York, D. G., Anderson Jr., J. E., Anderson, S. F., Annis, J., Bahcall, N. A., Bernardi, M., Briggs, J. W., Brinkmann, J., Brunner, R., Burles, S., Carey, L., Castander, F. J., Connolly, A. J., Crocker, J. H., , Csabai, I., et al. (2001). Colors of 2625 quasars at 0 < z < 5 measured in the sloan digital sky survey photometric system. Astronomical Journal, 121(5), 2308-2330.
• Richards, G. T., Weinstein, M. A., Schneider, D. P., Fan, X., Strauss, M. A., E., D., Annis, J., Burles, S., Laubacher, E. M., York, D. G., Frieman, J. A., Johnston, D., Scranton, R., Gunn, J. E., Ivezić, Ž., Nichol, R. C., Budavári, T., Csabai, I., Szalay, A. S., , Connolly, A. J., et al. (2001). Photometric redshifts of quasars. Astronomical Journal, 122(3), 1151-1162.
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  Abstract: We demonstrate that the design of the Sloan Digital Sky Survey (SDSS) filter system and the quality of the SDSS imaging data are sufficient for determining accurate and precise photometric redshifts of quasars. Using a sample of 2625 quasars, we show that "photo-z" determination is even possible for z ≤ 2.2 despite the lack of a strong continuum break, which robust photo-z techniques normally require. We find that, using our empirical method on our sample of objects known to be quasars, approximately 70% of the photometric redshifts are correct to within Δz = 0.2; the fraction of correct photometric redshifts is even better for z > 3. The accuracy of quasar photometric redshifts does not appear to be dependent upon magnitude to nearly 21st magnitude in i′. Careful calibration of the color-redshift relation to 21st magnitude may allow for the discovery of ∼106 quasar candidates in addition to the 105 quasars that the SDSS will confirm spectroscopically. We discuss the efficient selection of quasar candidates from imaging data for use with the photometric redshift technique and the potential scientific uses of a large sample of quasar candidates with photometric redshifts.
• Schneider, D. P., Fan, X., Strauss, M. A., Gunn, J. E., Richards, G. T., Hill, G. J., MacQueen, P. J., Ramsey, L. W., Adams, M. T., Booth, J. A., Hill, G. M., Knapp, G. R., Lupton, R. H., Saxe, D. H., Shetrone, M., Tufts, J. R., E., D., Wolf, M. J., York, D. G., , Anderson Jr., J. E., et al. (2001). High-redshift quasars found in sloan digital sky survey commissioning data. V. Hobby-Eberly telescope observations. Astronomical Journal, 121(3), 1232-1240.
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  Abstract: We report the discovery of 27 quasars with redshifts between 3.58 and 4.49. The objects were identified as high-redshift candidates based on their colors in Sloan Digital Sky Survey commissioning data. The redshifts were confirmed with low-resolution spectra obtained at the Hobby-Eberly Telescope. The quasars' i* magnitudes range from 18.55 to 20.97. Nearly 60% of the quasar candidates observed are confirmed spectroscopically as quasars. Two of the objects are broad absorption line quasars, and several other quasars appear to have narrow associated absorption features.
• Vignali, C., Brandt, W. N., Fan, X., Gunn, J. E., Kaspi, S., Schneider, D. P., & Strauss, M. A. (2001). Exploratory Chandra observations of the highest-redshift quasars: X-rays from the dawn of the modern universe. Astronomical Journal, 122(5), 2143-2155.
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  Abstract: We report exploratory Chandra observations of 14 high-redshift (z = 4.06-5.27), optically selected quasars. Ten of these quasars are detected, increasing the number of z > 4 X-ray detected quasars by 71%. Our detections include four of the five highest-redshift X-ray detected quasars to date, among them SDSSp J021043.17-001818.4, the highest-redshift (z = 4.77) radio-loud quasar detected in the X-ray band. The four undetected objects are the broad absorption-line quasars SDSSp J112956.10-014212.4 and SDSSp J160501.21-011220.0, the weak emission-line quasar SDSSp J153259.96-003944.1, and the quasar PSS 1435 + 3057. A comparison of the quasars' spectral energy distributions (by means of the optical-to-X-ray spectral index αox) with those of lower-redshift samples indicates that the Chandra quasars are X-ray fainter by a factor of ≈2. X-ray faintness could be associated with the presence of large amounts of gas in the primeval galaxies harboring these high-redshift quasars, as suggested by recent studies conducted on z > 4 quasars in other bands. Using the current Chandra data, predictions for the next generation of X-ray observatories, Constellation-X and XEUS, are also provided.
• Carilli, C. L., Bertoldi, F., Menten, K. M., Rupen, M. P., Kreysa, E., Fan, X., Strauss, M. A., Schneider, D. P., Bertarini, A., Yun, M. S., & Zylka, R. (2000). Dust emission from high-redshift QSOs. Astrophysical Journal Letters, 533(1 PART 2), L13-L16.
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  Abstract: We present detections of emission at 250 GHz (1.2 mm) from two high-redshift QSOs from the Sloan Digital Sky Survey sample using the bolometer array at the IRAM 30 m telescope. The sources are SDSSp 015048.83 + 004126.2 at z = 3.7 and SDSSp J033829.31 + 002156.3 at z = 5.0; the latter is the third highest redshift QSO known and the highest redshift millimeter-emitting source yet identified. We also present deep radio continuum imaging of these two sources at 1.4 GHz using the Very Large Array. The combination of centimeter and millimeter observations indicate that the 250 GHz emission is most likely thermal dust emission, with implied dust masses ≈ 108 M ⊙. We consider possible dust heating mechanisms, including UV emission from the active galactic nucleus (AGN) and a massive starburst concurrent with the AGN, with implied star formation rates greater than 103 M ⊙ yr-1.
• Fan, X., Knapp, G. R., Strauss, M. A., Gunn, J. E., Lupton, R. H., Ivezić, Ž., Rockosi, C. M., Yanny, B., Kent, S., Schneider, D. P., Kirkpatrick, J. D., Annis, J., Bastian, S., Berman, E., Brinkmann, J., Csabai, I., Federwitz, G. R., Fukugita, M., Gurbani, V. K., , Hennessy, G. S., et al. (2000). L dwarfs found in sloan digital sky survey commissioning imaging data. Astronomical Journal, 119(2), 928-935.
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  Abstract: This paper describes the discovery of seven dwarf objects of spectral type L (objects cooler than the latest M dwarfs) in commissioning imaging data taken by the Sloan Digital Sky Survey (SDSS). Low-resolution spectroscopy shows that they have spectral types from L0 to L8. Comparison of the SDSS and the Two Micron All Sky Survey (2MASS) photometry for several of these objects indicates the presence of significant opacity at optical wavelengths. This comparison also demonstrates the high astrometric accuracy (better than 1″ for these faint sources) of both surveys. The L dwarfs are shown to occupy a distinctive region of color-color space as measured in the SDSS filters, which should enables their identification in a straightforward way. This should lead eventually to a complete sample of many hundreds of these low-mass objects, or about 1 per 15 deg2 to i′ ≈ 20, in the complete SDSS data set.
• Fan, X., Strauss, M. A., Schneider, D. P., Gunn, J. E., Lupton, R. H., Anderson, S. F., Voges, W., Margon, B., Annis, J., Bahcall, N. A., Brinkmann, J., Brunner, R. J., Carr, M. A., Csabai, I., Doi, M., Frieman, J. A., Fukugita, M., Hennessy, G. S., Hindsley, R. B., , Ivezić, Ž., et al. (2000). High-redshift quasars found in sloan digital sky survey commissioning data. II. The spring equatorial stripe. Astronomical Journal, 119(1), 1-11.
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  Abstract: This is the second paper in a series aimed at finding high-redshift quasars from five-color (u′g′r′i′z′) imaging data taken along the Celestial Equator by the Sloan Digital Sky Survey (SDSS) during its commissioning phase. In this paper, we present 22 high-redshift quasars (z > 3.6) discovered from ∼250 deg2 of data in the spring Equatorial Stripe, plus photometry for two previously known high-redshift quasars in the same region of the sky. Our success rate in identifying high-redshift quasars is 68%. Five of the newly discovered quasars have redshifts higher than 4.6 (z = 4.62, 4.69, 4.70, 4.92, and 5.03). All the quasars have i* < 20.2 with absolute magnitude -28.8 < MB < -26.1 (h = 0.5, q0 = 0.5). Several of the quasars show unusual emission and absorption features in their spectra, including an object at z = 4.62 without detectable emission lines, and a broad absorption line (BAL) quasar at z = 4.92.
• Finlator, K., Ivezić, Ž., Fan, X., Strauss, M. A., Knapp, G. R., Lupton, R. H., Gunn, J. E., Rockosi, C. M., Anderson, J. E., Csabai, I., Hennessy, G. S., Hindsley, R. B., McKay, T. A., Nichol, R. C., Schneider, D. P., Smith, J. A., & York, D. G. (2000). Optical and infrared colors of stars observed by the Two Micron All Sky Survey and the Sloan Digital Sky Survey. Astronomical Journal, 120(5), 2615-2626.
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  Abstract: We discuss optical and infrared photometric properties of stars matched in the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) commissioning data for ∼50 deg2 of sky along the celestial equator centered at l = 150°, b = -60°. About 98% (∼63,000) of objects listed in the 2MASS Point Source Catalog in the analyzed area are matched within 2″ to an SDSS source. The matched sources represent 8% of the ∼800,000 SDSS sources in this area. They are predominantly red sources, as expected, and 15% of them are resolved in SDSS imaging data although they are detected as point sources in 2MASS data. The distribution of positional discrepancies for the matched sources and the astrometric statistics for the multiply observed SDSS sources imply that the astrometric accuracy of both surveys is about 0″.1 per coordinate (rms). For about 14,000 stars with the smallest photometric errors (≲10%) in both surveys, we present optical and infrared color-magnitude and color-color diagrams. We use optical (SDSS) colors to identify the stellar spectral sequence and show that stars of different spectral types can have similar infrared colors, thus making the classification of stars based on only 2MASS data very difficult. However, a broad separation into "early" and "late" spectral types (relative to type K0) is possible with a reliability of ∼95% even with 2MASS colors alone. The distributions of matched sources in color-magnitude and color-color diagrams are compared with the predictions of a stellar population synthesis code. We find that the models are in fair overall agreement with the data. In particular, the total number counts agree to better than 10%, and the morphologies of the color-magnitude and color-color diagrams appear similar. The most significant discrepancies are found for the number ratio of "early" to "late" type stars (by about a factor of 2) and in the colors of M stars (up to 0.2 mag). The first disagreement indicates that some parameters of the standard Galactic structure model and/or initial mass function can be improved, and the second disagreement emphasizes known difficulties with the modeling of stellar atmospheres for cool stars.
• Ivezić, Ž., Goldston, J., Finlator, K., Knapp, G. R., Yanny, B., Mckay, T. A., Amrose, S., Krisciunas, K., Willman, B., Anderson, S., Schaber, C., Erb, D., Logan, C., Stubbs, C., Chen, B., Neilsen, E., Uomoto, A., Pier, J. R., Fan, X., , Gunn, J. E., et al. (2000). Candidate RR Lyrae stars found in Sloan Digital Sky Survey commissioning data. Astronomical Journal, 120(2), 963-977.
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  Abstract: We present a sample of 148 candidate RR Lyrae stars selected from Sloan Digital Sky Survey (SDSS) commissioning data for about 100 deg2 of sky surveyed twice with Δt = 1.9946 days. Although the faint-magnitude limit of the SDSS allows us to detect RR Lyrae stars to large Galactocentric distances (∼100 kpc, or r* ∼ 21), we find no candidates fainter than r* ∼ 20, i.e., farther than ∼65 kpc from the Galactic center. On the assumption that all 148 candidates are indeed RR Lyrae stars (contamination by other species of variable star is probably less than 10%), we find that their volume density has roughly a power-law dependence on Galactocentric radius, R-2.7±0.2, between 10 and 50 kpc and drops abruptly at R ∼ 50-60 kpc, possibly indicating a sharp edge to the stellar halo as traced by RR Lyrae stars. The Galactic distribution of stars in this sample is very inhomogeneous and shows a clump of over 70 stars at about 45 kpc from the Galactic center. This clump is also detected in the distribution of nonvariable objects with RR Lyrae star colors. When sources in the clump are excluded, the best power-law fit becomes consistent with the R-3 distribution found from surveys of bright RR Lyrae stars. These results imply that the halo contains clumpy overdensities inhomogeneously distributed within a smooth R-3 background, with a possible cutoff at ∼50 kpc.
• Kong, X., Zhou, X., Chen, J., Cheng, F., Jiang, Z., Zhu, J., Zheng, Z., Mao, S., Shang, Z., Fan, X., Byun, Y., Chen, R., Chen, W., Deng, L., Hester, J. J., Yong, L. i., Lin, W., Hongjun, S. u., Sun, W., , Tsay, W., et al. (2000). Spatially resolved spectrophotometry of M81: Age, metallicity, and reddening maps. Astronomical Journal, 119(6), 2745-2756.
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  Abstract: In this paper we present a multicolor photometric study of the nearby spiral galaxy M81, using images obtained with the Beijing Astronomical Observatory 60/90 cm Schmidt telescope in 13 intermediate-band filters from 3800 to 10000 Å. The observations cover the whole area of M81, with a total integration of 51 hr from 1995 to 1997 February. This provides a multicolor map of M81 in pixels of 1″7. × 1.″7. Using theoretical stellar population synthesis models, we demonstrate that some BATC (Beijing-Arizona-Taiwan-Connecticut Multicolor Sky Survey) colors and color indices can be used to disentangle the age and metallicity effect. We compare in detail the observed properties of M81 with the predictions from population synthesis models and quantify the relative chemical abundance, age, and reddening distributions for different components of M81. We find that the metallicity of M81 is about Z = 0.03, with no significant difference over the whole galaxy. In contrast, an age gradient is found between stellar populations of the central regions and of the bulge and disk regions of M81: the stellar population in its central regions is older than 8 Gyr, while the disk stars are considerably younger (∼2 Gyr). We also give the reddening distribution in M81. Some dust lanes are found in the galaxy bulge region, and the reddening in the outer disk is higher than that in the central regions.
• Leggett, S. K., Geballe, T. R., Fan, X., Schneider, D. P., Gunn, J. E., Lupton, R. H., Knapp, G. R., Strauss, M. A., Mcdaniel, A., Golimowski, D. A., Henry, T. J., Peng, E., Tsvetanov, Z. I., Uomoto, A., Zheng, W., Hill, G. J., Ramsey, L. W., Anderson, S. F., Annis, J. A., , Bahcall, N. A., et al. (2000). The missing link: Early methane ("T") dwarfs in the sloan digital Sky Survey. Astrophysical Journal Letters, 536(1 PART 2), L35-L38.
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  Abstract: We report the discovery of three cool brown dwarfs that fall in the effective temperature gap between the latest L dwarfs currently known, with no methane absorption bands in the 1-2.5 μm range, and the previously known methane (T) dwarfs, whose spectra are dominated by methane and water. The newly discovered objects were detected as very red objects in the Sloan Digital Sky Survey imaging data and have JHK colors between the red L dwarfs and the blue Gl 229B-like T dwarfs. They show both CO and CH4 absorption in their near-infrared spectra in addition to H2O, with weaker CH4 absorption features in the H and K bands than those in all other methane dwarfs reported to date. Due to the presence of CH4 in these bands, we propose that these objects are early T dwarfs. The three form part of the brown dwarf spectral sequence and fill in the large gap in the overall spectral sequence from the hottest main-sequence stars to the coolest methane dwarfs currently known.
• Schneider, D. P., Fan, X., Strauss, M. A., Gunn, J. E., Richards, G. T., Knapp, G. R., Lupton, R. H., Saxe, D. H., Anderson Jr., J. E., Bahcall, N. A., Brinkmann, J., Brunner, R., Csabai, I., Fukugita, M., Hennessy, G. S., Hindsley, R. B., Ivezić, Ž., Nichol, R. C., Pier, J. R., & York, D. G. (2000). Discovery of a pair of z = 4.25 quasars from the Sloan Digital Sky Survey. Astronomical Journal, 120(5), 2183-2189.
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  Abstract: We report the discovery of a pair of z = 4.25 quasars with a separation of 33″. The brighter of the two objects was identified as a high-redshift quasar candidate from Sloan Digital Sky Survey multicolor imaging data, and the redshift was measured from a spectrum obtained with the Hobby-Eberly Telescope. The slit orientation of this observation by chance included another quasar, approximately 1 mag fainter and having the same redshift as the target. This is the third serendipitous discovery of a z > 4 quasar. The differences in the relative strengths and profiles of the emission lines suggest that this is a quasar pair and not a gravitational lens. The two objects are likely to be physically associated; the projected physical separation is approximately 210 h-150 kpc and the redshifts are identical to ≈0.01, implying a radial physical separation of 950 h-150 kpc or less. The existence of this pair is strong circumstantial evidence that z ∼ 4 quasars are clustered.
• Schneider, D. P., Hill, G. J., Fan, X., Ramsey, L. W., Macqueen, P. J., Weedman, D. W., Booth, J. A., Eracleous, M., Gunn, J. E., Lupton, R. H., Adams, M. T., Bastian, S., Bender, R., Berman, E., Brinkmann, J., Csabai, I., Federwitz, G., Gurbani, V., Hennessy, G. S., , Hill, G. M., et al. (2000). The Low-Resolution Spectrograph of the Hobby-Eberly Telescope. II. Observations of quasar candidates from the Sloan Digital Sky Survey. Publications of the Astronomical Society of the Pacific, 112(767), 6-11.
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  Abstract: This paper describes spectra of quasar candidates acquired during the commissioning phase of the Low-Resolution Spectrograph of the Hobby-Eberly Telescope. The objects were identified as possible quasars from multicolor image data from the Sloan Digital Sky Survey. The 10 sources had typical r′ magnitudes of 19-20, except for one extremely red object with r′ ≈ 23. The data, obtained with exposure times between 10 and 25 minutes, reveal that the spectra of four candidates are essentially featureless and are not quasars, five are quasars with redshifts between 2.92 and 4.15 (including one broad absorption line quasar), and the red source is a very late M star or early L dwarf.
• Tsvetanov, Z. I., Golimowski, D. A., Zheng, W., Geballe, T. R., Leggett, S. K., Ford, H. C., Davidsen, A. F., Uomoto, A., Fan, X., Knapp, G. R., Strauss, M. A., Brinkmann, J., Lamb, D. Q., Newberg, H. J., Rechenmacher, R., Schneider, D. P., York, D. G., Lupton, R. H., Pier, J. R., , Annis, J., et al. (2000). The discovery of a second field methane brown dwarf from sloan digital sky survey commissioning data. Astrophysical Journal Letters, 531(1 PART 2), L61-L65.
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  Abstract: We report the discovery of a second field methane brown dwarf from the commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS J134646.45-003150.4 (hereafter SDSS 1346-00), was selected because of its very red color and stellar appearance. Its spectrum between 0.8 and 2.5 μm is dominated by strong absorption bands of H2O and CH4 and closely mimics those of Gliese 229B and SDSS 162414.37+002915.6 (hereafter SDSS 1624+00), two other known methane brown dwarfs. SDSS 1346-00 is approximately 1.5 mag fainter than Gliese 229B, suggesting that it lies about 11 pc from the Sun. The ratio of flux at 2.1 μm to that at 1.27 μm is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which suggests that SDSS 1346-00 has a slightly higher effective temperature than the others. Based on a search area of 130 deg2 and a detection limit of z* = 19.8, we estimate a space density of 0.05 pc-3 for methane brown dwarfs with Teff ∼ 1000 K in the 40 pc3 volume of our search. This estimate is based on small-sample statistics and should be treated with appropriate caution.
• Xiaohui, F., White, R. L., Davis, M., Becker, R. H., Strauss, M. A., Haiman, Z., Schneider, D. P., Gregg, M. D., Gunn, J. E., Knapp, G. R., Lupton, R. H., Anderson Jr., J. E., Anderson, S. F., Annis, J., Bahcall, N. A., Boroski, W. N., Brunner, R. J., Chen, B., Connolly, A. J., , Csabai, I., et al. (2000). The discovery of a luminous z = 5.80 quasar from the sloan digital sky survey. Astronomical Journal, 120(3), 1167-1174.
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  Abstract: We present observations of SDSSp J104433.04-012502.2, a luminous quasar at z = 5.80 discovered from Sloan Digital Sky Survey (SDSS) multicolor imaging data. This object was selected as an i′-band dropout object, with i* = 21.8 ± 0.2 and z* = 19.2 ± 0.1. It has an absolute magnitude M1450 = -27.2 (H0 = 50 km s-1 Mpc-1, q0 = 0.5). The spectrum shows a strong and broad Lyα emission line, strong Lyα forest absorption lines with a mean continuum decrement DA = 0.91 and a Lyman limit system at z = 5.72. The spectrum also shows strong O I and Si IV emission lines similar to those of quasars at z ≲ 5, suggesting that these metals were produced at a redshift beyond 6. The lack of a Gunn-Peterson trough in the spectrum indicates that the universe is already highly ionized at z ∼ 5.8. Using a high-resolution spectrum in the Lyα forest region, we place a conservative upper limit on the optical depth because of the Gunn-Peterson effect of τ < < 0.5 in regions of minimum absorption. The Lyα forest absorption in this object is much stronger than that in quasars at z ≲ 5. The object is unresolved in a deep image with excellent seeing, implying that it is unlensed. The black hole mass of this quasar is ∼3 × 109 M⊙ if we assume no lensing amplification and that it is radiating at the Eddington luminosity, implying that it resides in a very massive dark matter halo. The discovery of one quasar at M1450 < -27 in a survey area of 600 deg2 is consistent with an extrapolation of the observed luminosity function at lower redshifts. The abundance and evolution of such quasars can provide sensitive tests for models of quasar and galaxy formation.
• Yan, H., Burstein, D., Fan, X., Zheng, Z., Chen, J., Byun, Y., Chen, R., Chen, W., Deng, L., Deng, Z., Fang, L. Z., Hester, J. J., Jiang, Z., Yong, L. i., Lin, W., Phillip, L. u., Shang, Z., Hongjun, S. u., Sun, W., , Tsay, W., et al. (2000). Calibration of the BATC survey: Methodology and accuracy. Publications of the Astronomical Society of the Pacific, 112(771), 691-702.
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  Abstract: We describe in detail the extinction correction procedures used for the Beijing-Arizona-Taiwan-Connecticut Sky Survey (BATC Survey). The survey covers the spectral range 3200-9900 Å by utilizing a set of 15 intermediate-band filters. These filters are specifically designed to exclude most of the bright and variable night-sky emission lines. We also present extinction coefficients for the filter passbands for typical photometric nights at the Xinglong Observing Station, Beijing Astronomical Observatory (where the observations of the survey are being carried out). Time-dependent, low-amplitude (∼1%), nightly extinction variation has been observed. Such variation is demonstrably independent of filter bandpass and air mass, with amplitudes ranging from ∼0.01 to ∼0.03 mag. The variation is plausibly caused by slowly varying (at ∼1%) atmospheric extinction, possibly related to changes in air pressure/temperature/humidity that occur during the night. An iterative fitting scheme has been developed to take this time-varying component into account. We conclude that the survey can achieve its stated observational goal, namely, an absolute photometric calibration that is tied to the ABV system to an accuracy of 1% in all filters.
• York, D. G., Adelman, J., Anderson Jr., J. E., Anderson, S. F., Annis, J., Bahcall, N. A., Bakken, J. A., Barkhouser, R., Bastian, S., Berman, E., Boroski, W. N., Bracker, S., Briegel, C., Briggs, J. W., Brinkmann, J., Brunner, R., Burles, S., Carey, L., Carr, M. A., , Castander, F. J., et al. (2000). The Sloan Digital Sky Survey: Technical summary. Astronomical Journal, 120(3), 1579-1587.
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  Abstract: The Sloan Digital Sky Survey (SDSS) will provide the data to support detailed investigations of the distribution of luminous and nonluminous matter in the universe : a photometrically and astrometrically calibrated digital imaging survey of π sr above about Galactic latitude 30° in five broad optical bands to a depth of g′ ∼ 23 mag, and a spectroscopic survey of the approximately 106 brightest galaxies and 105 brightest quasars found in the photometric object catalog produced by the imaging survey. This paper summarizes the observational parameters and data products of the SDSS and serves as an introduction to extensive technical on-line documentation. © 2000. The American Astronomical Society. All rights reserved.
• Zheng, W., Tsvetanov, Z. I., Schneider, D. P., Fan, X., Becker, R. H., Davis, M., White, R. L., Strauss, M. A., Anderson Jr., J. E., Annis, J., Bahcall, N. A., Connolly, A. J., Csabai, I., Davidsen, A. F., Fukugita, M., Gunn, J. E., Heckman, T. M., Hennessy, G. S., Ivezić, Ž., , Knapp, G. R., et al. (2000). Five high-redshift quasars discovered in commissioning imaging data of the sloan digital sky survey. Astronomical Journal, 120(4), 1607-1611.
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  Abstract: We report the discovery of five quasars with redshifts of 4.67-5.27 and z′-band magnitudes of 19.5-20.7 (MB ∼ -27). All were originally selected as distant quasar candidates in optical/near-infrared photometry from the Sloan Digital Sky Survey (SDSS), and most were confirmed as probable high-redshift quasars by supplementing the SDSS data with J and K measurements. The quasars possess strong, broad Lyα emission lines, with the characteristic sharp cutoff on the blue side produced by Lyα forest absorption. Three quasars contain strong, broad absorption features, and one of them exhibits very strong N v emission. The amount of absorption produced by the Lyα forest increases toward higher redshift, and that in the z = 5.27 object (DA ≈ 0.7) is consistent with a smooth extrapolation of the absorption seen in lower redshift quasars. The high luminosity of these objects relative to most other known objects at z ≳ 5 makes them potentially valuable as probes of early quasar properties and of the intervening intergalactic medium.
• Fan, X. (1999). Simulation of stellar objects in SDSS color space. Astronomical Journal, 117(5), 2528-2551.
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  Abstract: We present a simulation of the spatial, luminosity and spectral distributions of four types of stellar objects. We simulate (1) Galactic stars, based on a Galactic structure model, a stellar population synthesis model, stellar isochrones, and stellar spectral libraries; (2) white dwarfs, based on model atmospheres, the observed luminosity function, mass distribution, and Galactic distribution of white dwarfs; (3) quasars, based on their observed luminosity function and its evolution, and models of emission and absorption spectra of quasars; and (4) compact emission-line galaxies, based on the observed distribution of their spectral properties and sizes. The results are presented in the color system of the Sloan Digital Sky Survey (SDSS), with realistic photometric error and Galactic extinction. The simulated colors of stars and quasars are compared with observations in the SDSS system and show good agreement. The stellar simulation can be used as a tool to analyze star counts and constrain models of Galactic structure, as well as to identify stars with unusual colors. The simulation can also be used to establish the quasar target selection algorithm for the SDSS.
• Fan, X., Strauss, M. A., Gunn, J. E., Lupton, R. H., Carilli, C. L., Rupen, M. P., Schmidt, G. D., Moustakas, L. A., Davis, M., Annis, J., Bahcall, N. A., Brinkmann, J., Brunner, R. J., Csabai, I., Doi, M., Fukugita, M., Heckman, T. M., Hennessy, G. S., Hindsley, R. B., , Ivezić, Ž., et al. (1999). The discovery of a high-redshift quasar without emission lines from Sloan Digital Sky Survey commissioning data. Astrophysical Journal Letters, 526(2 PART 2), L57-L60.
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  Abstract: We report observations of a luminous unresolved object at redshift z = 4.62, with a featureless optical spectrum redward of the Lyα forest region, discovered from Sloan Digital Sky Survey commissioning data. The redshift is determined by the onset of the Lyα forest at λ ∼ 6800 Å and a Lyman limit system at λ = 5120 Å. A strong Lya absorption system with weak metal absorption lines at z = 4.58 is also identified in the spectrum. The object has a continuum absolute magnitude of -26.6 at 1450 Å in the rest frame (h0 = 0.5, q0 = 0.5) and therefore cannot be an ordinary galaxy. It shows no radio emission (the 3 σ upper limit of its flux at 6 cm is 60 μJy), indicating a radio-to-optical flux ratio at least as small as that of the radio-weakest BL Lacertae objects known. It is also not linearly polarized to a 3 σ upper limit of 4% in the observed I band. Therefore, it is either the most distant BL Lac object known to date, with very weak radio emission, or a new type of unbeamed quasar, whose broad emission line region is very weak or absent.
• Fan, X., Strauss, M. A., Schneider, D. P., Gunn, J. E., Lupton, R. H., Yanny, B., Anderson, S. F., Anderson Jr., J. E., Annis, J., Bahcall, N. A., Bakken, J. A., Bastian, S., Berman, E., Boroski, W. N., Briegel, C., Briggs, J. W., Brinkmann, J., Carr, M. A., Colestock, P. L., , Connolly, A. J., et al. (1999). High-redshift quasars found in Sloan Digital Sky Survey commissioning data. Astronomical Journal, 118(1), 1-13.
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  Abstract: We present photometric and spectroscopic observations of 15 high-redshift quasars (z > 3.6) discovered from ∼140 deg2 of five-color (u′, g′, r′, i′, and z′) imaging data taken by the Sloan Digital Sky Survey (SDSS) during its commissioning phase. The quasars are selected by their distinctive colors in SDSS multicolor space. Four of the quasars have redshifts higher than 4.6 (z = 4.63, 4.75, 4.90, and 5.00, the latter being the highest redshift quasar yet known). In addition, two previously known z > 4 objects were recovered from the data. The quasars all have i* < 20 and have luminosities comparable to that of 3C 273. The spectra of the quasars have similar features (strong, broad emission lines and substantial absorption blueward of the Lyα emission line) seen in previously known high-redshift quasars. Although the photometric accuracy and image quality fail to meet the final survey requirements, our success rate for identifying high-redshift quasars (17 quasars from 27 candidates) is much higher than that of previous multicolor surveys. However, the numbers of high-redshift quasars found is in close accord with the number density inferred from previous surveys.
• Kepner, J., Fan, X., Bahcall, N., Gunn, J., Lupton, R., & Guohong, X. u. (1999). An automated cluster finder: The adaptive matched filter. Astrophysical Journal Letters, 517(1 PART 1), 78-91.
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  Abstract: We describe an automated method for detecting clusters of galaxies in imaging and redshift galaxy surveys. The adaptive matched filter (AMF) method utilizes galaxy positions, magnitudes, and - when available - photometric or spectroscopic redshifts to find clusters and determine their redshift and richness. The AMF can be applied to most types of galaxy surveys, from two-dimensional (2D) imaging surveys, to multiband imaging surveys with photometric redshifts of any accuracy (2.5 dimensional [21/2D]), to three-dimensional (3D) redshift surveys. The AMF can also be utilized in the selection of clusters in cosmological N-body simulations. The AMF identifies clusters by finding the peaks in a cluster likelihood map generated by convolving a galaxy survey with a filter based on a model of the cluster and field galaxy distributions. In tests on simulated 2D and 21/2D data with a magnitude limit of r′ ≈ 3.5, clusters are detected with an accuracy of Az ≈ 0.02 in redshift and ∼ 10% in richness to z ≲ 0.5. Detecting clusters at higher redshifts is possible with deeper surveys. In this paper we present the theory behind the AMF and describe test results on synthetic galaxy catalogs.
• Newberg, H. J., Richards, G. T., Richmond, M., & Fan, X. (1999). Catalog of four-color photometry of stars, galaxies, and QSOs using SDSS filters. Astrophysical Journal, Supplement Series, 123(2), 377-435.
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  Abstract: We present a catalog containing the measurements of 2262 sources, including 334 extended sources, 1915 point sources, and 13 known QSOs, in five SDSS passbands. Of these objects, over 1600 are measured in 15 fields covering 0.5 deg2, with a limiting magnitude of r* < 19.5, similar to the photometric limit of the SDSS spectroscopic survey. Color plots of the data show that stars, galaxies, and quasars are fairly well separated by color alone. The stellar locus populates a ribbon-like subset of color-color-color space. It is shown that stars, galaxies, and QSOs tend toward the same fundamental plane in three-dimensional color space. The stars are compared with synthetic photometry from Kurucz models; the agreement is consistent with the errors in the data. The stellar locus moves in color space by about a tenth of a magnitude from r* = 14 to r* = 19.5. The shift is consistent with a shift in the metallicity from about [Me/H] = -1 to [Me/H] = -2. We compare this with previously measured metallicity gradients as a function of distance from the galactic plane.
• Strauss, M. A., Fan, X., Gunn, J. E., Leggett, S. K., Geballe, T. R., Pier, J. R., Lupton, R. H., Knapp, G. R., Annis, J., Brinkmann, J., Crocker, J. H., Csabai, I., Fukugita, M., Golimowski, D. A., Harris, F. H., Hennessy, G. S., Hindsley, R. B., Ivezić, Ž., Kent, S., , Lamb, D. Q., et al. (1999). The discovery of a field methane dwarf from sloan digital sky survey commissioning data. Astrophysical Journal Letters, 522(1 PART 2), L61-L64.
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  Abstract: We report the discovery of the coolest field dwarf yet known, selected as an unresolved object with extremely red colors from commissioning imaging data of the Sloan Digital Sky Survey. Its spectrum from 0.8 to 2.5 μm is dominated by strong bands of H2O and CH4. Its spectrum and colors over this range are very similar to those of Gl 229B. the only other known example of a methane dwarf. It is roughly 1.2 mag fainter than Gl 229B, suggesting that it lies at a distance of ∼10 pc. Such a cool object must have a mass well below the hydrogenburning limit of 0.08 M⊙ and therefore is a genuine brown dwarf, with a probable mass in the range 0.015-0.06 M⊙ for an age range of 0.3-5 Gyr. Subject headings: stars: low-mass, brown dwarfs - surveys.
• Zheng, Z., Shang, Z., Hongjun, S. u., Burstein, D., Chen, J., Deng, Z., Byun, Y., Chen, R., Chen, W., Deng, L., Fan, X., Fang, L., Hester, J. J., Jiang, Z., Yong, L. i., Lin, W., Sun, W., Tsay, W., Windhorst, R. A., , Hong, W. u., et al. (1999). Deep intermediate-band surface photometry of NGC 5907. Astronomical Journal, 117(6), 2757-2780.
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  Abstract: Intrigued by the initial report of an extended luminosity distribution perpendicular to the disk of the edge-on Sc galaxy NGC 5907, we have obtained very deep exposures of this galaxy with a Schmidt telescope, large-format CCD. and intermediate-band filters centered at 6660 Å and 8020 Å. These two filters, part of a 15-filter set, are custom designed to avoid the brightest (and most variable) night skylines. As a result, our images are able to go deeper with lower sky noise than those taken with broadband filters at similar effective wavelengths: e.g., 0.6 e- arcsec-2 s-1 for our observations versus 7.4 e- arcsec-2 s-1 for the R-band measures of Morrison et al. In our assessment of both random and systematic errors, we show that the flux level where the errors of observation reach 1 mag arcsec-2 are 29.00 mag arcsec-2 in the 6660 Å image (corresponding to 28.7 in the R band) and 27.4 mag arcsec-2 in the 8020 Å image (essentially on the I-band system). In a previous paper we have shown that NGC 5907 has a luminous ring around it, most plausibly caused by the tidal disruption of a dwarf spheroidal galaxy by the much more massive spiral. Here we show that, for values fainter than 27 R mag arcsec-2, the surface brightness around NGC 5907 is strongly asymmetric, being mostly brighter on the northwest (ring) side of the galactic midplane. This asymmetry rules out a halo as the cause of the faint surface brightness we see. We find this asymmetry is likely an artifact resulting from a combination of ring light and residual surface brightness at faint levels from stars that our star-masking procedure cannot completely eliminate. The possible existence of an optical face-on warp in NGC 5907, suggested by our Very Large Array H I observations, is too confused with foreground star contamination to be independently studied. Good agreement with the surface photometry of NGC 5907 by other observers leads us to conclude that their data are similarly affected at faint levels by ring light and the residual effects of star masking procedures. Inspection of published images confirm this to be the case. Thus, we conclude that NGC 5907 does not have a faint extended halo.
• Bahcall, N. A., & Fan, X. (1998). A lightweight universe?. Proceedings of the National Academy of Sciences of the United States of America, 95(11), 5956-5959.
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  PMID: 9600898;PMCID: PMC27566;Abstract: How much matter is there in the universe? Does the universe have the critical density needed to stop its expansion, or is the universe underweight and destined to expand forever? We show that several independent measures, especially those utilizing the largest bound systems known - clusters of galaxies - all indicate that the mass-density of the universe is insufficient to halt the expansion. A promising new method, the evolution of the number density of clusters with time, provides the most powerful indication so far that the universe has a subcritical density. We show that different techniques reveal a consistent picture of a lightweight universe with only ~20-30% of the critical density. Thus, the universe may expand forever.
• Bahcall, N. A., & Fan, X. (1998). The most massive distant clusters: Determining Ω and σ₈. Astrophysical Journal Letters, 504(1 PART I), 1-6.
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  Abstract: The existence of the three most massive clusters of galaxies observed so far at z > 0.5 is used to constrain the mass density parameter of the universe, Ω, and the amplitude of mass fluctuations, σ8. We find Ω = 0.2-0.1+0.3 and σ8 = 1.2-0.4+0.5 (95%). We show that the existence of even the single most distant cluster at z = 0.83, MS 1054-03, with its large gravitational lensing mass, high temperature, and large velocity dispersion, is sufficient to establish powerful constraints. High-density, Ω = 1 (σ8 ≃ 0.5-0.6) Gaussian models are ruled out by these data (10-6 probability); the Ω = 1 models predict only 10-5 massive clusters at z > 0.65 (∼10-3 at z > 0.5) instead of the one (three) clusters observed. © 1998. The American Astronomical Society. All rights reserved.
• Shang, Z., Zheng, Z., Brinks, E., Chen, J., Burstein, D., Hongjun, S. U., Byun, Y., Deng, L., Deng, Z., Fan, X., Jiang, Z., Yong, L. I., Lin, W., Feng, M. a., Sun, W., Wills, B., Windhorst, R. A., Hong, W. U., Xia, X., , Wen, X. U., et al. (1998). Ring structure and warp of NGC 5907: Interaction with dwarf galaxies. Astrophysical Journal Letters, 504(1 PART II), L23-L26.
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  Abstract: The edge-on, nearby spiral galaxy NGC 5907 has long been used as the prototype of a "noninteracting" warped galaxy. We report here the discovery of two interactions with companion dwarf galaxies that substantially change this picture. First, a faint ring structure is discovered around this galaxy that is likely due to the tidal disruption of a companion dwarf spheroidal galaxy. The ring is elliptical in shape with the center of NGC 5907 close to one of the ring's foci. This suggests that the ring material is in orbit around NGC 5907. No gaseous component to the ring has been detected either with deep Hα images or in Very Large Array H i 21 cm line maps. The visible material in the ring has an integrated luminosity ≤108 L⊙, and its brightest part has a color R - I ∼ 0.9. All of these properties are consistent with the ring being a tidally disrupted dwarf spheroidal galaxy. Second, we find that NGC 5907 has a dwarf companion galaxy, PGC 54419, which is projected to be only 36.9 kpc from the center of NGC 5907, close in radial velocity (ΔV = 45 km s-1) to the giant spiral galaxy. This dwarf is seen at the tip of the H I warp and in the direction of the warp. Hence, NGC 5907 can no longer be considered noninteracting but is obviously interacting with its dwarf companions much as the Milky Way interacts with its dwarf galaxies. These results, coupled with the finding by others that dwarf galaxies tend to be found around giant galaxies, suggest that tidal interaction with companions, even if containing a mere 1% of the mass of the parent galaxy, might be sufficient to excite the warps found in the disks of many large spiral galaxies. © 1998. The American Astronomical Society. All rights reserved.
• Bahcall, N. A., Fan, X., & Cen, R. (1997). Constraining Ω with cluster evolution. Astrophysical Journal Letters, 485(2 PART II), L53-L56.
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  Abstract: We show that the evolution of the number density of rich clusters of galaxies breaks the degeneracy between Ω (the mass density ratio of the universe) and σ8 (the normalization of the power spectrum), σ8Ω0.5 ≃ 0.5, that follows from the observed present-day abundance of rich clusters. The evolution of high-mass (Coma-like) clusters is strong in Ω = 1, low-σ8 models (such as the standard biased cold dark matter model with σ8 ≃ 0.5), where the number density of clusters decreases by a factor of about 103 from z = 0 to z ≃ 0.5; the same clusters show only mild evolution in low-Ω, high-σ8 models, where the decrease is a factor of about 10. This diagnostic provides a most powerful constraint on Ω. Using observations of clusters to z ≃ 0.5-1, we find only mild evolution in the observed cluster abundance. We find Ω = 0.3 ± 0.1 and σ8 = 0.85 ± 0.15 (for A = 0 models; for Ω + Λ = 1 models, Ω = 0.34 ± 0.13). These results imply, if confirmed by future surveys, that we live in a low-density, low-bias universe. © 1997. The American Astronomical Society. All rights reserved.
• Fan, X., Bahcall, N. A., & Cen, R. (1997). Determining the amplitude of mass fluctuations in the universe. Astrophysical Journal Letters, 490(2 PART II), L123-L126.
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  Abstract: We present a method for determining the amplitude of mass fluctuations on 8 h-1 Mpc scale, σ8. The method utilizes the rate of evolution of the abundance of rich clusters of galaxies. Using the Press-Schechter approximation, we show that the cluster abundance evolution is a strong function of σ8: d log nldz ∝ -1/σ82; low-σ8 models evolve exponentially faster than high-σ8 models, for a given mass cluster. For example, the number density of Coma-like clusters decreases by a factor of ∼103 from z = 0 to z ≃ 0.5 for σ8 = 0.5 models, while the decrease is only a factor of ∼5 for σ8 ≃ 1. The strong exponential dependence on σ8 arises because clusters represent rarer density peaks in low-σ8 models. We show that the evolution rate at z ∝ 1 is insensitive to the density parameter Ω or to the exact shape of the power spectrum. Cluster evolution therefore provides a powerful constraint on σ8. Using available cluster data to z ∼ 0.8, we find σ8 = 0.83 ± 0.15. This amplitude implies a bias parameter b ∼ σ8-1 = 1.2 ± 0.2, i.e., a nearly unbiased universe with mass approximately tracing light on large scales. When combined with the present-day cluster abundance normalization, σ8Ω0.5 ≃ 0.5, the cosmological density parameter can be determined: Ω ≃ 0.3 ± 0.1. © 1997. The American Astronomical Society. All rights reserved.
• Lu, P. K., Tsay, W. S., Chen, A. B., Chen, R., Sun, W. H., Byun, Y. I., Chen, W. P., Chiueh, T. H., Kuo, H. J., Burstein, D., Hester, J. J., Windhorst, R. A., Fang, L. Z., Chen, J. S., Zhu, J., Deng, L. C., Fan, X. H., Jiang, Z. J., Li, Y., , Wu, H., et al. (1997). Multiband photometry of selected areas in a study of galactic structure. Baltic Astronomy, 6(1), 33-40.
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  Abstract: The BATC (Beijing-Arizona-Taipei-Connecticut) intermediate-band 15 filter photometry system has been tested by using the Michigan Curtis Schmidt telescope at CTIO taking one-degree fields around NGC 288, SA 92, 94, 95, Lp 543-32/33. BVRI and uvby, Hβ filter observations of these same fields were also made using the 0.9 m CTIO telescope. The main purpose of these observations is to compare the results from the standard filter color systems (UBVRI and ubvy, Hβ) to those we can construct from the BATC filter system. These fields were chosen based on available filter color data in those systems. The BATC data are being taken on a Schmidt telescope similar to that at CTIO, so that these observations can provide an independent source of calibration for the BATC data. This is a report on the status of this study.
• Richards, G. T., Yanny, B., Annis, J., J., H., McKay, T. A., York, D. G., & Fan, X. (1997). Quasar photometry with the SDSS monitor telescope. Publications of the Astronomical Society of the Pacific, 109(731), 39-46.
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  Abstract: Photometric data were obtained for a set of known QSOs in five bands with a filter system similar to the Sloan Digital Sky Survey (SDSS) filter system for the purpose of testing the ability of the SDSS system to separate QSO colors from the colors of the stellar locus. Data were obtained from two similar telescopes and camera systems, and were intercompared. The F stars BD+26 2606 and BD+17 4708 were used as primary standards to set the zero points for the rest of the observed objects. The standard star magnitudes were found to be in reasonable agreement with the theoretically predicted magnitudes, and the QSO colors were well separated from the stellar locus, defined by random stars in the QSO fields.
• Fan, X., Burstein, D., Chen, J., Zhu, J., Jiang, Z., Hong, W. u., Yan, H., Zheng, Z., Zhou, X., Fang, L., Chen, F., Deng, Z., Chu, Y., Hester, J. J., Windhorst, R. A., Yong, L. i., Phillip, L. u., Sun, W., Chen, W., , Tsay, W., et al. (1996). Deep wide-field spectrophotometry of the open cluster M67. Astronomical Journal, 112(2), 628-648.
• Xiao-hui, F., & Jian-sheng, C. (1994). Variation of QSO color indices with redshift. Chinese Astronomy and Astrophysics, 18(4), 393-402.
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  Abstract: In this paper we discuss the effect of features in the spectrum of QSOs on their color indices and the consequent variation of the latter with the redshift. As the redshift increases, the various absorption features shortwards of the Lyα emission line enter into the visible region and alter the way in which the color indices vary with the redshift, while the effects of the intrinsic power-law spectrum and emission lines on the color indices become secondary. The largest effect is due to the Lyman limit absorption system. These results are checked using the color indices calculated from IUE spectra. © 1994.

Proceedings Publications

• Lawther, D., Vestergaard, M., & Fan, X. (2016). "FeLoBAL quasars: evolution, orientation or just oddBALs?". In From Interstellar Clouds to Star-Forming Galaxies: Universal Processes?, 315.
• Wu, X., Wang, F., Fan, X., Yi, W., Zuo, W., Bian, F., Jiang, L., McGreer, I., Wang, R., Yang, J., Yang, Q., Thompson, D., & Beletsky, Y. (2016). "Discovery of a 12 billion solar mass black hole at redshift 6.3 and its challenge to the black hole/galaxy coevolution at cosmic dawn". In Galaxies at High Redshift and Their Evolution Over Cosmic Time, 319.
• Ba\~nados, E., Venemans, B. P., Morganson, E., Decarli, R., Walter, F., Chambers, K. C., Rix, H., Farina, E. P., Fan, X., Jiang, L., McGreer, I., De Rosa, G., Simcoe, R., Wei\ss, A., Price, P. A., Morgan, J. S., Burgett, W. S., Greiner, J., Kaiser, N., , Kudritzki, R., et al. (2014, jul). The discovery of eight z \~ 6 quasars from Pan-STARRS1. In IAU Symposium, 304, 19-22.
• Cai, Z., Fan, X., Bian, F., McGreer, I. D., Frye, B. L., Yang, Y., Zabludoff, A. I., & Zheng, Z. (2014, jan). MApping the Most Massive Overdensity Through Hydrogen (MAMMOTH). In American Astronomical Society Meeting Abstracts, 223, #358.21.
• Eftekharzadeh, S., Myers, A. D., White, M., Bovy, J., Fan, X., Le Goff, J., Laurent, P., McBride, C., Miralda-Escude, J., Palanque-Delabrouille, N., Petitjean, P., Ross, N. P., Schneider, D. P., Shen, Y., Strauss, M. A., Streblyanska, A., Weinberg, D. H., Wood-Vasey, W. M., Viel, M., , Yeche, C., et al. (2014, jun). The Clustering of Quasars at Redshift 2.5 from the Final SDSS-III/BOSS Sample. In American Astronomical Society Meeting Abstracts, 224, #221.01.
• Ivezi\'c, \. Z., Brandt, W. N., Fan, X., MacLeod, C. L., Richards, G. T., & Yoachim, P. (2014, jul). Optical selection of quasars: SDSS and LSST. In IAU Symposium, 304, 11-17.
• Stein, M., Shemmer, O., Anderson, S. F., Brandt, W. N., Diamond-Stanic, A. M., Fan, X., Luo, B., Plotkin, R., Richards, G. T., Schneider, D. P., Strauss, M. A., & Wu, J. (2014, jan). The XMM-Newton View of Weak Emission-Line Quasars. In American Astronomical Society Meeting Abstracts, 223, #150.02.
• Bian, F., Fan, X., Jiang, L., McGreer, I. D., Dey, A., Green, R. F., Walter, F., Maiolino, R., Lee, K., & Dave, R. (2013, jan). Luminous 3 Lyman Break Galaxies in Deep and Wide Field Surveys. In American Astronomical Society Meeting Abstracts, 221, #207.03.
• Lundgren, B., Brammer, G., Van Dokkum, P. G., Bezanson, R., Franx, M., Fumagalli, M., Momcheva, I. G., Nelson, E., Skelton, R., Wake, D., Whitaker, K. E., Cunha, E., Erb, D., Fan, X., Kriek, M., Labbe, I., Marchesini, D., Patel, S., Rix, H., , Schmidt, K., et al. (2013, jan). Large-Scale Star Formation-Driven Outflows at 1 ltz lt2 in the 3D-HST Survey. In American Astronomical Society Meeting Abstracts, 221, #147.39.
• McGreer, I. D., Fan, X., Richards, G. T., Haiman, Z., Strauss, M. A., Jiang, L., & Schneider, D. P. (2013, jan). An HST Snapshot Survey for Gravitationally Lensed z=6 Quasars. In American Astronomical Society Meeting Abstracts, 221, #133.03.
• Mechtley, M., Koekemoer, A. M., Jahnke, K., Smith, B. M., Windhorst, R. A., Cohen, S. H., Fan, X., Hathi, N. P., Jansen, R., Jiang, L., Keel, W. C., R "ottgering, H., Ryan, R. E., Scannapieco, E., Schneider, D. P., Schneider, G., Strauss, M. A., & Yan, H. (2013, jan). Quasar Host Galaxies at z=2 and z=6: Point Source Subtraction With MCMC. In American Astronomical Society Meeting Abstracts, 221, #339.31.
• Shemmer, O., Anderson, S. F., Ballantyne, D. R., Barth, A. J., Brandt, W. N., Brunner, R. J., Chartas, G., Coppi, P. S., Vries, W. H., Eracleous, M., Fan, X., Gibson, R., Gray, A. G., Green, R. F., Kimball, A. E., Lacy, M., Lira, P., Madejski, G. M., Newman, J., , Richards, G. T., et al. (2013, jan). AGN Science with the LSST. In American Astronomical Society Meeting Abstracts, 221, #247.10.
• Wang, R., Carilli, C. L., Walter, F., Fan, X., Wagg, J., Riechers, D. A., Bertoldi, F., Omont, A., Cox, P., Strauss, M. A., Menten, K., Narayanan, D. T., Knudsen, K., & Jiang, L. (2013, jan). [C II] Line Emission and Star Formation from Quasar Host Galaxies at 6. In American Astronomical Society Meeting Abstracts, 221, #221.04.
• Bian, F., Fan, X., Jiang, L., Dey, A., Green, R. F., Maiolino, R., Walter, F., Jester, S., Lee, K., & Mcgreer, I. (2012, jan). The Lbt Bootes Field Survey: Luminosity Function And Clustering Of Z 3 Lbgs At The Bright End.. In American Astronomical Society Meeting Abstracts #219, 219, #340.13.
• Jang, M., Im, M., Lee, I., Urata, Y., Huang, K., Hirashita, H., Fan, X., & Jiang, L. (2012, jan). Dust Properties In Afterglow Of GRB 071025 At z 5. In American Astronomical Society Meeting Abstracts #219, 219, #142.01.
• McGreer, I. D., Jiang, L., Fan, X., Ross, N. P., Eskew, M. R., Myers, A. D., & Schneider, D. P. (2012, jan). The z 5 Quasar Luminosity Function From SDSS Stripe 82. In American Astronomical Society Meeting Abstracts #219, 219, #209.07.
• Mechtley, M., Windhorst, R. A., Ryan, R. E., Cohen, S. H., Schneider, G., Fan, X., Hathi, N. P., Jansen, R. A., Keel, W. C., Koekemoer, A. M., R "ottgering, H., Scannapieco, E., Schneider, D. P., Strauss, M. A., & Yan, H. (2012, jan). WFC3 Imaging Of z=6 Quasars: Examining The Host Galaxies Of AGN In The Early Universe. In American Astronomical Society Meeting Abstracts #219, 219, #243.17.
• Myers, A. D., Aubourg, E., Bailey, S., Bovy, J., Fan, X., Ho, S., Jiang, L., Miralda-Escude, J., Palanque-Delabrouille, N., Petitjean, P., Ross, N., Schlegel, D., Schneider, D., Strauss, M., Weinberg, D., White, M., Yeche, C., & Zehavi, I. (2012, jan). Clustering Near the Epoch of Peak Quasar Activity with SDSS-III/BOSS. In American Astronomical Society Meeting Abstracts #219, 219, #324.04.
• Myers, A. D., Palanque-Delabrouille, N., Schlegel, D. J., Yeche, C., Aubourg, E., Bailey, S., Dey, A., Eftekharzadeh, S., Fan, X., Magneville, C., Paris, I., Petitjean, P., & Ross, N. P. (2012, jan). The BigBOSS QSO Pilot Survey. In American Astronomical Society Meeting Abstracts #219, 219, #335.12.
• Ross, N., McGreer, I. D., White, M., Myers, A. M., Richards, G. T., Strauss, M. A., Anderson, S. F., Bailey, S., Fan, X., Palanque-Delabrouille, N., Petitjean, P., Schawinski, K., Schneider, D. P., Silverman, J., Weinberg, D. H., Yeche, C., & York, D. G. (2012, jan). The SDSS-III BOSS DR9 Quasar Luminosity Function. In American Astronomical Society Meeting Abstracts #219, 219, #209.06.