Christopher D Impey

Interests

Teaching

Astronomy, cosmology, astrobiology, science literacy

Research

Galaxies, quasars, supermassive black holes

Courses

Scholarly Contributions

Books

• Impey, C. D. (2020). Astronomy Education Volume 1: Evidence-Based Instruction for Introductory Courses. USA: American Institute of Physics.
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  This was the first education publication in the new IoP/AAS series of astronomy titles, covering the state-of-the-art in instruction for introductory astronomy.
• Impey, C. D. (2020). Astronomy Education Volume 2: Best Practices for Online Learning Environments. USA: American Institute of Physics.
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  This is the second education book ever published by the IoP/AAS collaboration, on the subject of best teaching practices for online astronomy.
• Impey, C. D. (2018). Einstein’s Monsters: The Life and Times of Black Holes. Norton.
• Impey, C. D., & Campion, N. (2018). Imagining Other Worlds: Explorations in Astronomy and Culture. Sophia Center Press.
• Impey, C. D. (2014). Humble Before the Void. Philadelphia: Templeton Press.
• Impey, C. D. (2013). Shadow World. Dark Skies Press.
• Impey, C. D., & Henry, H. (2013). Dreams of Other Worlds: The Amazing Story of Unmanned Space Exploration. Princeton University Press.
• Impey, C. D. (2013). How It Began: A Time Travelers Guide to the Universe. Norton.
• Impey, C. D. (2011). The Living Cosmos: Our Search for Life in the Universe. Cambridge University Press.
• Impey, C. D. (2010). How It Ends: From You to the Universe. Norton.
• Impey, C. D. (2007). The Living Cosmos: Our Search for Life in the Universe. Random House.
• Impey, C. D., & Hartmann, W. H. (2000). The Universe Revealed. Cole Publishing.
• Impey, C. D., & Hartmann, W. H. (1994). Astronomy: The Cosmic Journey. Wadsworth Publishing.

Chapters

• Impey, C. D. (2021). Astrobiology Education: Inspiring Diverse Audiences with the Search for Life in the Universe. In Astrobiology: Science, Ethics and Public Policy(p. 15). Hoboken, NJ: Wiley Scrivener.
• Impey, C. D. (2020). Models and Uncertainty in Astrophysics. In Dialogues Around Models and Uncertainty(p. 28). Singapore: World Scientific Publishing.
• Impey, C. D. (2017). Cosmology and the Void. In Cosmology and Consciousness IV(p. 34). The Library of Tibetan Works and Archives: Emptiness and Quantum Mechanics.
• Impey, C. D. (2014). Cosmology and the Human Condition. In The Science and Religion Dialog: Past and Future(pp 173-193). Frankfurt: Peter Lang.
• Impey, C. (2013). Science Literacy of Undergraduates in the United States. In Organizations, People and Strategies in Astronomy Vol.~2(pp 353-364).
• Impey, C. (2013). The First Thousand Exoplanets: Twenty Years of Excitement and Discovery. In Astrobiology, History, and Society(p. 201).
• Brissenden, G., Prather, E. E., & Impey, C. (2012). Astronomy CATS. In Organizations, People and Strategies in Astronomy Vol.~1, Edited by Andre Heck, Venngeist, Duttlenheim (2012) pp.~149-162(pp 149-162).
• Impey, C. (2012). Astronomy in the United States: Workforce Development and Public Engagement. In Organizations, People and Strategies in Astronomy Vol.~1, Edited by Andre Heck, Venngeist, Duttlenheim (2012) pp.~77-91(pp 77-91).

Journals/Publications

• Impey, C. D. (2020). Higher Education Online and the Developing World. Journal of Education and Human Development, 9(2), 17. doi:10.15640/jehd.v9n2a3
• Impey, C. D. (2020). Visions of a Martian Future. Futures, 117(102514), 21. doi:10.1016/j.futures.2020.102514
• Impey, C. D. (2019). Pluto Redux. Scientific American Blog, 4.
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  The faraway world’s demotion more than a decade ago opened a celestial can of worms
• Impey, C. D. (2019). Tibetan Monks Meet Science near the Roof of the World. Scientific American Blog, 8.
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  Astronomical and cosmological questions get an airing in India’s Sikkim province in a program started 20 years ago by the Dalai Lama
• Impey, C. D., & Szocik, K. (2019). Human Enhancement and Mars Settlement – Biological Necessity or Science Fiction?. Human Enhancements in Lunar, Martian, and Future missions to the Outer Planets.
• Impey, C. D., & Szocik, K. (2019). Mars and Beyond: The Feasibility of Living in the Solar System, in The Human Factor. A Mission to Mars: An Interdisciplinary Approach, 93. doi:https://philpapers.org/go.pl?id=IMPMAB&proxyId=5208&u=http%3A%2F%2Fdx.doi.org%2F10.1007%2F978-3-030-02059-0_5
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  Mars has occupied a distinctive place in the popular imagination for a century. Science fiction and the wishful thinking of Percival Lowell primed us to think of Mars as a living world, but the first landers and orbiters witnessed a frigid and arid desert landscape. Now the pendulum has swung back toward habitability, with evidence of the red planet’s warmer and wetter history and hints of sub-surface aquifers. A Mars base with short-term occupants would be a first step toward eventual colonization. NASA is planning to send astronauts to Mars, subject to its limited and uncertain budget. Meanwhile, the private sector has ambitious plans for establishing a foothold on Mars, with the Mars One plans viewed more skeptically by experts than the plans of SpaceX. Mars colonists would face challenges caused by isolation, radiation, reduced gravity, and an unforgiving external environment. Over time, they would diverge culturally, psychologically, and genetically from the inhabitants of Earth.
• Impey, C. D., Formanek, M., Wenger, M. C., & Buxner, S. R. (2019). Relationship Between Learners’ Motivation and Course Engagement in an Astronomy Massive Open Online Class. Research Gate, 15, 15. doi:10.1103/PhysRevPhysEducRes.15.020140
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  In this work we study how different motivations of learners relate to their engagement in a massive open online course (MOOC). The research was conducted in a cohort-based astronomy MOOC “Astronomy: Exploring Time and Space” provided through the Coursera platform. In order to discern motivations of course participants, the Science Motivation Questionnaire II was administered as a first assignment of the course. The survey data were collected from December 2016 until December 2018 and responses from 3701 learners were acquired. In order to determine the relationship of motivation to course performance, four course engagement indicators are defined: course completion, use of video lectures, participation in the discussion forums, and participation in peer-grading assignments. Self-determination and self-efficacy are identified as the main motivational factors influencing all aspects of the course engagement except for the video usage. We show that both of these decrease with age, but increase with the number of previously taken science courses. The main reasons for signing up for the course which had an effect on course engagement were the goal of getting a certificate and perceived importance of the reputation of the instructor. There was no statistically significant difference in motivations between people who dropped after the first peer graded writing assignment and those who successfully completed the course, suggesting that after the first writing assignment learners quit from other reasons than lack of motivation. Unexpectedly, learners with high career, grade, and social motivation watched statistically significantly fewer instructional videos, and social motivation did not play a role in the forum usage.
• Impey, C. D., Jannuzi, B. T., Stocke, J., Keeney, B., Danforth, C., Oppenheimer, B., Pratt, C., & Berlind, A. (2019). The Ultraviolet Detection of Diffuse Gas in Galaxy Groups. The Astrophysical Journal Supplement Series, 240(1). doi:10.3847/1538-4365/aaf73d
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  A small survey of the UV-absorbing gas in 12 low-z galaxy groups has been conducted using the Cosmic Origins Spectrograph on board the Hubble Space Telescope. Targets were selected from a large, homogeneously selected sample of groups found in the Sloan Digital Sky Survey. A critical selection criterion excluded sight lines that pass close (
• Impey, C. D., Prather, E. E., Simon, M. N., & Buxner, S. R. (2019). The Development and Validation of the Planet Formation Concept Inventory (PFCI). International Journal of Science Education, 2448-2464.
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  The discovery and characterisation of planets orbiting distant stars has shed light on the origin of our own Solar System. It is important that college-level introductory astronomy students have a general understanding of the planet formation process before they are able to draw parallels between extrasolar systems and our own Solar System. In this work, we introduce the Planet Formation Concept Inventory (PFCI), an educational research tool used to assess student learning on the topic of planet formation. The PFCI Version 3 was administered to N = 561 students pre-instruction and N = 374 students post-instruction. Here, we present a Classical Test Theory (CTT) analysis of the PFCI Version 3. Ultimately, we conclude that the PFCI is a reliable and valid instrument that can differentiate experts from novices, and can be used to assess college-level introductory astronomy students' learning on the topic of planet formation. Initial findings on class normalised gain scores indicate that the PFCI may be capable of assessing the effectiveness of different instructional models. In the future, we recommend a national study of the PFCI to discern its ability to provide insight regarding the ascribed characteristics of learners and the effectiveness of different instructional strategies being used to teach this topic.
• Impey, C. D., Stocke, J., Keeney, B., Danforth, C., Jannuzi, B. T., Oppenheimer, B., Berlind, A., & Pratt, C. (2019). VizieR Online Data Catalog: HST/COS UV obs. of low-z SDSS galaxy groups. The Astrophysical Journal Supplement Series, 240, 15.
• Impey, C. D. (2018). Our Future Off-Earth and the Road to the Stars. Mediterranean Archaeology and Archaeometry, 18(4), 296.
• Impey, C. D., & Jasensky, D. (2018). Visualizing the Universe – the Intersection of Astronomy and Art. Mediterranean Archaeology and Archaeometry, 18(4), 307.
• Impey, C. D., Buxner, S. R., Romine, J., & Nieberding, M. (2018). Linking Introductory Astronomy Students' Basic Science Knowledge, Beliefs, Attitudes, Sources of Information, and Information Literacy. Physical Review Physics Education Research, 14(010142).
• Impey, C. D., Danehy, A., & Mocherla, S. (2018). Evaluation of Naïve Bayes and Support Vector Machines for Wikipedia. Applied Artificial Intelligence, 11. doi:https://doi.org/10.1080/08839514.2018.1440907
• Impey, C. D., Prather, E., & Brock, L. S. (2018). Finding the Time: Exploring a New Perspective on Students’ Perceptions of Cosmological Time and Efforts to Improve Temporal Frameworks in Astronomy. Physical Review Physics Education Research, 14(010138).
• Impey, C. D., Simon, M. N., & Buxner, S. R. (2018). A Survey and Analysis of College Students’ Understanding of Planet Formation Before Instruction. Astrobiology, 18(12), 1.
• Impey, C. D., Wenger, M., Deustua, S., & Eastwood, K. (2018). Online Astronomy for Formal and Informal Learners, in International Symposium on Education in Astronomy and Astrobiology. EPJ Web of Conferences, Vol. 200, 10.
• Impey, C. D., & Danehy, A. (2017). A Website for Astronomy Education and Outreach. Communicating Astronomy with the Public Journal, 22, 30.
• Impey, C. D., Formanek, M., Buxner, S. R., & Wenger, M. (2017). Twenty Seven Years of Tracking Undergraduate Science Knowledge and Beliefs. Electronic Journal of Science Education, 21(4).
• Impey, C. D., Sonam, T., Formanek, M., Buxner, S. R., & Wenger, M. (2017). Insights About Large-Scale Online Peer Assessment from an Analysis of an Astronomy MOOC. Computers and Education, 113, 243.
• Impey, C. D., & Galbany, L. (2016). UBVRIz Light Curves of 51 Type II Supernovae. NA, 33.
• Impey, C. D., & Hardegree-Ullman, K. (2016). Teach Astronomy – A Comprehensive Online Astronomy Education and Outreach Resource, International Journal for Innovation Education and Research. NA, 4(10), 117.
• Impey, C. D., & Wenger, M. (2016). Bringing the Universe to the World: Lessons Learned from a Massive Open Online Class in Astronomy, Communicating Astronomy with the Public Journal. NA, 21, 20.
• Impey, C. D. (2015). Astronomy for Astronomical Numbers: A Worldwide Massive Open Online Class. The International Review of Research in Open and Distributed Learning, 16(1).
• Hao, H., Elvis, M., Civano, F., Zamorani, G., Ho, L. C., Comastri, A., Brusa, M., Bongiorno, A., Merloni, A., Trump, J. R., Salvato, M., Impey, C. D., Koekemoer, A. M., Lanzuisi, G., Celotti, A., Jahnke, K., Vignali, C., Silverman, J. D., Urry, C. M., , Schawinski, K., et al. (2014). Spectral energy distributions of type 1 AGN in XMM-COSMOS - II. Shape evolution. Monthly Notices of the Royal Astronomical Society, 438(2), 1288-1304.
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  Abstract: The mid-infrared-to-ultraviolet (0.1-10 μm) spectral energy distribution (SED) shapes of 407 X-ray-selected radio-quiet type 1 active galactic nuclei (AGN) in the wide-field 'Cosmic Evolution Survey' (COSMOS) have been studied for signs of evolution. For a sub-sample of 200 radio-quiet quasars with black hole mass estimates and host galaxy corrections, we studied their mean SEDs as a function of a broad range of redshift, bolometric luminosity, black hole mass and Eddington ratio, and compared them with the Elvis et al. (E94) type 1 AGN mean SED. We found that the mean SEDs in each bin are closely similar to each other, showing no statistical significant evidence of dependence on any of the analysed parameters. We also measured the SED dispersion as a function of these four parameters, and found no significant dependences. The dispersion of the XMM-COSMOS SEDs is generally larger than E94 SED dispersion in the ultraviolet, which might be due to the broader 'window function' for COSMOS quasars, and their X-ray-based selection. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
• Hao, H., Elvis, M., Bongiorno, A., Zamorani, G., Merloni, A., Kelly, B. C., Civano, F., Celotti, A., Ho, L. C., Jahnke, K., Comastri, A., Trump, J. R., Mainieri, V., Salvato, M., Brusa, M., Impey, C. D., Koekemoer, A. M., Lanzuisi, G., Vignali, C., , Silverman, J. D., et al. (2013). A quasar-galaxy mixing diagram: Quasar spectral energy distribution shapes in the optical to near-infrared. Monthly Notices of the Royal Astronomical Society, 434(4), 3104-3121.
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  Abstract: We define a quasar-galaxy mixing diagram using the slopes of their spectral energy distributions (SEDs) from 1 μm to 3000 å and from 1 to 3 μm in the rest frame. The mixing diagram can easily distinguish among quasar-dominated, galaxy-dominated and reddening-dominated SED shapes. By studying the position of the 413 XMM-selected type 1 AGN in the wide-field 'Cosmic Evolution Survey' in the mixing diagram, we find that a combination of the Elvis et al. mean quasar SED with various contributions from galaxy emission and some dust reddening is remarkably effective in describing the SED shape from 0.3 to 3μm for large ranges of redshift, luminosity, black hole mass and Eddington ratio of type 1 AGN. In particular, the location in the mixing diagram of the highest luminosity AGN is very close (within 1σ) to that of the Elvis et al. SED template. The mixing diagram can also be used to estimate the host galaxy fraction and reddening in quasar. We also show examples of some outliers which might be AGN in different evolutionary stages compared to the majority of AGN in the quasar-host galaxy co-evolution cycle. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
• Impey, C. (2013). The first thousand exoplanets: Twenty years of excitement and discovery. Advances in Astrobiology and Biogeophysics, 201-212.
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  Abstract: The recent "explosion" in the number of extrasolar planets, or exoplanets, is perhaps the most exciting phenomenon in all of science. Two decades ago, no planets were known beyond the Solar System, and now there are more than 770 confirmed exoplanets and several thousand more candidates, while the mass detection limit has marched steadily downwards from Jupiter mass in 1995 to Neptune mass in the early 2000s to Earth mass now. The vast majority of these exoplanets are detected indirectly, by their gravitational influence on the parent star or the partial eclipse they cause when they periodically pass in front of it. Doppler detection of the planet's reflex motion yields a period and an estimate of the mass, while transits or eclipses yield the size. Exoplanet detection taxes the best observatories in space, yet useful contributions can be made by amateur astronomers armed with 6-inch telescopes. The early discoveries were surprising; no one predicted "hot Jupiters" or the wild diversity of exoplanet properties that has been seen. It is still unclear if the Solar System is "typical" or not, but at current detection limits at least 10 % of Sun-like stars harbor planets and architectures similar to the Solar System are now being found. Over a hundred multiple planet systems are known and the data are consistent with every star in the Milky Way having at least one planet, with an implication of millions of habitable, Earth-like planets, and of which could harbor life. Doppler and transit data can be combined to give average density, and additional methods are beginning to give diagnostics of atmospheric composition. When this work can be extended to rocky and low mass exoplanets, and the imprint of biology on a global atmosphere can be measured, this might be the way that life beyond Earth is finally detected for the first time. © 2013 Springer-Verlag Berlin Heidelberg.
• Impey, C. D., Hardegree-Ullman, K., Patikkal, A., Srinathan, A., Austin, C. L., Ganesan, N. K., & Guvenen, B. C. (2013). A new online astronomy resource for education and outreach. Astronomy Education Review, 12(1).
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  Abstract: A new web site called Teach Astronomy (http://www.teachastronomy.com) has been created to serve astronomy instructors and their students, amateur astronomers, and members of the public interested in astronomy. The content includes an online textbook of 400 000 words and 1200 images, 40 000 articles from Wikipedia, over 1100 video clips covering all topics in astronomy, over 6300 images from Astronomy Picture of the Day and AstroPix, over 1400 podcasts from 365 Days of Astronomy, and 25-30 astronomy news stories each week from Science Daily. All these resources are navigable by a keyword search and via a unique visual interface called a Wikimap, which presents the items clustered by degree of keyword overlap and allows surfing among related items. The site can serve as a replacement for a textbook and as an information resource for formal and informal learners. Future development includes building an instructor and learner community and creating homework assignments that utilize different content areas within Teach Astronomy. © 2013 The American Astronomical Society.
• Schneider, E. E., Impey, C. D., Trump, J. R., & Salvato, M. (2013). Steps toward unveiling the true population of active galactic nuclei: Photometric characterization of active galactic nuclei in cosmos. Astrophysical Journal, 766(2).
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  Abstract: Using a physically motivated, model-based active galactic nucleus (AGN) characterization technique, we fit a large sample of X-ray-selected AGNs with known spectroscopic redshifts from the Cosmic Evolution Survey field. We identify accretion disks in the spectral energy distributions of broad- and narrow-line AGNs, and infer the presence or absence of host galaxy light in the SEDs. Based on infrared and UV excess AGN selection techniques, our method involves fitting a given SED with a model consisting of three components: infrared power-law emission, optical-UV accretion disk emission, and host galaxy emission. Each component can be varied in relative contribution, and a reduced chi-square minimization routine is used to determine the optimum parameters for each object. Using this technique, both broad- and narrow-line AGNs fall within well-defined and plausible bounds on the physical parameters of the model, allowing trends with luminosity and redshift to be determined. In particular, based on our sample of spectroscopically confirmed AGNs, we find that approximately 95% of the broad-line AGNs and 50% of the narrow-line AGNs in our sample show evidence of an accretion disk, with maximum disk temperatures ranging from 1 to 10 eV. Because this fitting technique relies only on photometry, we hope to apply it in future work to the characterization and eventually the selection of fainter AGNs than are accessible in wide-field spectroscopic surveys, and thus probe a population of less luminous and/or higher redshift objects without prior redshift or X-ray data. With the abundant availability of photometric data from large surveys, the ultimate goal is to use this technique to create large samples that will complement and complete AGN catalogs selected by X-ray emission alone. © 2013. The American Astronomical Society. All rights reserved.
• Civano, F., Elvis, M., Brusa, M., Comastri, A., Salvato, M., Zamorani, G., Aldcroft, T., Bongiorno, A., Capak, P., Cappelluti, N., Cisternas, M., Fiore, F., Fruscione, A., Hao, H., Kartaltepe, J., Koekemoer, A., Gilli, R., Impey, C. D., Lanzuisi, G., , Lusso, E., et al. (2012). The Chandra COSMOS Survey. III. Optical and infrared identification of X-ray point sources. Astrophysical Journal, Supplement Series, 201(2).
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  Abstract: The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that has imaged the central 0.9deg 2 of the COSMOS field down to limiting depths of 1.9 × 10 -16ergcm -2s -1 in the soft (0.5-2keV) band, 7.3 × 10 -16ergcm -2s -1 in the hard (2-10keV) band, and 5.7 × 10 -16ergcm -2s -1 in the full (0.5-10keV) band. In this paper we report the i, K, and 3.6μm identifications of the 1761 X-ray point sources. We use the likelihood ratio technique to derive the association of optical/infrared counterparts for 97% of the X-ray sources. For most of the remaining 3%, the presence of multiple counterparts or the faintness of the possible counterpart prevented a unique association. For only 10 X-ray sources we were not able to associate a counterpart, mostly due to the presence of a very bright field source close by. Only two sources are truly empty fields. The full catalog, including spectroscopic and photometric redshifts and classification described here in detail, is available online. Making use of the large number of X-ray sources, we update the "classic locus" of active galactic nuclei (AGNs) defined 20years ago in soft X-ray surveys and define a new locus containing 90% of the AGNs in the survey with full-band luminosity >10 42ergs -1. We present the linear fit between the total i-band magnitude and the X-ray flux in the soft and hard bands, drawn over two orders of magnitude in X-ray flux, obtained using the combined C-COSMOS and XMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and we test its known correlation with redshift and luminosity, and a recently introduced anti-correlation with the concentration index (C). We find a strong anti-correlation (though the dispersion is of the order of 0.5 dex) between X/O computed in the hard band and C and that 90% of the obscured AGNs in the sample with morphological information live in galaxies with regular morphology (bulgy and disky/spiral), suggesting that secular processes govern a significant fraction of the black hole growth at X-ray luminosities of 10 43-10 44.5ergs -1. We also investigate the degree of obscuration of the sample using the hardness ratio, and we compare the X-ray color with the near-infrared to optical color. © © 2012. The American Astronomical Society. All rights reserved.
• Donley, J. L., Koekemoer, A. M., Brusa, M., Capak, P., Cardamone, C. N., Civano, F., Ilbert, O., Impey, C. D., Kartaltepe, J. S., Miyaji, T., Salvato, M., Sanders, D. B., Trump, J. R., & Zamorani, G. (2012). Identifying luminous active galactic nuclei in deep surveys: Revised IRAC selection criteria. Astrophysical Journal, 748(2).
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  Abstract: Spitzer/IRAC selection is a powerful tool for identifying luminous active galactic nuclei (AGNs). For deep IRAC data, however, the AGN selection wedges currently in use are heavily contaminated by star-forming galaxies, especially at high redshift. Using the large samples of luminous AGNs and high-redshift star-forming galaxies in COSMOS, we redefine the AGN selection criteria for use in deep IRAC surveys. The new IRAC criteria are designed to be both highly complete and reliable, and incorporate the best aspects of the current AGN selection wedges and of infrared power-law selection while excluding high-redshift star-forming galaxies selected via the BzK, distant red galaxy, Lyman-break galaxy, and submillimeter galaxy criteria. At QSO luminosities of log L 2-10 keV(ergs -1)≥44, the new IRAC criteria recover 75% of the hard X-ray and IRAC-detected XMM-COSMOS sample, yet only 38% of the IRAC AGN candidates have X-ray counterparts, a fraction that rises to 52% in regions with Chandra exposures of 50-160ks. X-ray stacking of the individually X-ray non-detected AGN candidates leads to a hard X-ray signal indicative of heavily obscured to mildly Compton-thick obscuration (log N H (cm -2) = 23.5 0.4). While IRAC selection recovers a substantial fraction of luminous unobscured and obscured AGNs, it is incomplete to low-luminosity and host-dominated AGNs. © 2012. The American Astronomical Society. All rights reserved..
• Elvis, M., Hao, H., Civano, F., Brusa, M., Salvato, M., Bongiorno, A., Capak, P., Zamorani, G., Comastri, A., Jahnke, K., Lusso, E., Mainieri, V., Trump, J. R., Ho, L. C., Aussel, H., Cappelluti, N., Cisternas, M., Frayer, D., Gilli, R., , Hasinger, G., et al. (2012). Spectral energy distributions of type 1 active galactic nuclei in the Cosmos survey. I. the XMM-Cosmos sample. Astrophysical Journal, 759(1).
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  Abstract: The "Cosmic Evolution Survey" (COSMOS) enables the study of the spectral energy distributions (SEDs) of active galactic nuclei (AGNs) because of the deep coverage and rich sampling of frequencies from X-ray to radio. Here we present an SED catalog of 413 X-ray (XMM-Newton)-selected type 1 (emission line FWHM > 2000 km s -1) AGNs with Magellan, SDSS, or VLT spectrum. The SEDs are corrected for Galactic extinction, broad emission line contributions, constrained variability, and host galaxy contribution. We present the mean SED and the dispersion SEDs after the above corrections in the rest-frame 1.4GHz to 40 keV, and show examples of the variety of SEDs encountered. In the near-infrared to optical (rest frame ∼8 μm-4000 Å), the photometry is complete for the whole sample and the mean SED is derived from detections only. Reddening and host galaxy contamination could account for a large fraction of the observed SED variety. The SEDs are all available online. © 2012. The American Astronomical Society. All rights reserved.
• Impey, C. (2012). Calculating Catastrophe by Gordon Woo. Mathematical Intelligencer, 34(2), 76-.
• Iwasawa, K., Mainieri, V., Brusa, M., Comastri, A., Gilli, R., Vignali, C., Hasinger, G., Sanders, D. B., Cappelluti, N., Impey, C. D., Koekemoer, A., Lanzuisi, G., Lusso, E., Merloni, A., Salvato, M., Taniguchi, Y., & Trump, J. R. (2012). Fe K emission from active galaxies in the COSMOS field. Astronomy and Astrophysics, 537.
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  Abstract: We present a rest-frame spectral stacking analysis of ∼1000 X-ray sources detected in the XMM-COSMOS field to investigate the iron-K line properties of active galaxies beyond redshift z ∼ 1. In Type I AGN that have a typical X-ray luminosity of L X ∼ 1.5 × 10 44 (erg s -1) and z ∼ 1.6 the cold Fe K at 6.4 keV is weak (EW ∼ 0.05 keV), which agrees with the known trend. In contrast, high-ionization lines of Fe xxv and Fe xxvi are pronounced. These high-ionization Fe K lines appear to have a connection with high accretion rates. While no broad Fe emission is detected in the total spectrum, it might be present, albeit at low significance (∼2σ), when the X-ray luminosity is restricted to the range below 3 × 10 44 erg s -1, or when an intermediate range of Eddington ratio around λ ∼ 0.1 is selected. In Type II AGN, both cold and high-ionzation lines become weak with increasing X-ray luminosity. However, we detected strong high-ionization Fe K (EW ∼ 0.3 keV) in the spectrum of objects at z > 2, while we found no 6.4 keV line. We also found that the primary source of the high-ionization Fe K emission are those objects detected with Spitzer-MIPS at 24 μm. Given their median redshift of z ≈ 2.5, their bolometric luminosity is likely to reach 10 13 L ⊙ and the MIPS-detected emission most likely originates from hot dust heated by embedded AGN, probably accreting at high Eddington ratio. These properties match those of rapidly growing black holes in ultra-luminous infrared galaxies at the interesting epoch (z ∼ 2-3) of galaxy formation. © 2012 ESO.
• Lusso, E., Comastri, A., Simmons, B. D., Mignoli, M., Zamorani, G., Vignali, C., Brusa, M., Shankar, F., Lutz, D., Trump, J. R., Maiolino, R., Gilli, R., Bolzonella, M., Puccetti, S., Salvato, M., Impey, C. D., Civano, F., Elvis, M., Mainieri, V., , Silverman, J. D., et al. (2012). Bolometric luminosities and Eddington ratios of X-ray selected active galactic nuclei in the XMM-COSMOS survey. Monthly Notices of the Royal Astronomical Society, 425(1), 623-640.
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  Abstract: Bolometric luminosities and Eddington ratios of both X-ray selected broad-line (Type-1) and narrow-line (Type-2) active galactic nuclei (AGN) from the XMM-Newton survey in the Cosmic Evolution Survey field are presented. The sample is composed of 929 AGN (382 Type-1 AGN and 547 Type-2 AGN) and it covers a wide range of redshifts, X-ray luminosities and absorbing column densities. About 65per cent of the sources are spectroscopically identified as either Type-1 or Type-2 AGN (83 and 52per cent, respectively), while accurate photometric redshifts are available for the rest of the sample. The study of such a large sample of X-ray selected AGN with a high-quality multiwavelength coverage from the far-infrared (now with the inclusion of Herschel data at 100 and 160 μm) to the optical-ultraviolet allows us to obtain accurate estimates of bolometric luminosities, bolometric corrections and Eddington ratios. The k bol - L bol relations derived in this work are calibrated for the first time against a sizable AGN sample, and rely on observed redshifts, X-ray luminosities and column density distributions. We find that k bol is significantly lower at high L bol with respect to previous estimates by Marconi et al. and Hopkins et al. Black hole (BH) masses and Eddington ratios are available for 170 Type-1 AGN, while BH masses for Type-2 AGN are computed for 481 objects using the BH mass-stellar mass relation and the morphological information. We confirm a trend between k bol and λ Edd, with lower hard X-ray bolometric corrections at lower Eddington ratios for both Type-1 and Type-2 AGN. We find that, on average, the Eddington ratio increases with redshift for all types of AGN at any given M BH, while no clear evolution with redshift is seen at any given L bol. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.
• Masters, D., Capak, P., Salvato, M., Civano, F., Mobasher, B., Siana, B., Hasinger, G., Impey, C. D., Nagao, T., Trump, J. R., Ikeda, H., Elvis, M., & Scoville, N. (2012). Evolution of the quasar luminosity function over 3 < z < 5 in the cosmos survey field. Astrophysical Journal, 755(2).
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  Abstract: We investigate the high-redshift quasar luminosity function (QLF) down to an apparent magnitude of I AB = 25 in the Cosmic Evolution Survey (COSMOS). Careful analysis of the extensive COSMOS photometry and imaging data allows us to identify and remove stellar and low-redshift contaminants, enabling a selection that is nearly complete for type-1 quasars at the redshifts of interest. We find 155 likely quasars at z > 3.1, 39 of which have prior spectroscopic confirmation. We present our sample in detail and use these confirmed and likely quasars to compute the rest-frame UV QLF in the redshift bins 3.1 < z < 3.5 and 3.5 < z < 5. The space density of faint quasars decreases by roughly a factor of four from z 3.2 to z 4, with faint-end slopes of β -1.7 at both redshifts. The decline in space density of faint optical quasars at z > 3 is similar to what has been found for more luminous optical and X-ray quasars. We compare the rest-frame UV luminosity functions found here with the X-ray luminosity function at z > 3, and find that they evolve similarly between z 3.2 and z 4; however, the different normalizations imply that roughly 75% of X-ray bright active galactic nuclei (AGNs) at z 3-4 are optically obscured. This fraction is higher than found at lower redshift and may imply that the obscured, type-2 fraction continues to increase with redshift at least to z 4. Finally, the implications of the results derived here for the contribution of quasars to cosmic reionization are discussed. © 2012. The American Astronomical Society. All rights reserved.
• Race, M., Denning, K., Bertka, C. M., Dick, S. J., Harrison, A. A., Impey, C., & Mancinelli, R. (2012). Astrobiology and society: Building an interdisciplinary research community. Astrobiology, 12(10), 958-965.
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  PMID: 23046203;PMCID: PMC3484766;Abstract: This paper reports recent efforts to gather experts from the humanities and social sciences along with astrobiologists to consider the cultural, societal, and psychological implications of astrobiology research and exploration. We began by convening a workshop to draft a research roadmap on astrobiology's societal implications and later formed a Focus Group on Astrobiology and Society under the auspices of the NASA Astrobiology Institute (NAI). Just as the Astrobiology Science Roadmap and various astrobiology science focus groups have helped researchers orient and understand their work across disciplinary contexts, our intent was to apply the same approach to examine areas beyond the physical and life sciences and expand interdisciplinary interaction and scholarly understanding. These efforts continue as an experiment in progress, with an open invitation to interested researchers-astrobiologists as well as scholars in the humanities and social sciences-to become involved in research, analysis, and proactive discussions concerning the potential impacts of astrobiology on society as well as the possible impacts of society on progress in astrobiology. Key Words: Astrobiology-Extraterrestrial life-Life detection. Astrobiology 12, 958-965. © Copyright 2012, Mary Ann Liebert, Inc. 2012.
• Cisternas, M., Jahnke, K., Bongiorno, A., Inskip, K. J., Impey, C. D., Koekemoer, A. M., Merloni, A., Salvato, M., & Trump, J. R. (2011). Secular evolution and A non-evolving black-hole-to-galaxy mass ratio in the last 7Gyr. Astrophysical Journal Letters, 741(1).
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  Abstract: We present new constraints on the ratio of black hole (BH) mass to total galaxy stellar mass at 0.3 < z < 0.9 for a sample of 32 type-1 active galactic nuclei (AGNs) from the XMM-COSMOS survey covering the range M BH 107.2 - 8.7 M⊙. Virial M BH estimates based on Hβ are available from the COSMOS Magellan/IMACS survey. We use high-resolution Hubble Space Telescope (HST) imaging to decompose the light of each type-1 AGN and host galaxy, and employ a specially built mass-to-light ratio to estimate the stellar masses (M *). The M BH-M * ratio shows a zero offset with respect to the local relation for galactic bulge masses, and we also find no evolution in the mass ratio M BH/M *(1 + z)0.02 0.34 up to z 0.9. Interestingly, at the high-M BH end there is a positive offset from the z = 0 relation, which can be fully explained by a mass function bias with a cosmic scatter of σμ = 0.3, reaffirming that the intrinsic distribution is consistent with zero evolution. From our results we conclude that since z 0.9 no substantial addition of stellar mass is required: the decline in star formation rates and merger activity at z < 1 support this scenario. Nevertheless, given that a significant fraction of these galaxies show a disk component, their bulges are indeed undermassive. This is a direct indication that for the last 7Gyr the only essential mechanism required for these galaxies to obey the z = 0 relation is a redistribution of stellar mass to the bulge, likely driven by secular processes, i.e., internal instabilities and minor merging. © 2011. The American Astronomical Society. All rights reserved.
• Cisternas, M., Jahnke, K., Inskip, K. J., Kartaltepe, J., Koekemoer, A. M., Lisker, T., Robaina, A. R., Scodeggio, M., Sheth, K., Trump, J. R., Andrae, R., Miyaji, T., Lusso, E., Brusa, M., Capak, P., Cappelluti, N., Civano, F., Ilbert, O., Impey, C. D., , Leauthaud, A., et al. (2011). The bulk of the black hole growth since z ∼ 1 occurs in a secular universe: No major merger-AGN connection. Astrophysical Journal, 726(2).
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  Abstract: What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGNs) activity? To answer this long-standing question, we analyze 140 XMM-Newton-selected AGN host galaxies and a matched control sample of 1264 inactive galaxies over z ∼ 0.3-1.0 and M* < 1011.7 M⊙ with high-resolution Hubble Space Telescope/Advanced Camera for Surveys imaging from the COSMOS field. The visual analysis of their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies in the AGN and control samples, i.e., quantifying the signature of recent mergers which might potentially be responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies do not show strong distortions and (2) there is no significant difference in the distortion fractions between active and inactive galaxies. Our findings provide the best direct evidence that, since z ∼ 1, the bulk of black hole (BH) accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e., internal secular processes and minor interactions, are the leading triggers for the episodes of major BH growth. We also exclude an alternative interpretation of our results: a substantial time lag between merging and the observability of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to (1) recent major mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGNs, (2) the lack of a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low levels of soft X-ray obscuration for AGNs hosted by interacting galaxies, in contrast to model predictions. © 2011. The American Astronomical Society.
• Impey, C. D., Buxner, S. R., Antonellis, J., Johnson, E., & King, C. (2011). A twenty-year survey of science literacy among college undergraduates.. Journal of College Science Teaching, 40(4), 31-37.
• Impey, H. S., Impey, C. D., & Buxner, S. R. (2011). Astrology beliefs among undergraduate students.. Astronomy Education Review, 10(1), 010101.
• Mainieri, V., Bongiorno, A., Merloni, A., Aller, M., Carollo, M., Iwasawa, K., Koekemoer, A. M., Mignoli, M., Silverman, J. D., Bolzonella, M., Brusa, M., Comastri, A., Gilli, R., Halliday, C., Ilbert, O., Lusso, E., Salvato, M., Vignali, C., Zamorani, G., , Contini, T., et al. (2011). Black hole accretion and host galaxies of obscured quasars in XMM-COSMOS. Astronomy and Astrophysics, 535.
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  Abstract: Aims. We explore the connection between black hole growth at the center of obscured quasars selected from the XMM-COSMOS survey and the physical properties of their host galaxies. We study a bolometric regime ( Lbol) = 8 × 1045 erg s-1) where several theoretical models invoke major galaxy mergers as the main fueling channel for black hole accretion. Methods. To derive robust estimates of the host galaxy properties, we use an SED fitting technique to distinguish the AGN and host galaxy emission. We evaluate the effect on galaxy properties estimates of being unable to remove the nuclear emission from the SED. The superb multi-wavelength coverage of the COSMOS field allows us to obtain reliable estimates of the total stellar masses and star formation rates (SFRs) of the hosts. We supplement this information with a morphological analysis of the ACS/HST images, optical spectroscopy, and an X-ray spectral analysis. Results. We confirm that obscured quasars mainly reside in massive galaxies (M* > 1010M ⊙) and that the fraction of galaxies hosting such powerful quasars monotonically increases with the stellar mass. We stress the limitation of the use of rest-frame color-magnitude diagrams as a diagnostic tool for studying galaxy evolution and inferring the influence that AGN activity can have on such a process. We instead use the correlation between SFR and stellar mass found for star-forming galaxies to discuss the physical properties of the hosts. We find that at z ∼ 1, ≈ 62% of Type-2 QSOs hosts are actively forming stars and that their rates are comparable to those measured for normal star-forming galaxies. The fraction of star-forming hosts increases with redshift: ≈ 71% at z ∼ 2, and 100% at z ∼ 3. We also find that the evolution from z ∼ 1 to z ∼ 3 of the specific SFR of the Type-2 QSO hosts is in excellent agreement with that measured for star-forming galaxies. From the morphological analysis, we conclude that most of the objects are bulge-dominated galaxies, and that only a few of them exhibit signs of recent mergers or disks. Finally, bulge-dominated galaxies tend to host Type-2 QSOs with low Eddington ratios (λ < 0.1), while disk-dominated or merging galaxies have at their centers BHs accreting at high Eddington ratios (λ > 0.1). © 2011 ESO.
• Salvato, M., Ilbert, O., Hasinger, G., Rau, A., Civano, F., Zamorani, G., Brusa, M., Elvis, M., Vignali, C., Aussel, H., Comastri, A., Fiore, F., Floc'H, E. L., Mainieri, V., Bardelli, S., Bolzonella, M., Bongiorno, A., Capak, P., Caputi, K., , Cappelluti, N., et al. (2011). Dissecting photometric redshift for active galactic nucleus using XMM- and Chandra-cosmos samples. Astrophysical Journal, 742(2).
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  Abstract: In this paper, we release accurate photometric redshifts for 1692 counterparts to Chandra sources in the central square degree of the Cosmic Evolution Survey (COSMOS) field. The availability of a large training set of spectroscopic redshifts that extends to faint magnitudes enabled photometric redshifts comparable to the highest quality results presently available for normal galaxies. We demonstrate that morphologically extended, faint X-ray sources without optical variability are more accurately described by a library of normal galaxies (corrected for emission lines) than by active galactic nucleus (AGN) dominated templates, even if these sources have AGN-like X-ray luminosities. Preselecting the library on the bases of the source properties allowed us to reach an accuracy with a fraction of outliers of 5.8% for the entire Chandra-COSMOS sample. In addition, we release revised photometric redshifts for the 1735 optical counterparts of the XMM-detected sources over the entire 2deg2 of COSMOS. For 248 sources, our updated photometric redshift differs from the previous release by Δz > 0.2. These changes are predominantly due to the inclusion of newly available deep H-band photometry (H AB = 24mag). We illustrate once again the importance of a spectroscopic training sample and how an assumption about the nature of a source together, with the number and the depth of the available bands, influences the accuracy of the photometric redshifts determined for AGN. These considerations should be kept in mind when defining the observational strategies of upcoming large surveys targeting AGNs, such as eROSITA at X-ray energies and the Australian Square Kilometre Array Pathfinder Evolutionary Map of the Universe in the radio band. © 2011. The American Astronomical Society. All rights reserved.
• Trump, J. R., Impey, C. D., Kelly, B. C., Civano, F., Gabor, J. M., Diamond-Stanic, A. M., Merloni, A., Urry, C. M., Hao, H., Jahnke, K., Nagao, T., Taniguchi, Y., Koekemoer, A. M., Lanzuisi, G., Liu, C., Mainieri, V., Salvato, M., & Scoville, N. Z. (2011). Accretion rate and the physical nature of unobscured active galaxies. Astrophysical Journal, 733(1).
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  Abstract: We show how accretion rate governs the physical properties of a sample of unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rates by using accurate intrinsic accretion luminosities (L int) from well-sampled multiwavelength spectral energy distributions from the Cosmic Evolution Survey, and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L int/L Edd > 10-2), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L int/L Edd < 10-2) are unobscured and yet lack a broad-line region. The disappearance of the broad emission lines is caused by an expanding radiatively inefficient accretion flow (RIAF) at the inner radius of the accretion disk. The presence of the RIAF also drives L int/L Edd < 10-2 narrow-line and lineless AGNs to have ratios of radio-to-optical/UV emission that are 10 times higher than L int/L Edd > 10-2 broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L int/L Edd < 10-2 AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together, these results suggest that specific accretion rate is an important physical "axis" of AGN unification, as described by a simple model. © 2011. The American Astronomical Society. All rights reserved..
• Trump, J. R., Nagao, T., Ikeda, H., Murayama, T., Impey, C. D., Stocke, J. T., Civano, F., Elvis, M., Jahnke, K., Kelly, B. C., Koekemoer, A. M., & Taniguchi, Y. (2011). Spectropolarimetric evidence for radiatively inefficient accretion in an optically dull active galaxy. Astrophysical Journal, 732(1).
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  Abstract: We present Subaru/FOCAS spectropolarimetry of two active galaxies in the Cosmic Evolution Survey. These objects were selected to be optically dull, with the bright X-ray emission of an active galactic nucleus (AGN) but missing optical emission lines in our previous spectroscopy. Our new observations show that one target has very weak emission lines consistent with an optically dull AGN, while the other object has strong emission lines typical of a host-diluted Type 2 Seyfert galaxy. In neither source do we observe polarized emission lines, with 3σ upper limits of P BLR ≲ 2%. This means that the missing broad emission lines (and weaker narrow emission lines) are not due to simple anisotropic obscuration, e.g., by the canonical AGN torus. The weak-lined optically dull AGN exhibits a blue polarized continuum with P = 0.78% ± 0.07% at 4400 Å < λrest < 7200 Å (P = 1.37% ± 0.16% at 4400 Å < λrest < 5050Å). The wavelength dependence of this polarized flux is similar to that of an unobscured AGN continuum and represents the intrinsic AGN emission, either as synchrotron emission or the outer part of an accretion disk reflected by a clumpy dust scatterer. Because this intrinsic AGN emission lacks emission lines, this source is likely to have a radiatively inefficient accretion flow. © 2011. The American Astronomical Society. All rights reserved.
• Brusa, M., Civano, F., Comastri, A., Miyaji, T., Salvato, M., Zamorani, G., Cappelluti, N., Fiore, F., Hasinger, G., Mainieri, V., Merloni, A., Bongiorno, A., Capak, P., Elvis, M., Gilli, R., Hao, H., Jahnke, K., Koekemoer, A. M., Ilbert, O., , Floc'H, E. L., et al. (2010). The XMM-newton wide-field survey in the cosmos field (XMM-COSMOS): Demography and multiwavelength properties of obscured and unobscured luminous active galactic nuclei. Astrophysical Journal Letters, 716(1), 348-369.
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  Abstract: We report the final optical identifications of the medium-depth (∼60ks), contiguous (2deg2) XMM-Newton survey of the COSMOS field. XMM-Newton has detected ∼1800 X-ray sources down to limiting fluxes of ∼5 × 10-16, ∼3 × 10-15, and ∼7 × 10-15ergcm-2s-1 in the 0.5-2keV, 2-10keV, and 5-10keV bands, respectively (∼1 × 10-15, 6 × 10-15, and 1 × 10-14ergcm-2s -1, in the three bands, respectively, over 50% of the area). The work is complemented by an extensive collection of multiwavelength data from 24 μm to UV, available from the COSMOS survey, for each of the X-ray sources, including spectroscopic redshifts for ≳50% of the sample, and high-quality photometric redshifts for the rest. The XMM and multiwavelength flux limits are well matched: 1760 (98%) of the X-ray sources have optical counterparts, 1711 (95%) have IRAC counterparts, and 1394 (78%) have MIPS 24μm detections. Thanks to the redshift completeness (almost 100%) we were able to constrain the high-luminosity tail of the X-ray luminosity function confirming that the peak of the number density of log LX > 44.5 active galactic nuclei (AGNs) is at z ∼ 2. Spectroscopically identified obscured and unobscured AGNs, as well as normal and star-forming galaxies, present well-defined optical and infrared properties. We devised a robust method to identify a sample of ∼150 high-redshift (z > 1), obscured AGN candidates for which optical spectroscopy is not available. We were able to determine that the fraction of the obscured AGN population at the highest (LX > 10 44ergs-1) X-ray luminosity is ∼15%-30% when selection effects are taken into account, providing an important observational constraint for X-ray background synthesis. We studied in detail the optical spectrum and the overall spectral energy distribution of a prototypical Type 2 QSO, caught in a stage transitioning from being starburst dominated to AGN dominated, which was possible to isolate only thanks to the combination of X-ray and infrared observations. © 2010 The American Astronomical Society.
• Civano, F., Elvis, M., Lanzuisi, G., Jahnke, K., Zamorani, G., Blecha, L., Bongiorno, A., Brusa, M., Comastri, A., Hao, H., Leauthaud, A., Loeb, A., Mainieri, V., Piconcelli, E., Salvato, M., Scoville, N., Trump, J., Vignali, C., Aldcroft, T., , Bolzonella, M., et al. (2010). A runaway black hole in cosmos: Gravitational wave or slingshot recoil?. Astrophysical Journal Letters, 717(1), 209-222.
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  Abstract: We present a detailed study of a peculiar source detected in the COSMOS survey at z = 0.359. Source CXOC J100043.1+020637, also known as CID-42, has two compact optical sources embedded in the same galaxy. The distance between the two, measured in the HST/ACS image, is 0495 0005 that, at the redshift of the source, corresponds to a projected separation of 2.46 ± 0.02 kpc. A large (∼1200kms-1) velocity offset between the narrow and broad components of Hβ has been measured in three different optical spectra from the VLT/VIMOS and Magellan/IMACS instruments. CID-42 is also the only X-ray source in COSMOS, having in its X-ray spectra a strong redshifted broad absorption iron line and an iron emission line, drawing an inverted P-Cygni profile. The Chandra and XMM-Newton data show that the absorption line is variable in energy by ΔE = 500eV over four years and that the absorber has to be highly ionized in order not to leave a signature in the soft X-ray spectrum. That these features - the morphology, the velocity offset, and the inverted P-Cygni profile - occur in the same source is unlikely to be a coincidence. We envisage two possible explanations, both exceptional, for this system: (1) a gravitational wave (GW) recoiling black hole (BH), caught 1-10 Myr after merging; or (2) a Type 1/Type 2 system in the same galaxy where the Type 1 is recoiling due to the slingshot effect produced by a triple BH system. The first possibility gives us a candidate GW recoiling BH with both spectroscopic and imaging signatures. In the second case, the X-ray absorption line can be explained as a BAL-like outflow from the foreground nucleus (a Type 2 AGN) at the rearer one (a Type 1 AGN), which illuminates the otherwise undetectable wind, giving us the first opportunity to show that fast winds are present in obscured active galactic nuclei (AGNs), and possibly universal in AGNs. © 2010. The American Astronomical Society. All rights reserved..
• Hao, H., Elvis, M., Civano, F., Lanzuisi, G., Brusa, M., Lusso, E., Zamorani, G., Comastri, A., Bongiorno, A., Impey, C. D., Koekemoer, A. M., Floc'h, E. L., Salvato, M., Sanders, D., Trump, J. R., & Vignali, C. (2010). Hot-dust-poor type 1 active galactic nuclei in the COSMOS survey. Astrophysical Journal Letters, 724(1 PART 2), L59-L63.
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  Abstract: We report a sizable class of type 1 active galactic nuclei (AGNs) with unusually weak near-infrared (1-3μm) emission in the XMM-COSMOS type 1 AGN sample. The fraction of these "hot-dust-poor" AGNs increases with redshift from 6%at lowredshift (z < 2) to 20% at moderate high redshift (2 < z < 3.5). There is no clear trend of the fraction with other parameters: bolometric luminosity, Eddington ratio, black hole mass, and X-ray luminosity. The 3μm emission relative to the 1μmemission is a factor of 2-4 smaller than the typical Elvis et al.AGNspectral energy distribution (SED), which indicates a "torus" covering factor of 2%-29%, a factor of 3-40 smaller than required by unified models. The weak hot dust emission seems to expose an extension of the accretion disk continuum in some of the source SEDs. We estimate the outer edge of their accretion disks to lie at (0.3-2.0) × 104 Schwarzschild radii, ∼10-23 times the gravitational stability radii. Formation scenarios for these sources are discussed. © 2010 The American Astronomical Society. All rights reserved.
• Impey, C. D., Trump, J. R., & Gabor, J. M. (2010). AGN and host galaxies in the COSMOS survey. Proceedings of the International Astronomical Union, 6(S277), 21-25.
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  Abstract: The Cosmological Evolution Survey (COSMOS) is a unique tool for studying low level AGN activity and the co-evolution of galaxies and supermassive black holes. COSMOS involves the largest contiguous region of the sky ever imaged by HST; it includes very complete multiwavelength coverage, and the largest joint samples of galaxy and AGN redshifts in any deep survey. The result is a search for AGN with low black hole mass, low accretion rates, and levels of obscuration that can remove them from optical surveys. A complete census of intermediate mass black holes at redshifts of 1 to 3 is required to tell the story of the co-evolution of galaxies and their embedded, and episodically active, black holes. © Copyright International Astronomical Union 2011.
• Impey, C., & Green, H. (2010). The living cosmos: A fabric that binds art and science. Leonardo, 43(5), 435-441.
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  Abstract: The authors, an astronomer and an artist, have collaborated on a series of seven mixedmedia constructions and prose pieces that follow the flow and themes of Impey's book on astrobiology, The Living Cosmos. The book summarizes recent research on astrobiology, from the origin of life on Earth and its environmental range on this planet to the search for life in the solar system and beyond. The artist's work encapsulates these ideas with its use of material objects, textures, images and metaphors that mirror the elements of the scientific approach to astrobiology. © ISAST.
• Mainieri, V., Vignali, C., Merloni, A., Civano, F., Puccetti, S., Brusa, M., Gilli, R., Bolzonella, M., Comastri, A., Zamorani, G., Aller, M., Carollo, M., Scarlata, C., Elvis, M., Aldcroft, T. L., Cappelluti, N., Fabbiano, G., Finoguenov, A., Fiore, F., , Fruscione, A., et al. (2010). Ultraluminous X-ray sources out to z ∼ 0.3 in the COSMOS field. Astronomy and Astrophysics, 514(12).
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  Abstract: Context. Using Chandra observations we identified a sample of seven off-nuclear X-ray sources in the redshift range z = 0.072-0.283, located within optically bright galaxies in the COSMOS Survey. All of them, if associated with their closest bright galaxy, would have L[0.5-7 keV] > 1039 erg s-1 and therefore can be classified as ultraluminous X-ray sources (ULXs). Aims. Using the multi-wavelength coverage available in the COSMOS field, we studied the properties of the host galaxies of these ULXs. In detail, we derived their star formation rate from Hα measurements and their stellar masses using SED fitting techniques with the aim to compute the probability to have an off-nuclear source based on the host galaxy properties. We divided the host galaxies in different morphological classes with the available ACS/HST imaging. Methods. We selected off-nuclear candidates with the following criteria: 1) the distance between the X-ray and the optical centroid has to be larger than 0.9, larger than 1.8 times the radius of the Chandra positional error circle and smaller than the Petrosian radius of the host galaxy; 2) the optical counterpart is a bright galaxy (RAB
• Merloni, A., Bongiorno, A., Bolzonella, M., Brusa, M., Civano, F., Comastri, A., Elvis, M., Fiore, F., Gilli, R., Hao, H., Jahnke, K., Koekemoer, A. M., Lusso, E., Mainieri, V., Mignoli, M., Miyaji, T., Renzini, A., Salvato, M., Silverman, J., , Trump, J., et al. (2010). On the cosmic evolution of the scaling relations between black holes and their host galaxies: Broad-line active galactic nuclei in the zCosmos survey. Astrophysical Journal Letters, 708(1), 137-157.
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  Abstract: We report on the measurement of the physical properties (rest-frame K-band luminosity and total stellar mass) of the hosts of 89 broad-line (type-1) active galactic nuclei (AGNs) detected in the zCOSMOS survey in the redshift range 1 < z < 2.2. The unprecedented multi-wavelength coverage of the survey field allows us to disentangle the emission of the host galaxy from that of the nuclear black hole in their spectral energy distributions (SEDs). We derive an estimate of black hole masses through the analysis of the broad Mg II emission lines observed in the medium-resolution spectra taken with VIMOS/VLT as part of the zCOSMOS project. We found that, as compared to the local value, the average black hole to host-galaxy mass ratio appears to evolve positively with redshift, with a best-fit evolution of the form (1+z)0.68±0.12 -0.3+0.6, where the large asymmetric systematic errors stem from the uncertainties in the choice of initial mass function, in the calibration of the virial relation used to estimate BH masses and in the mean QSO SED adopted. On the other hand, if we consider the observed rest-frame K-band luminosity, objects tend to be brighter, for a given black hole mass, than those on the local M BH-MK relation. This fact, together with more indirect evidence from the SED fitting itself, suggests that the AGN hosts are likely actively star-forming galaxies. A thorough analysis of observational biases induced by intrinsic scatter in the scaling relations reinforces the conclusion that an evolution of the M BH-M * relation must ensue for actively growing black holes at early times: either its overall normalization, or its intrinsic scatter (or both) appear to increase with redshift. This can be interpreted as signature of either a more rapid growth of supermassive black holes at high redshift, a change of structural properties of AGN hosts at earlier times, or a significant mismatch between the typical growth times of nuclear black holes and host galaxies. In any case, our results provide important clues on the nature of the early co-evolution of black holes and galaxies and challenging tests for models of AGN feedback and self-regulated growth of structures. © 2010 The American Astronomical Society.
• Peng, C. Y., Ho, L. C., Impey, C. D., & Rix, H. (2010). Detailed decomposition of galaxy images. II. beyond axisymmetric models. Astronomical Journal, 139(6), 2097-2129.
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  Abstract: We present a two-dimensional (2D) fitting algorithm (GALFIT, ver. 3) with new capabilities to study the structural components of galaxies and other astronomical objects in digital images. Our technique improves on previous 2D fitting algorithms by allowing for irregular, curved, logarithmic and power-law spirals, ring, and truncated shapes in otherwise traditional parametric functions like the Sérsic, Moffat, King, Ferrer, etc., profiles. One can mix and match these new shape features freely, with or without constraints, and apply them to an arbitrary number of model components of numerous profile types, so as to produce realistic-looking galaxy model images. Yet, despite the potential for extreme complexity, the meaning of the key parameters like the Sérsic index, effective radius, or luminosity remains intuitive and essentially unchanged. The new features have an interesting potential for use to quantify the degree of asymmetry of galaxies, to quantify low surface brightness tidal features beneath and beyond luminous galaxies, to allow more realistic decompositions of galaxy subcomponents in the presence of strong rings and spiral arms, and to enable ways to gauge the uncertainties when decomposing galaxy subcomponents. We illustrate these new features by way of several case studies that display various levels of complexity. © 2010. The American Astronomical Society. All rights reserved.
• Smolić, V., Zamorani, G., Schinnerer, E., Bardelli, S., Bondi, M., Brzan, L., Carilli, C. L., Ciliegi, P., Elvis, M., Impey, C. D., Koekemoer, A. M., Merloni, A., Paglione, T., Salvato, M., Scodeggio, M., Scoville, N., & Trump, J. R. (2010). Erratum: Cosmic evolution of radio selected active galactic nuclei in the cosmos field (The Astrophysical Journal (2009) 696 (24)). Astrophysical Journal Letters, 708(1), 909-.
• Elvis, M., Civano, F., Vignali, C., Puccetti, S., Fiore, F., Cappelluti, N., Aldcroft, T. L., Fruscione, A., Zamorani, G., Comastri, A., Brusa, M., Gilli, R., Miyaji, T., Damiani, F., Koekemoer, A. M., Finoguenov, A., Brunner, H., Urry, C. M., Silverman, J., , Mainieri, V., et al. (2009). The Chandra COSMOS Survey. I. overview and point source catalog. Astrophysical Journal, Supplement Series, 184(1), 158-171.
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  Abstract: The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that has imaged the central 0.5 deg2 of the COSMOS field (centered at 10h, +02o) with an effective exposure of ∼160 ks, and an outer 0.4 deg2 area with an effective exposure of ∼80 ks. The limiting source detection depths are 1.9 × 10-16 erg cm -2 s-1 in the soft (0.5-2 keV) band, 7.3 × 10 -16 erg cm-2 s-1 in the hard (2-10 keV) band, and 5.7 × 10-16 erg cm-2 s-1 in the full (0.5-10 keV) band. Here we describe the strategy, design, and execution of the C-COSMOS survey, and present the catalog of 1761 point sources detected at a probability of being spurious of
• Fiore, F., Puccetti, S., Brusa, M., Salvato, M., Zamorani, G., Aldcroft, T., Aussel, H., Brunner, H., Capak, P., Cappelluti, N., Civano, F., Comastri, A., Elvis, M., Feruglio, C., Finoguenov, A., Fruscione, A., Gilli, R., Hasinger, G., Koekemoer, A., , Kartaltepe, J., et al. (2009). Chasing highly obscured QSOs in the cosmos field. Astrophysical Journal, 693(1), 447-462.
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  Abstract: A large population of heavily obscured, Compton-thick active galactic nuclei (AGNs) is predicted by AGN synthesis models for the cosmic X-ray background and by the "relic" supermassive black hole mass function measured from local bulges. However, even the deepest X-ray surveys are inefficient to search for these elusive AGNs. Alternative selection criteria, combining mid-infrared with near-infrared, and optical photometry, have instead been successful in pinpointing a large population of Compton-thick AGNs. We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs) in the Cosmic Evolution Survey (COSMOS) field to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density, and therefore large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24 μm fluxes higher than 550 μJy. For the MIPS sources not directly detected in the Chandra images, we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population, we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations, including both obscured AGN and star-forming galaxies. The volume density of Compton-thick QSOs (log L(2-10 keV) = 44-45 erg s-1, or logλL λ(5.8 μm) = 44.79-46.18 erg s-1 for a typical infrared to X-ray luminosity ratio) evaluated in this way is (4.8 ± 1.1) × 10-6 Mpc-3 in the redshift bin 1.2-2.2. This density is ∼44% of all X-ray-selected QSOs in the same redshift and luminosity bin, and it is consistent with the expectation of the most up-to-date AGN synthesis models for the cosmic X-ray background (Gilli et al. 2007). The density of lower luminosity Compton-thick AGNs (log L(2-10 keV) = 43.5-44) at z = 0.7-1.2 is (3.7 ± 1.1) × 10-5 Mpc-3, corresponding to ∼67% of X-ray-selected AGNs. The comparison between the fraction of infrared-selected, Compton-thick AGNs to the X-ray selected, unobscured, and moderately obscured AGNs at high and low luminosity suggests that Compton-thick AGNs follow a luminosity dependence similar to that discovered for Compton-thin AGNs, becoming relatively rarer at high luminosities. We estimate that the fraction of AGNs (unobscured, moderately obscured, and Compton thick) to the total MIPS source population is 49 ± 10%, a value significantly higher than that previously estimated at similar 24 μm fluxes. We discuss how our findings can constrain AGN feedback models. © 2009. The American Astronomical Society. All rights reserved..
• Gabor, J. M., Impey, C. D., Jahnke, K., Simmons, B. D., Trump, J. R., Koekemoer, A. M., Brusa, M., Cappelluti, N., Schinnerer, E., Smolčić, V., Salvato, M., Rhodes, J. D., Mobasher, B., Capak, P., Massey, R., Leauthaud, A., & Scoville, N. (2009). Active galactic nucleus host galaxy morphologies in COSMOS. Astrophysical Journal, 691(1), 705-722.
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  Abstract: We use Hubble Space Telescope/Advanced Camera for Surveys images and a photometric catalog of the Cosmic Evolution Survey (COSMOS) field to analyze morphologies of the host galaxies of 400 active galactic nucleus (AGN) candidates at redshifts 0.3 < z < 1.0. We compare the AGN hosts with a sample of nonactive galaxies drawn from the COSMOS field to match the magnitude and redshift distribution of the AGN hosts. We perform two-dimensional surface brightness modeling with GALFIT to yield host galaxy and nuclear point source magnitudes. X-ray-selected AGN host galaxy morphologies span a substantial range that peaks between those of early-type, bulge-dominated and late-type, disk-dominated systems. We also measure the asymmetry and concentration of the host galaxies. Unaccounted for, the nuclear point source can significantly bias results of these measured structural parameters, so we subtract the best-fit point source component to obtain images of the underlying host galaxies. Our concentration measurements reinforce the findings of our two-dimensional morphology fits, placing X-ray AGN hosts between early- and late-type inactive galaxies. AGN host asymmetry distributions are consistent with those of control galaxies. Combined with a lack of excess companion galaxies around AGN, the asymmetry distributions indicate that strong interactions are no more prevalent among AGN than normal galaxies. In light of recent work, these results suggest that the host galaxies of AGN at these X-ray luminosities may be in a transition from disk-dominated to bulge-dominated, but that this transition is not typically triggered by major mergers. © 2009. The American Astronomical Society. All rights reserved.
• Gilli, R., Zamorani, G., Miyaji, T., Silverman, J., Brusa, M., Mainieri, V., Cappelluti, N., Daddi, E., Porciani, C., Pozzetti, L., Civano, F., Comastri, A., Finoguenov, A., Fiore, F., Salvato, M., Vignali, C., Hasinger, G., Lilly, S., Impey, C., , Trump, J., et al. (2009). The spatial clustering of X-ray selected AGN in the XMM-COSMOS field. Astronomy and Astrophysics, 494(1), 33-48.
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  Abstract: We study the spatial clustering of 538 X-ray selected AGN in the 2 deg 2 XMM-COSMOS field that are spectroscopically identified with I AB < 23 and span the redshift range z = 0.2-3.0. The median redshift and X-ray luminosity of the sample are z = 0.98 and L0.5-10 = 6.3 × 1043 erg s-1, respectively. A strong clustering signal is detected at ∼ 18σ level, which is the most significant measurement obtained to date for clustering of X-ray selected AGN. By fitting the projected correlation function w(rp) with, a power law on scales of rp = 0.3-40 h-1 Mpc, we derive a best-fit comoving con-elation length of r0 = 8.6 ± 0.5 h-1 Mpc and slope of γ = 1.88 ± 0.07 (Poissonian errors; bootstrap errors are about a factor of 2 larger). An excess signal is observed in the range rp ∼ 5-15 h-1 Mpc, which is due to a large-scale structure at z ∼ 0.36 containing about 40 AGN, a feature which is evident over many wavelengths in the COSMOS field. When removing the z ∼ 0.36 structure or computing w(rp) in a narrower range around the peak of the redshift distribution (e.g. z = 0.4-1.6), the correlation length decreases to r0 ∼ 5-6 h-1 Mpc, which is consistent with what is observed for bright optical QSOs at the same redshift. We investigate the clustering properties of obscured and unobscured AGN separately, adopting different definitions for the source obscuration. For the first time, we are able to provide a significant measurement for the spatial clustering of obscured AGN at z ∼ 1. Within the statistical uncertainties, we do not find evidence that AGN with broad optical lines (BLAGN) cluster differently from AGN without broad optical lines (non-BLAGN). Based on these results, which are limited by object statistics, however, obscured and unobscured AGN are consistent with inhabiting similar environments. The evolution of AGN clustering with redshift is also investigated. No significant difference is found between the clustering properties of XMM-COSMOS AGN at redshifts below or above z = 1. The con-elation length measured for XMM-COSMOS AGN at z ∼ 1 is similar to that of massive galaxies (stellar mass M* ≳ 3 × 1010 M ⊙) at the same redshift. This suggests that AGN at z ∼ 1 are preferentially hosted by massive galaxies, as observed both in the local and in the distant (z ∼ 2) Universe. According to a simple clustering evolution scenario, we find that the relics of AGN are expected to have a correlation length as large as r0 ∼ 8 h-1 Mpc by z = 0, and hence to be hosted by local bright (L ∼ L*) ellipticals. We make use of dark matter halo catalogs from the Millennium simulation to determine the typical halo hosting moderately luminous z ∼ 1 AGN. We find that XMM-COSMOS AGN live in halos with masses M ≳ 2.5 × 1012 M⊙ h-1. By combining the number density of XMM-COSMOS AGN to that of the hosting dark matter halos we estimate the AGN duty cycle and lifetimes. We find lifetimes approximately of 1 Gyr for AGN at z ∼ 1, which are longer than those estimated for optically bright QSOs at the same redshift. These longer lifetimes mainly reflect the higher number density of AGN selected by X-ray samples. © ESO 2009.
• Jahnke, K., Bongiorno, A., Brusa, M., Capak, P., Cappelluti, N., Cisternas, M., Civano, F., Colbert, J., Comastri, A., Elvis, M., Hasinger, G., Ilbert, O., Impey, C., Inskip, K., Koekemoer, A. M., Lilly, S., Maier, C., Merloni, A., Riechers, D., , Salvato, M., et al. (2009). Massive galaxies in cosmos: Evolution of black hole versus bulge mass but not versus total stellar mass over the last 9 Gyr?. Astrophysical Journal Letters, 706(2 PART 2), L215-L220.
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  Abstract: We constrain the ratio of black hole (BH) mass to total stellar mass of type-1 active galactic nuclei (AGNs) in the COSMOS survey at 1 < z < 2. For 10 AGNs at mean redshift z ∼ 1.4 with both Hubble Space Telescope (HST)/ACS and HST/NICMOS imaging data, we are able to compute the total stellar mass M *,total, based on rest-frame UV-to-optical host galaxy colors which constrain mass-to-light ratios. All objects have virial M BH estimates available from the COSMOS Magellan/IMACS and zCOSMOS surveys. We find within errors zero difference between the M BH-M *,total relation at z ∼ 1.4 and the M BH-M *,bulge relation in the local universe. Our interpretation is (1) if our objects were purely bulge-dominated, the M BH-M *,bulge relation has not evolved since z ∼ 1.4. However, (2) since we have evidence for substantial disk components, the bulges of massive galaxies (M *,total = 11.1 ± 0.3 or log M BH ∼ 8.3 ± 0.2) must have grown over the last 9 Gyr predominantly by redistribution of the disk into the bulge mass. Since all necessary stellar mass exists in galaxies at z = 1.4, no star formation or addition of external stellar material is required, but only a redistribution, e.g., induced by minor and major merging or through disk instabilities. Merging, in addition to redistributing mass in the galaxy, will add both BH and stellar/bulge mass, but does not change the overall final M BH/M *,bulge ratio. Since the overall cosmic stellar and BH mass buildup trace each other tightly over time, our scenario of bulge formation in massive galaxies is independent of any strong BH feedback and means that the mechanism coupling BH and bulge mass until the present is very indirect. © 2009. The American Astronomical Society. All rights reserved.
• Salvato, M., Hasinger, G., Ilbert, O., Zamorani, G., Brusa, M., Scoville, N. Z., Rau, A., Capak, P., Arnouts, S., Aussel, H., Bolzonella, M., Buongiorno, A., Cappelluti, N., Caputi, K., Civano, F., Cook, R., Elvis, M., Gilli, R., Jahnke, K., , Kartaltepe, J. S., et al. (2009). Photometric redshift and classification for the XMM-cosmos sources. Astrophysical Journal Letters, 690(2), 1250-1263.
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  Abstract: We present photometric redshifts and spectral energy distribution (SED) classifications for a sample of 1542 optically identified sources detected with XMM in the COSMOS field. Our template fitting classifies 46 sources as stars and 464 as nonactive galaxies, while the remaining 1032 require templates with an active galactic nucleus (AGN) contribution. High accuracy in the derived photometric redshifts was accomplished as the result of (1) photometry in up to 30 bands with high-significance detections, (2) a new set of SED templates, including 18 hybrids covering the far-UV to mid-infrared, which have been constructed by the combination of AGNs and nonactive galaxies templates, and (3) multiepoch observations that have been used to correct for variability (most important for type 1 AGNs). The reliability of the photometric redshifts is evaluated using the subsample of 442 sources with measured spectroscopic redshifts. We achieved an accuracy of for i*AB < 22.5 ( for i*AB < 24.5). The high accuracies were accomplished for both type 2 (where the SED is often dominated by the host galaxy) and type 1 AGNs and QSOs out to z = 4.5. The number of outliers is a large improvement over previous photometric redshift estimates for X-ray-selected sources (4.0% and 4.8% outliers for i*AB < 22.5 and i*AB < 24.5, respectively). We show that the intermediate band photometry is vital to achieving accurate photometric redshifts for AGNs, whereas the broad SED coverage provided by mid-infrared (Spitzer/IRAC) bands is important to reduce the number of outliers for normal galaxies. © 2009. The American Astronomical Society. All rights reserved.
• Shioya, Y., Taniguchi, Y., Sasaki, S. S., Nagao, T., Murayama, T., Saito, T., Ideue, Y., Nakajima, A., Matsuoka, K., Trump, J., Scoville, N. Z., Sanders, D. B., Mobasher, B., Aussel, H., Capak, P., Kartaltepe, J., Koekemoer, A., Carilli, C., Ellis, R. S., , Garilli, B., et al. (2009). Photometric properties of Lyα emitters at z ≈ 4.86 in the cosmos 2 square degree field. Astrophysical Journal, 696(1), 546-561.
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  Abstract: We present results of a survey for Lyα emitters at z ≈ 4.86 based on optical narrowband (λc = 7126 Å, Δλ = 73 Å) and broadband (B, V, r', i', and z') observations of the Cosmic Evolution Survey field using Suprime-Cam on the Subaru Telescope. We find 79 Lyα emitter (LAE) candidates at z ≈ 4.86 over a contiguous survey area of 1.83 deg2, down to the Lyα line flux of 1.47 × 10 -17erg s-1 cm-2. We obtain the Lyα luminosity function with a best-fit Schechter parameters of logL * = 42.9+0.5-0.3 erg s-1 and φ* = 1.2+8.0-1.1 × 10 -4 Mpc-3 for α = -1.5 (fixed). The two-point correlation function for our LAE sample is ξ (r) = (r/4.4+5.7-2.9 Mpc)-1.90±0.22. In order to investigate the field-to-field variations of the properties of Lyα emitters, we divide the survey area into nine tiles of 0°5 × 0°5 each. We find that the number density varies with a factor of ≃2 from field to field with high statistical significance. However, we find no significant field-to-field variance when we divide the field into four tiles with 0.7 × 0°7 each. We conclude that at least 0.5 deg2 survey area is required to derive averaged properties of LAEs at z ∼ 5, and our survey field is wide enough to overcome the cosmic variance. © 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
• Smolčić, V., Zamorani, G., Schinnerer, E., Bardelli, S., Bondi, M., Bîrzan, L., Carilli, C. L., Ciliegi, P., Elvis, M., Impey, C. D., Koekemoer, A. M., Merloni, A., Paglione, T., Salvato, M., Scodeggio, M., Scoville, N., & Trump, J. R. (2009). Cosmic evolution of radio selected active galactic nuclei in the cosmos field. Astrophysical Journal, 696(1), 24-39.
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  Abstract: We explore the cosmic evolution of radio luminous active galactic nuclei (AGNs) with low radio powers (L1.4GHz ≲ 5×1025 W Hz-1) out to z = 1.3 using to date the largest sample of ∼600 low-luminosity radio AGN at intermediate redshift drawn from the VLA-COSMOS survey. We derive the radio-luminosity function for these AGNs, and its evolution with cosmic time assuming two extreme cases: (1) pure luminosity and (2) pure density evolution. The former and latter yield L* α (1+z)0.8±0.1 and φ* α (1+z)1.1±0.1, respectively, both implying a fairly modest change in properties of low-radio-power AGNs since z = 1.3. We show that this is in stark contrast with the evolution of powerful (L1.4GHz > 5 × 1025 W Hz-1) radio AGN over the same cosmic time interval, constrained using the 3CRR, 6CE, and 7CRS radio surveys by Willot et al. We demonstrate that this can be explained through differences in black hole fueling and triggering mechanisms, and a dichotomy in host galaxy properties of weak and powerful AGNs. Our findings suggest that high- and low-radio-power AGN activities are triggered in different stages during the formation of massive red galaxies. We show that weak radio AGN occur in the most massive galaxies already at z ∼ 1, and they may significantly contribute to the heating of their surrounding medium and thus inhibit gas accretion onto their host galaxies, as recently suggested for the "radio mode" in cosmological models. © 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
• Taniguchi, Y., Murayama, T., Scoville, N. Z., Sasaki, S. S., Nagao, T., Shioya, Y., Saito, T., Ideue, Y., Nakajima, A., Matsuoka, K., Sanders, D. B., Mobasher, B., Aussel, H., Capak, P., Salvato, M., Koekemoer, A., Carilli, C., Cimatti, A., Ellis, R. S., , Garilli, B., et al. (2009). Hubble space telescope/advanced camera for surveys morphology of lyα emitters at redshift 5.7 in the cosmos field. Astrophysical Journal Letters, 701(2), 915-944.
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  Abstract: We present detailed morphological properties of Lyα emitters (LAEs) at z 5.7 in the COSMOS field based on Hubble Space Telescope Advanced Camera for Surveys (ACS) data. The ACS imaging in the F814W filter covered 85 LAEs of the 119 LAEs identified in the full two square degree field, and 47 LAEs of them are detected in the ACS images. Nearly half of them are spatially extended with a size larger than 0.15 arcsec (0.88 kpc at z = 5.7) and up to 0.4 arcsec (2.5 kpc at z = 5.7). The others are nearly unresolved compact objects. Two LAEs show double-component structures indicating interaction or merging of building components to form more massive galaxies. By stacking the ACS images of all the detected sources, we obtain a Sersic parameter of n 0.7 with a half-light radius of 0.13 arcsec (0.76 kpc), suggesting that the majority of ACS detected LAEs have not spheroidal-like but disk-like or irregular light profiles. Comparing ACS F814W magnitudes (I 814) with Subaru/Suprime-Cam magnitudes in the NB816, i′, and z′ bands, we find that the ACS imaging in the F814W band mainly probes UV continuum rather than Lyα line emission. UV continuum sizes tend to be larger for LAEs with larger Lyα emission regions as traced by the NB816 imaging. The nondetection of 38 LAEs in the ACS images is likely due to the fact that their surface brightness is too low both in the UV continuum and Lyα emission. Estimating I 814 for the ACS-undetected LAEs from the z′ and NB816 magnitudes, we find that 16 of these are probably LAEs with a size larger than 0.15 arcsec in UV continuum. All these results suggest that our LAE sample contains systematically larger LAEs in UV continuum size than those previously studied at z 6. © 2009. The American Astronomical Society. All rights reserved.
• Trump, J. R., Impey, C. D., Elvis, M., McCarthy, P. J., Huchra, J. P., Brusa, M., Salvato, M., Capak, P., Cappelluti, N., Civano, F., Comastri, A., Gabor, J., Hao, H., Hasinger, G., Jahnke, K., Kelly, B. C., Lilly, S. J., Schinnerer, E., Scoville, N. Z., & Smolčić, V. (2009). The cosmos active galactic nucleus spectroscopic survey. I. XMM-newton counterparts. Astrophysical Journal, 696(2), 1195-1212.
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  Abstract: We present optical spectroscopy for an X-ray and optical flux-limited sample of 677 XMM-Newton selected targets covering the 2 deg 2 Cosmic Evolution Survey field, with a yield of 485 high-confidence redshifts. The majority of the spectra were obtained over three seasons (2005-2007) with the Inamori Magellan Areal Camera and Spectrograph instrument on the Magellan (Baade) telescope. We also include in the sample previously published Sloan Digital Sky Survey spectra and supplemental observations with MMT/Hectospec. We detail the observations and classification analyses. The survey is 90% complete to flux limits of f 0.5-10 keV > 8 × 10 -16 erg cm -2 s -1 and i +AB < 22, where over 90% of targets have high-confidence redshifts. Making simple corrections for incompleteness due to redshift and spectral type allows for a description of the complete population to i +AB < 23. The corrected sample includes a 57% broad emission line (Type 1, unobscured) active galactic nucleus (AGN) at 0.13 < z < 4.26, 25% narrow emission line (Type 2, obscured) AGN at 0.07 < z < 1.29, and 18% absorption line (host-dominated, obscured) AGN at 0 < z < 1.22 (excluding the stars that made up 4% of the X-ray targets). We show that the survey's limits in X-ray and optical fluxes include nearly all X-ray AGNs (defined by L 0.5-10 keV > 3 × 10 42 erg s -1) to z < 1, of both optically obscured and unobscured types. We find statistically significant evidence that the obscured-to-unobscured AGN ratio at z < 1 increases with redshift and decreases with luminosity. © 2009. The American Astronomical Society. All rights reserved.
• Trump, J. R., Impey, C. D., Taniguchi, Y., Brusa, M., Civano, F., Elvis, M., Gabor, J. M., Jahnke, K., Kelly, B. C., Koekemoer, A. M., Nagao, T., Salvato, M., Shioya, Y., Capak, P., Huchra, J. P., Kartaltepe, J. S., Lanzuisi, G., McCarthy, P. J., Maineri, V., & Scoville, N. Z. (2009). The nature of optically dull active galactic nuclei in cosmos. Astrophysical Journal Letters, 706(1), 797-809.
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  Abstract: We present infrared, optical, and X-ray data of 48 X-ray bright, optically dull active galactic nuclei (AGNs) in the COSMOS field. These objects exhibit the X-ray luminosity of an AGN but lack broad and narrow emission lines in their optical spectrum. We show that despite the lack of optical emission lines, most of these optically dull AGNs are not well described by a typical passive red galaxy spectrum: instead they exhibit weak but significant blue emission like an unobscured AGN. Photometric observations over several years additionally show significant variability in the blue emission of four optically dull AGNs. The nature of the blue and infrared emission suggest that the optically inactive appearance of these AGNs cannot be caused by obscuration intrinsic to the AGNs. Instead, up to ∼70% of optically dull AGNs are diluted by their hosts, with bright or simply edge-on hosts lying preferentially within the spectroscopic aperture. The remaining ∼30% of optically dull AGNs have anomalously high fX /fO ratios and are intrinsically weak, not obscured, in the optical. These optically dull AGNs are best described as a weakly accreting AGN with a truncated accretion disk from a radiatively inefficient accretion flow. © 2009. The American Astronomical Society. All rights reserved.
• Casey, C. M., Impey, C. D., Petry, C. E., Marble, A. R., & Davé, R. (2008). PC 1643+4631a, B: The lyman-α forest at the edge of coherence. Astronomical Journal, 136(1), 181-196.
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  Abstract: This is the first measurement and detection of coherence in the intergalactic medium (IGM) at substantially high redshift (z 3.8) and on large physical scales (2.5 h -170 Mpc). We perform the measurement by presenting new observations of the high-redshift quasar pair PC 1643+4631A, B and their Lyα-absorber coincidences. With data collected from Keck I Low Resolution Imaging Spectrometer (LRIS) in a 10,200 s integration, we have full coverage of the Lyα forest over the redshift range 2.6 < z < 3.8 at a resolution of 3.6 (220 km s-1). This experiment extends multiple sight line quasar absorber studies to higher redshift, higher opacity, larger transverse separation, and into a regime where coherence across the IGM becomes weak and difficult to detect. Noteworthy features from these spectra are the strong damped Lyα absorbers (DLAs) just blueward of both Lyα emission peaks, each within 1000 km s -1 of the emission redshift but separated by 2500 km s-1 from each other. The coherence is measured by fitting discrete Lyα absorbers and by using pixel flux statistics. The former technique results in 222 Lyα absorbers in the A sight line and 211 in B. Relative to a Monte Carlo pairing test (using symmetric, nearest-neighbor matching) the data exhibit a 4σ excess of pairs at low velocity splitting (Δv < 150 km s-1), thus detecting coherence on transverse scales of 2.5 h -170 Mpc. We use spectra extracted from a smoothed particle hydrodynamic (SPH) simulation to analyze symmetric pair matching, transmission distributions as a function of redshift and compute zero-lag cross-correlations to compare with the quasar pair data. The simulations agree with the data with the same strength (4σ) at similarly low velocity splitting above random chance pairings. In cross-correlation tests, the simulations agree when the mean flux (as a function of redshift) is assumed to follow the prescription given by Kirkman et al. (2005). While the detection of flux correlation (measured through coincident absorbers and cross-correlation amplitude) is only marginally significant, the agreement between data and simulations is encouraging for future work in which even better quality data will provide the best insight into the overarching structure of the IGM and its understanding as shown by SPH simulations. © 2008. The American Astronomical Society. All rights reserved..
• Casey, C. M., Impey, C. D., Trump, J. R., Gabor, J., Abraham, R. G., Capak, P., Scoville, N. Z., Brusa, M., & Schinnerer, E. (2008). Optical selection of faint active galactic nuclei in the cosmos field. Astrophysical Journal, Supplement Series, 177(1), 131-147.
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  Abstract: We outline a strategy to select faint (iAB < 24.5) type 1 active galactic nucleus (AGN) candidates down to the Seyfert/QSO boundary for spectroscopic targeting in the COSMOS field. Our selection process picks candidates by their nonstellar colors in uBVRizK broadband photometry from the Subaru and CFH Telescopes and morphological properties extracted from Hubble Space Telescope (HST) ACS i-band data. Although the COSMOS field has been used extensively to survey the faint galaxy population out to z ∼ 6, AGN optical color selection has not been applied to so faint a level in such a large continuous part of the sky. Hot stars are known to be the dominant contaminant for bright AGN candidate selection at z < 2, but we anticipate the highest color contamination rate at all redshifts to be from faint starburst and compact galaxies. Morphological selection via the Gini Coefficient separates most potential AGNs from these faint blue galaxies. Recent models of the quasar luminosity function (QLF) from Hopkins et al. are used to estimate quasar surface densities, and a recent study of stellar populations in the COSMOS field is applied to infer stellar surface densities and contamination. We use 292 spectroscopically confirmed type 1 broad-line AGN and quasar templates to predict AGN colors as a function of redshift, and then contrast those predictions with the colors of known contaminating populations. Since the number of galaxy contaminants cannot be reliably identified with respect to stellar and predicted QLF numbers, the completeness and efficiency of the selection cannot be calculated before gathering confirming spectroscopic observations. Instead we offer an upper limit estimate to selection efficiency (about 50%) for low-z and 20%-40% for intermediate-z and high-z) as well as the completeness and efficiency with respect to an X-ray point source population (from the COSMOS AGN Survey), in the range 20%-50%. The motivation of this study and subsequent spectroscopic follow-up is to populate and refine the faint end of the QLF, at both low and high redshifts, where the population of type 1 AGNs is presently not well known. The anticipated AGN observations will add to the ~300 already known AGNs in the COSMOS field, making COSMOS a densely packed field of quasars to be used to understand supermassive black holes and probe the structure of the intergalactic medium in the intervening volume. © 2008. The American Astronomical Society. All rights reserved.
• Marble, A. R., Eriksen, K. A., Impey, C. D., Lei, B., & Miller, L. (2008). The flux auto- and cross-correlation of the Lyα forest. I. Spectroscopy of QSO pairs with arcminute separations and similar redshifts. Astrophysical Journal, Supplement Series, 175(1), 29-47.
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  Abstract: The Lyα forest has opened a new redshift regime for cosmological investigation. At z > 2 it provides a unique probe of cosmic geometry and an independent constraint on dark energy that is not subject to standard candle or ruler assumptions. In Paper I of this series on using the Lya forest observed in pairs of QSOs for a new application of the Alcock-Paczynski test, we present and discuss the results of a campaign to obtain moderate-resolution spectroscopy (FWHM ≃ 2.5 Å) of the Lyα forest in pairs of QSOs with small redshift differences (Δz < 0.25, z > 2.2) and arc-minute separations (θ < 5′). This data set, composed of seven individual QSOs, 35 pairs, and one triplet, is also well suited for future investigations of the coherence of Lya absorbers on ∼1 Mpc transverse scales and the transverse proximity effect. We note seven revisions for previously published QSO identifications and/or redshifts. © 2008. The American Astronomical Society. All rights reserved.
• Marble, A. R., Eriksen, K. A., Impey, C. D., Oppenheimer, B. D., & Davé, R. (2008). The flux auto- and cross-correlation of the Lyα forest. II. Modeling anisotropies with cosmological hydrodynamic simulations. Astrophysical Journal, 675(2), 946-959.
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  Abstract: The isotropy of the Lyα forest in real-space uniquely provides a measurement of cosmic geometry at z > 2. The angular diameter distance for which the correlation function along the line of sight and in the transverse direction agree corresponds to the correct cosmological model. However, the Lyα forest is observed in redshift-space where distortions due to Hubble expansion, bulk flows, and thermal broadening introduce anisotropy. Similarly, a spectrograph's line-spread function affects the autocorrelation and cross-correlation differently. In this the second paper of a series on using the Lyα forest observed in pairs of quasi-stellar objects for a new application of the Alcock-Paczyński test, these anisotropies and related sources of potential systematic error are investigated with cosmological hydrodynamic simulations. Three prescriptions for galactic outflow were compared and found to have only a marginal effect on the Lyα flux correlation (which changed by at most 7% with use of the currently favored variable-momentum wind model vs. no winds at all). An approximate solution for obtaining the zero-lag cross-correlation corresponding to arbitrary spectral resolution directly from the zero-lag cross-correlation computed at full resolution (good to within 2% at the scales of interest) is presented. Uncertainty in the observationally determined mean flux decrement of the Lyα forest was found to be the dominant source of systematic error; however, this is reduced significantly when considering correlation ratios. We describe a simple scheme for implementing our results, while mitigating systematic errors, in the context of a future application of the Alcock-Paczyński test. © 2008. The American Astronomical Society. All rights reserved.
• Shioya, Y., Taniguchi, Y., Sasaki, S. S., Nagao, T., Murayama, T., Takahashi, M. I., Ajiki, M., Ideue, Y., Mihara, S., Nakajima, A., Scoville, N. Z., Mobasher, B., Aussel, H., Giavalisco, M., Guzzo, L., Hasinger, G., Impey, C., Fèvre, O., Lilly, S., , Renzini, A., et al. (2008). The Hα luminosity function and star formation rate at z ≈ 0.24 in the COSMOS 2 square degree field. Astrophysical Journal, Supplement Series, 175(1), 128-137.
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  Abstract: To derive a new Hα luminosity function and to understand the clustering properties of star-forming galaxies at z ≈ 0.24, we have made a narrowband imaging survey for Hα-emitting galaxies in the Hubble Space Telescope (HST) COSMOS 2 square degree field. We used the narrowband filter NB816 (λc = 8150 Å, Δλ = 120 Å) and sampled Hα emitters with EWobs( Hα+ [N II]) > 12 Å in a redshift range between z = 0.233 and 0.251 corresponding to a depth of 70 Mpc. We obtained 980 Hα-emitting galaxies in a sky area of 5540 arcmin2, corresponding to a survey volume of 3.1 × 10 4 Mpc3. We derive a Hα luminosity function with a best-fit Schechter function parameter set of α = -1.35 -0.13+0.11, log φ* = -2.65-0.38+0.27 and log L*(ergs s-1) = 41.94 -0.23+0.38. The Hα luminosity density is 2.7 -0.6+0.7 × 1039 ergs s-1 Mpc-3. After subtracting the AGN contribution (15%) to the Hα luminosity density, the star formation rate density is evaluated as 1.8 -0.4+0.7 × 10-2 M⊙ yr-1 Mpc-3. The angular two-point correlation function of Hα-emitting galaxies of log L(Hα) > 39.8 is well fit by a power-law form of w(θ) = 0.013-0.001+0.002θ-0.88±0.03, corresponding to the correlation function of ξ(r) = (r/1.9 Mpc)-1.88. We also find that the Hα emitters with higher Hα luminosity are more strongly clustered than those with lower luminosity. © 2008. The American Astronomical Society. All rights reserved.
• Brusa, M., Zamorani, G., Comastri, A., Hasinger, G., Cappelluti, N., Civano, F., Finoguenov, A., Mainieri, V., Salvato, M., Vignali, C., Elvis, M., Fiore, F., Gilli, R., Impey, C. D., Lilly, S. J., Mignoli, M., Silverman, J., Trump, J., Urry, C. M., , Bender, R., et al. (2007). The XMM-Newton wide-field survey in the COSMOS field. III. Optical identification and multiwavelength properties of a large sample of X-ray-selected sources. Astrophysical Journal, Supplement Series, 172(1), 353-367.
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  Abstract: We present the optical identification of a sample of 695 X-ray sources detected in the first 1.3 deg 2 of the COSMOS XMM-Newton survey, down to a 0.5-2 keV (2-10 keV) limiting flux of ∼10 -15 erg cm -2 s -1 (∼5 × 10 -15 erg cm -2 s -1). In order to identify the correct optical counterparts and to assess the statistical significance of the X-ray - to - optical associations we have used the "likelihood ratio technique." Here we present the identification method and its application to the CFHT I-band and photometric catalogs. We were able to associate a candidate optical counterpart to ∼90% (626) of the X-ray sources, while for the remaining ∼10% of the sources we were not able to provide a unique optical association due to the faintness of the possible optical counterparts (I AB > 25) or to the presence of multiple optical sources, with similar likelihoods of being the correct identification, within the XMM-Newton error circles. We also cross-correlated the candidate optical counterparts with the Subaru multicolor and ACS catalogs and with the Magellan/IMACS, zCOSMOS, and literature spectroscopic data; the spectroscopic sample comprises 248 objects (∼40% of the full sample). Our analysis of this statistically meaningful sample of X-ray sources reveals that for ∼80% of the counterparts there is a very good agreement between the spectroscopic classification, the morphological parameters as derived from ACS data, and the optical - to - near-infrared colors: the large majority of spectroscopically identified broad-line active galactic nuclei (BL AGNs) have a pointlike morphology on ACS data, blue optical colors in color-color diagrams, and an X-ray - to - optical flux ratio typical of optically selected quasars. Conversely, sources classified as narrow line AGNs or normal galaxies are on average associated with extended optical sources, have significantly redder optical - to - near-infrared colors, and span a larger range of X-ray - to - optical flux ratios. However, about 20% of the sources show an apparent mismatch between the morphological and spectroscopic classifications. All the "extended" BL AGNs lie at redshift < 1.5, while the redshift distribution of the full BL AGN population peaks at z ∼ 1.5. The most likely explanation is that in these objects the nuclear emission is not dominant with respect to the host galaxy emission in the observed ACS band. Our analysis also suggests that the type 2/type 1 ratio decreases toward high luminosities, in qualitative agreement with the results from X-ray spectral analysis and the most recent modeling of the X-ray luminosity function evolution. © 2007. The American Astronomical Society. All rights reserved.
• Capak, P., Aussel, H., Ajiki, M., McCracken, H. J., Mobasher, B., Scoville, N., Shopbell, P., Taniguchi, Y., Thompson, D., Tribiano, S., Sasaki, S., Blain, A. W., Brusa, M., Carilli, C., Comastri, A., Carollo, C. M., Cassata, P., Colbert, J., Ellis, R. S., , Elvis, M., et al. (2007). The first release COSMOS optical and near-IR data and catalog. Astrophysical Journal, Supplement Series, 172(1), 99-116.
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  Abstract: We present imaging data and photometry for the COSMOS survey in 15 photometric bands between 0.3 and 2.4 μm. These include data taken on the Subaru 8.3 m telescope, the KPNO and CTIO 4 m telescopes, and the CFHT 3.6 m telescope. Special techniques are used to ensure that the relative photometric calibration is better than 1 % across the field of view. The absolute photometric accuracy from standard-star measurements is found to be 6%. The absolute calibration is corrected using galaxy spectra, providing colors accurate to 2% or better. Stellar and galaxy colors and counts agree well with the expected values. Finally, as the first step in the scientific analysis of these data we construct panchromatic number counts which confirm that both the geometry of the universe and the galaxy population are evolving. © 2007. The American Astronomical Society. All rights reserved.
• Finoguenov, A., Guzzo, L., Hasinger, G., Scoville, N. Z., Aussel, H., Böhringer, H., Brusa, M., Capak, P., Cappelluti, N., Comastri, A., Giodini, S., Griffiths, R. E., Impey, C., Koekemoer, A. M., Kneib, J. -., Leauthaud, A., Fèvre, O. L., Lilly, S., Mainieri, V., , Massey, R., et al. (2007). The XMM-Newton wide-field survey in the COSMOS field: Statistical properties of clusters of galaxies. Astrophysical Journal, Supplement Series, 172(1), 182-195.
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  Abstract: We present the results of a search for galaxy clusters in the first 36 XMM-Newton pointings on the Cosmic Evolution Survey (COSMOS) field. We reach a depth for a total cluster flux in the 0.5-2 keVband of 3 × 10 -15 ergs cm -2 s -1, having one of the widest XMM-Newton contiguous raster surveys, covering an area of 2.1 deg 2. Cluster candidates are identified through a wavelet detection of extended X-ray emission. Verification of the cluster candidates is done based on a galaxy concentration analysis in redshift slices of thickness 0.1 -0.2 in redshift, using the multiband photometric catalog of the COSMOS field and restricting the search to z < 1.3 and i AB < 25. We identify 72 clusters and derive their properties based on the X-ray cluster scaling relations. A statistical description of the survey in terms of the cumulative log (N > S)-log S distribution compares well with previous results, although yielding a somewhat higher number of clusters at similar fluxes. The X-ray luminosity function of COSMOS clusters matches well the results of nearby surveys, providing a comparably tight constraint on the faint-end slope of α = 1.93 ±0.04. For the probed luminosity range of (8 × 10 42)-(2 × 10 44) ergs s -1, our survey is in agreement with and adds significantly to the existing data on the cluster luminosity function at high redshifts and implies no substantial evolution at these luminosities to z = 1.3. © 2007. The American Astronomical Society. All rights reserved.
• Impey, C. D. (2007). Dim baryons in the cosmic web. Proceedings of the International Astronomical Union, 3(S244), 157-166.
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  Abstract: The distribution of baryons beyond galaxies is descibed. The majority of the baryons, which represent 4% of the cosmic mass and energy budget, lie far from individual galaxies in the diffuse intergalactic medium (IGM). Many of these baryons are in a warm phase that can be probed by quasar absorption in the Lyman- line of hydrogen. The mature field of quasar spectroscopy can diagnose the location, physical state, metallicity, and general geometry of this gas, which is called the cosmic web. The remainder of the gas is kept very hot by infall and shocks and is mostly in higher density regions such as filaments, groups and clusters. The hot gas is only detectable via X-rays and the absorption of highly ionized species of heavy elements. The baryons in low density regions of space are excellent tracers of underlying dark matter. The evolution of the cosmic web indicates where to look for the baryons in collapsed objects but the overall inefficiency of galaxy formation has conspired to keep most baryons dark. © 2008 International Astronomical Union.
• Lilly, S. J., Fèvre, O. L., Renzini, A., Zamorani, G., Scodeggio, M., Contini, T., Carollo, C. M., Hasinger, G., Kneib, J. -., Iovino, A., Brun, V. L., Maier, C., Mainieri, V., Mignoli, M., Silverman, J., Tasca, L. A., Bolzonella, M., Bongiorno, A., Bottini, D., , Capak, P., et al. (2007). zCOSMOS: A large VLT/VIMOS redshift survey covering 0 < z < 3 in the COSMOS field ¹. Astrophysical Journal, Supplement Series, 172(1), 70-85.
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  Abstract: zCOSMOS is a large-redshift survey that is being undertaken in the COSMOS field using 600 hr of observation with the VIMOS spectrograph on the 8 m VLT. The survey is designed to characterize the environments of COSMOS galaxies from the 100 kpc scales of galaxy groups up to the 100 Mpc scale of the cosmic web and to produce diagnostic information on galaxies and active galactic nuclei. The zCOSMOS survey consists of two parts: (1) zCOSMOSbright, a magnitude-limited I-band I AB < 22.5 sample of about 20,000 galaxies with 0.1 < z < 1.2 covering the whole 1.7 deg 2 COSMOS ACS field, for which the survey parameters at z ∼ 0.7 are designed to be directly comparable to those of the 2dFGRS at z ∼ 0.1 ; and (2) zCOSMOS-deep, a survey of approximately 10,000 galaxies selected through color-selection criteria to have 1.4 < z < 3.0, within the central 1 deg 2. This paper describes the survey design and the construction of the target catalogs and briefly outlines the observational program and the data pipeline. In the first observing season, spectra of 1303 zCOSMOS-bright targets and 977 zCOSMOS-deep targets have been obtained. These are briefly analyzed to demonstrate the characteristics that may be expected from zCOSMOS, and particularly zCOSMOS-bright, when it is finally completed between 2008 and 2009. The power of combining spectroscopic and photometric redshifts is demonstrated, especially in correctly identifying the emission line in single-line spectra and in determining which of the less reliable spectroscopic redshifts are correct and which are incorrect. These techniques bring the overall success rate in the zCOSMOS-bright so far to almost 90% and to above 97% in the 0.5 < z < 0.8 redshift range. Our zCOSMOS-deep spectra demonstrate the power of our selection techniques to isolate high-redshift galaxies at 1.4 < z < 3.0 and of VIMOS to measure their redshifts using ultraviolet absorption lines. © 2007. The American Astronomical Society. All rights reserved.
• Mainieri, V., Hasinger, G., Cappelluti, N., Brusa, M., Brunner, H., Civano, F., Comastri, A., Elvis, M., Finoguenov, A., Fiore, F., Gilli, R., Lehmann, I., Silverman, J., Tasca, L., Vignali, C., Zamorani, G., Schinnerer, E., Impey, C., Trump, J., , Lilly, S., et al. (2007). The XMM-Newton wide-field survey in the COSMOS field. IV. X-ray spectral properties of active galactic nuclei. Astrophysical Journal, Supplement Series, 172(1), 368-382.
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  Abstract: We present a detailed spectral analysis of pointlike X-ray sources in the XMM-Newton COSMOS field. Our sample of 135 sources only includes those that have more than 100 net counts in the 0.3-10 keV energy band and have been identified through optical spectroscopy. The majority of the sources are well described by a simple power-law model with either no absorption (76%) or a significant intrinsic, absorbing column (20%). The remaining ∼4% of the sources require a more complex modeling by incorporating additional components to the power law. For sources with more than 180 net counts (bright sample), we allowed both the photon spectral index Γ and the equivalent hydrogen column N H to be free parameters. For fainter sources, we fix Γ to the average value and allow N H to vary. The mean spectral index of the 82 sources in the bright sample is (Γ) = 2.06 ± 0.08, with an intrinsic dispersion of ∼0.24. Each of these sources has fractional errors on the value of T below 20%. As expected, the distribution of intrinsic absorbing column densities is markedly different between AGNs with or without broad optical emission lines. We find within our sample four type 2 QSO candidates (L X > 10 44 ergs s -1, N H > 10 22 cm -2), with a spectral energy distribution well reproduced by a composite Seyfert 2 spectrum, that demonstrates the strength of the wide-field COSMOS XMM-Newton survey to detect these rare and underrepresented sources. In addition, we have identified a Compton-thick (N H > 1.5 × 10 24 cm -2) AGN at z = 0.1248. Its X-ray spectrum is well fitted by a pure reflection model and a significant Fe Kα line at rest-frame energy of 6.4 keV. © 2007. The American Astronomical Society. All rights reserved.
• Minchin, R., Davies, J., Disney, M., Grossi, M., Sabatini, S., Boyce, P., Garcia, D., Impey, C., Jordan, C., Lang, R., Marble, A., Roberts, S., & Driel, W. V. (2007). 21 cm Synthesis observations of VIRGOHI 21 - A possible dark galaxy in the virgo cluster. Astrophysical Journal, 670(2), 1056-1064.
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  Abstract: Many observations indicate that dark matter dominates the extragalactic universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously, we have suggested that VIRGOHI 21, a 21 cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line width (∼200 km s-1) unaccompanied by any visible gravitational source to account for it. We have now imaged VERGOHI 21 in the neutral hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ∼108 yr ago with a perturber of ∼1011 M⊙. This we take as additional evidence for the massive nature of VERGOHI 21, as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the H I and find none down to an I-band surface brightness limit of 31.1 ± 0.2 mag arcsec-2. © 2007. The American Astronomical Society. All rights reserved.
• Sanders, D. B., Salvato, M., Aussel, H., Ilbert, O., Scoville, N., Surace, J. A., Frayer, D. T., Sheth, K., Helou, G., Brooke, T., Bhattacharya, B., Yan, L., Kartaltepe, J. S., Barnes, J. E., Blain, A. W., Calzetti, D., Capak, P., Carilli, C., Carollo, C. M., , Comastri, A., et al. (2007). S-COSMOS: The Spitzer legacy survey of the Hubble Space Telescope ACS 2 deg ² COSMOS field I: Survey strategy and first analysis. Astrophysical Journal, Supplement Series, 172(1), 86-98.
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  Abstract: The COSMOS Spitzer survey (S-COSMOS) is a Legacy program (Cycles 2+3) designed to carry out a uniform deep survey of the full 2 deg 2 COSMOS field in all seven Spitzer bands (3.6,4.5,5.6,8.0,24.0,70.0, and 160.0 μm). This paper describes the survey parameters, mapping strategy, data reduction procedures, achieved sensitivities to date, and the complete data set for future reference. We show that the observed infrared backgrounds in the S-COSMOS field are within 10% of the predicted background levels. The fluctuations in the background at 24 μm have been measured and do not show any significant contribution from cirrus, as expected. In addition, we report on the number of asteroid detections in the low Galactic latitude COSMOS field. We use the Cycle 2 S-COSMOS data to determine preliminary number counts, and compare our results with those from previous Spitzer Legacy surveys (e.g., SWIRE, GOODS). The results from this "first analysis" confirm that the S-COSMOS survey will have sufficient sensitivity with IRAC to detect ∼L* disks and spheroids out to z ≳ 3, and with MIPS to detect ultraluminous starbursts and AGNs out to z ∼ 3 at 24 μm and out to z ∼ 1.5-2 at 70 and 160 μm. © 2007. The American Astronomical Society. All rights reserved.
• Sasaki, S. S., Taniguchi, Y., Scoville, N., Mobasher, B., Aussel, H., Sanders, D. B., Koekemoer, A., Ajiki, M., Komiyama, Y., Miyazaki, S., Kaifu, N., Karoji, H., Okamura, S., Arimoto, N., Ohta, K., Shioya, Y., Murayama, T., Nagao, T., Koda, J., , Hainline, L., et al. (2007). A potential galaxy threshing system in the COSMOS field. Astrophysical Journal, Supplement Series, 172(1), 511-517.
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  Abstract: We report on the discovery of a new potential galaxy threshing system in the COSMOS 2 square degree field using the prime-focus camera, Suprime-Cam, on the 8.2 m Subaru Telescope. This system consists of a giant elliptical galaxy with M v ≈ -21.6 and a tidally disrupted satellite galaxy with M v PS -17.7 at a photometric redshift of z ≈ 0.08. This redshift is consistent with the spectroscopic redshift of 0.079 for the giant elliptical galaxy obtained from the Sloan Digital Sky Survey (SDSS) archive. The luminosity masses of the two galaxies are 3.7 × 10 12 and 3.7 × 10 9 M ⊙, respectively. The distance between the two galaxies is greater than 100 kpc. The two tidal tails emanating from the satellite galaxy extend over 150 kpc. This system would be the second well-defined galaxy threshing system found so far. © 2007. The American Astronomical Society. All rights reserved.
• Schinnerer, E., Smolčič, V., Carilli, C. L., Bondi, M., Ciliegi, P., Jahnke, K., Scoville, N. Z., Aussel, H., Bertoldi, F., Blain, A. W., Impey, C. D., Koekemoer, A. M., Fevre, O. L., & Urry, C. M. (2007). The VLA-COSMOS survey II. Source catalog of the large project. Astrophysical Journal, Supplement Series, 172(1), 46-69.
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  Abstract: The VLA-COSMOS Large Project is described and its scientific objective is discussed. We present a catalog of ∼3600 radio sources found in the 2 deg 2 COSMOS field at 1.4 GHz. The observations in the VLA A and C configuration resulted in a resolution of 1.5″ × 1.4″ and a mean rms noise of ∼10.5 (15) μJy beam -1 in the central 1 (2) deg 2. Eighty radio sources are clearly extended consisting of multiple components, and most of them appear to be double-lobed radio galaxies. The astrometry of the catalog has been thoroughly tested, and the uncertainty in the relative and absolute astrometry are 130 and
• Scoville, N., Abraham, R. G., Aussel, H., Barnes, J. E., Benson, A., Blain, A. W., Calzetti, D., Comastri, A., Capak, P., Carilli, C., Carlstrom, J. E., Carollo, C. M., Colbert, J., Daddi, E., Ellis, R. S., Elvis, M., Ewald, S. P., Fall, M., Franceschini, A., , Giavalisco, M., et al. (2007). Cosmos: Hubble space telescope observations. Astrophysical Journal, Supplement Series, 172(1), 38-45.
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  Abstract: The Cosmic Evolution Survey (COSMOS) was initiated with an extensive allocation (590 orbits in Cycles 12-13) using the Hubble Space Telescope (HST) for high-resolution imaging. Here we review the characteristics of the HST imaging with the Advanced Camera for Surveys (ACS) and parallel observations with NICMOS and WFPC2. A square field (1.8 deg 2) has been imaged with single-orbit ACS I-band F814W exposures with 50% completeness for sources 0.5″ in diameter at I AB = 26.0 mag. The ACS is a key part of the COSMOS survey, providing very high sensitivity and high-resolution (0.09″ FWHM and 0.05″ pixels) imaging and detecting a million objects. These images yield resolved morphologies for several hundred thousand galaxies. The small HST PSF also provides greatly enhanced sensitivity for weak-lensing investigations of the dark matter distribution. © 2007. The American Astronomical Society. All rights reserved.
• Scoville, N., Aussel, H., Brusa, M., Capak, P., Carollo, C. M., Elvis, M., Giavalisco, M., Guzzo, L., Hasinger, G., Impey, C., Kneib, J. -., LeFevre, O., Lilly, S. J., Mobasher, B., Renzini, A., Rich, R. M., Sanders, D. B., Schinnerer, E., Schminovich, D., , Shopbell, P., et al. (2007). The Cosmic Evolution Survey (COSMOS): Overview. Astrophysical Journal, Supplement Series, 172(1), 1-8.
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  Abstract: The Cosmic Evolution Survey (COSMOS) is designed to probe the correlated evolution of galaxies, star formation, active galactic nuclei (AGNs), and dark matter (DM) with large-scale structure (LSS) over the redshift range z > 0.5-6. The survey includes multiwavelength imaging and spectroscopy from X-ray-to-radio wavelengths covering a 2 deg 2 area, including HST imaging. Given the very high sensitivity and resolution of these data sets, COSMOS also provides unprecedented samples of objects at high redshift with greatly reduced cosmic variance, compared to earlier surveys. Here we provide a brief overview of the survey strategy, the characteristics of the major COSMOS data sets, and a summary the science goals. © 2007. The American Astronomical Society. All rights reserved.
• Smolčić, V., Schinnerer, E., Finoguenov, A., Sakelliou, I., Carilli, C. L., Botzler, C. S., Brusa, M., Scoville, N., Ajiki, M., Capak, P., Guzzo, L., Hasinger, G., Impey, C., Jahnke, K., Kartaltepe, J. S., McCracken, H. J., Mobasher, B., Murayama, T., Sasaki, S. S., , Shioya, Y., et al. (2007). A wide-angle tail radio galaxy in the COSMOS field: Evidence for cluster formation. Astrophysical Journal, Supplement Series, 172(1), 295-313.
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  Abstract: We have identified a complex galaxy cluster system in the COSMOS field via a wide-angle tail (WAT) radio galaxy, consistent with the idea that WAT galaxies can be used as tracers of clusters. The WAT galaxy, CWAT-01, is coincident with an elliptical galaxy resolved in the HSTACS image. Using the COSMOS multiwavelength data set, we derive the radio properties of CWAT-01 and use the optical and X-ray data to investigate its host environment. The cluster hosting CWAT-01 is part of a larger assembly consisting of a minimum of four X-ray luminous clusters within ∼2 Mpc distance. We apply hydrodynamic models that combine ram pressure and buoyancy forces on CWAT-01. These models explain the shape of the radio jets only if the galaxy's velocity relative to the intracluster medium (ICM) is in the range of about 300-550 km s-1, which is higher than expected for brightest cluster galaxies (BCGs) in relaxed systems. This indicates that the CWAT-01 host cluster is not relaxed but is possibly dynamically young. We argue that such a velocity could have been induced through subcluster merger within the CWAT-01 parent cluster and/or cluster-cluster interactions. Our results strongly indicate that we are witnessing the formation of a large cluster from an assembly of multiple clusters, consistent with the hierarchical scenario of structure formation. We estimate the total mass of the final cluster to be approximately 20% of the mass of the Coma Cluster. © 2007. The American Astronomical Society. All rights reserved.
• Takahashi, M. I., Shioya, Y., Taniguchi, Y., Murayama, T., Ajiki, M., Sasaki, S. S., Koizumi, O., Nagao, T., Scoville, N. Z., Mobasher, B., Aussel, H., Capak, P., Carilli, C., Ellis, R. S., Garilli, B., Giavalisco, M., Guzzo, L., Hasinger, G., Impey, C., , Kitzbichler, M. G., et al. (2007). The [O II] λ3727 luminosity function and star formation rate at z ≈ 1.2 in the COSMOS 2 square degree field and the Subaru deep field. Astrophysical Journal, Supplement Series, 172(1), 456-467.
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  Abstract: We have carried out a wide-field imaging survey for [O II] λ3727 emitting galaxies at z ≈ 1.2 in the HST COSMOS 2 square degree field using the Suprime-Cam on the Subaru Telescope. The survey covers a sky area of 6700 arcmin 2 in the COSMOS field, and a redshift range between 1.17 and 1.20 (Δz = 0.03), corresponding to a survey volume of 5.56 × 10 5 Mpc 3. We obtain a sample of 3176 [O II] emitting galaxies with observed emission-line equivalent widths greater than 26 Å. Since our survey tends to sample brighter [O II] emitting galaxies, we also analyze a sample of fainter [O II] emitting galaxies found in the Subaru Deep Field (SDF). We find an extinction-corrected [O II] luminosity density of 10 40.35-0.06+0.08 ergs s -1 Mpc -3, corresponding to star formation rate density of 0.32 -0.04+0.06 M ⊙ yr -1 Mpc -3 in the COSMOS field at z ≈ 1.2. This is the largest survey for [O II] emitters beyond z = 1 currently available. © 2007. The American Astronomical Society. All rights reserved.
• Taniguchi, Y., Scoville, N., Murayama, T., Sanders, D. B., Mobasher, B., Aussel, H., Capak, P., Ajiki, M., Miyazaki, S., Komiyama, Y., Shioya, Y., Nagao, T., Sasaki, S. S., Koda, J., Carilli, C., Giavalisco, M., Guzzo, L., Hasinger, G., Impey, C., , LeFevre, O., et al. (2007). The Cosmic Evolution Survey (COSMOS): Subaru observations of the HST cosmos field. Astrophysical Journal, Supplement Series, 172(1), 9-28.
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  Abstract: We present deep optical imaging observations of 2 square degree area, covered by the Cosmic Evolution Survey (COSMOS), made by the prime-focus camera (Supreme-Cam) on the 8.2 m Subaru Telescope. Observations were done in six broadband [B (4459.7 ̊), g′ (4723.1 ̊), V (5483.8 ̊), r′ (6213.0 ̊), i′(7640.8 ̊),z′ (8855.0 ̊)] and one narrowband (NB816) filters. A total of 10 6 galaxies were detected to i′∼ 26.5 mag. These data, combined with observations at u* and K-band are used to construct the photometric catalogs for the COSMOS, to measure their photometric redshifts, multiband spectral energy distributions, and stellar masses, and to identify high-redshift candidates. This catalog provides multi-wave band data for scientific analysis of the COSMOS survey. © 2007. The American Astronomical Society. All rights reserved.
• Trump, J. R., Impey, C. D., McCarthy, P. J., Elvis, M., Huchra, J. P., Brusa, M., Hasinger, G., Schinnerer, E., Capak, P., Lilly, S. J., & Scoville, N. Z. (2007). Magellan spectroscopy of AGN candidates in the COSMOS field. Astrophysical Journal, Supplement Series, 172(1), 383-395.
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  Abstract: We present spectroscopic redshifts for the first 466 X-ray- and radio-selected AGN targets in the 2 deg 2 COSMOS field. Spectra were obtained with the IMACS instrument on the Magellan (Baade) telescope, using the nod-and-shuffle technique. We identify a variety of type 1 and type 2 AGNs, as well as red galaxies with no emission lines. Our redshift yield is 72% down to i AB = 24, although the yield is >90% for i AB < 22, We expect the completeness to increase as the survey continues. When our survey is complete and additional redshifts from the zCOSMOS project are included, we anticipate ∼1100 AGNs with redshifts over the entire COSMOS field. Our redshift survey is consistent with an obscured AGN population that peaks at z ∼ 0.7, although further work is necessary to disentangle the selection effects. © 2007. The American Astronomical Society. All rights reserved.
• Impey, C. (2006). Borders in astronomy: Facing the end. Western Humanities Review, 60(2), 175-187.
• Impey, C. D., Trump, J. R., McCarthy, P. J., Elvis, M., Huchra, J. P., Scoville, N. Z., Lilly, S. J., Brusa, M., Hasinger, G., Schinnerer, E., Capak, P., & Gabor, J. (2006). A survey of AGN and supermassive black holes in the COSMOS Survey. Proceedings of the International Astronomical Union, 2(S238), 287-290.
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  Abstract: The Cosmic Evolution Survey (COSMOS) is an HST/ACS imaging survey of 2 square degrees centered on RA = 10:00:28.6, Dec = + 02:12:21 (J2000). While the primary goal of the survey is to study evolution of galaxy morphology and large scale structure, an extensive multi-wavelength data set allows for a sensitive survey of AGN. Spectroscopy of optical counterparts to faint X-ray and radio sources is being carried out with the Magallen (Baade) Telescope and the ESO VLT. By achieving 80 redshift completeness down to I AB = 3, the eventual yield of AGN will be 1100 over the whole field. Early results on supermassive black holes are described. The goals of the survey include a bolometric census of AGN down to moderate luminosities, the cosmic evolution and fueling history of the central engines, and a study of AGN environments on scales ranging from the host galaxy to clusters and superclusters. © 2007 International Astronomical Union.
• Peng, C. Y., Impey, C. D., Ho, L. C., Barton, E. J., & Rix, H. (2006). Probing the coevolution of supermassive black holes and quasar host galaxies. Astrophysical Journal Letters, 640(1 I), 114-125.
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  Abstract: At low redshift, there are fundamental correlations between the mass of supermassive black holes (script M sign BH) and the mass (script M sign bulge) and luminosity of the host galaxy bulge. We investigate the same relation at z ≳ 1. Using virial mass estimates for 11 quasars at z ≳ 2 to measure their black hole mass, we find that black holes at high z fall nearly on the same script M signBH versus R-band magnitude (MR) relation (to ∼0.3 mag) as low-redshift active and inactive galaxies, without making any correction for luminosity evolution. Using a set of conservative assumptions about the host galaxy stellar population, we show that at z ≳ 2 (10 Gyr ago), the ratio of script M signBH/script M signbulge was 3-6 times larger than today. Barring unknown systematic errors on the measurement of script M signBH, we also rule out scenarios in which moderately luminous quasar hosts at z ≳ 2 were fully formed bulges that passively faded to the present epoch. On the other hand, five quasar hosts at z ≈ 1 are consistent with the current-day script M sign BH-MR relationship after taking into account evolution that is appropriate for E/S0 galaxies. Therefore, z ≈ 1 host galaxies appear to fit the hypothesis that they are fully formed early-type galaxies. We also find that most quasar hosts with absolute magnitudes brighter than MR = -23 cannot fade below L* galaxies today, regardless of their stellar population makeup, because their black hole masses are too high and they must arrive at the local script M signBH-MR relationship by z = 0. © 2006. The American Astronomical Society. All rights reserved.
• Peng, C. Y., Impey, C. D., Rix, H., Falco, E. E., Keeton, C. R., Kochanek, C. S., Lehár, J., & McLeod, B. A. (2006). Lensed quasar hosts. New Astronomy Reviews, 50(9-10), 689-693.
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  Abstract: Gravitational lensing assists in the detection of quasar hosts by amplifying and distorting the host light away from the unresolved quasar core images. We present the results of HST observations of 30 quasar hosts at redshifts 1 < z < 4.5. The hosts are small in size (re ≲ 6 kpc), and span a range of morphologies consistent with early-types (though smaller in mass) to disky/late-type. The ratio of the black hole mass (MBH, from the virial technique) to the bulge mass (Mbulge, from the stellar luminosity) at 1 ≲ z ≲ 1.7 is broadly consistent with the local value; while MBH / Mbulge at z ≳ 1.7 is a factor of 3-6 higher than the local value. But, depending on the stellar content the ratio may decline at z ≳ 4 (if E/S0-like), flatten off to 6-10 times the local value (if Sbc-like), or continue to rise (if Im-like). We infer that galaxy bulge masses must have grown by a factor of 3-6 over the redshift range 3 ≳ z ≳ 1, and then changed little since z ∼ 1. This suggests that the peak epoch of galaxy formation for massive galaxies is above z ∼ 1. We also estimate the duty cycle of luminous AGNs at z ≳ 1 to be ∼1%, or 107 yr, with sizable scatter. © 2006 Elsevier B.V. All rights reserved.
• Peng, C. Y., Impey, C. D., Rix, H., Kochanek, C. S., Keeton, C. R., Falco, E. E., Lehár, J., & McLeod, B. A. (2006). Probing the coevolution of supermassive black holes and galaxies using gravitationally lensed quasar hosts. Astrophysical Journal Letters, 649(2 I), 616-634.
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  Abstract: In the present-day universe, supermassive black hole masses (M BH) appear to be strongly correlated with their galaxy's bulge luminosity, among other properties. In this study we explore the analogous relationship between MBH, derived using the virial method, and the stellar (LR)-band bulge luminosity (LR) or stellar bulge mass (M*) at epochs of 1 ≲ z ≲ 4.5, using a sample of 31 gravitationally lensed AGNs and 20 nonlensed AGNs. At redshifts z > 1.7 (10-12 Gyr ago), we find that the observed MBH-LR relation is nearly the same (to within ∼0.3 mag) as it is today. When the observed LR are corrected for luminosity evolution, this means that the black holes grew in mass faster than their hosts, with the MBH/M* mass ratio being a factor of ≳4-1+2 times larger at z > 1.7 than it is today. By the redshift range 1 ≲ z ≲ 1.7 (8-10 Gyr ago), the MBH/M* ratio is at most 2 times higher than today, but it may be consistent with no evolution. Combining the results, we conclude that the ratio MBH/M* rises with look-back time, although it may saturate at ≈6 times the local value. Scenarios in which moderately luminous quasar hosts at z ≳ 1.7 were fully formed bulges that passively faded to the present epoch are ruled out. © 2006. The American Astronomical Society. All rights reserved.
• Petry, C. E., Impey, C. D., Fenton, J. L., & Foltz, C. B. (2006). Lyα absorption in the quasar triplet Q0107-025A, Q0107-025B, and Q0107-0232: Data calibration and line selection. Astronomical Journal, 132(5), 2046-2073.
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  Abstract: We present spectroscopic observations for quasars Q0107-025A, Q0107-025B, and Q0107-0232. For the first time, all data obtained with the Hubble Space Telescope's GHRS and FOS for these three quasars are combined. Lya absorbers selected from the spectra can be used to trace the coherence of the intergalactic medium (IGM) on scales of 1-2 h70-1 Mpc over the redshift range 0.4 < za < 0.9. A new analysis of double and triple absorber coincidences derived from the improved spectra is performed. Automated line-selection algorithms were used to detect absorption lines in the QSO spectra; coincident absorbers were identified across two and three sight lines based on their proximity to each other in velocity space. A control simulation designed to generate artificial absorption-line spectra was used to gauge the significance of the coincidences. Four intervening metal-line absorption systems are detected, with three of the four coincident to the A and B sight lines and one coincident across all three sight lines. This is evidence for substantial clustering among the objects responsible for the metal-enriched gas. By contrast, large-scale coherence of the IGM gas is detectable but weak. Fewer than half of the absorbers are coincident on scales of 1-2 h70-1 Mpc, a result that is significant at the 99% confidence level, and coherence arises preferentially in the higher column density gas. Triple absorber coincidences occur at a level that is significant at the 99% confidence level, which indicates that the higher column density gas has a planar geometry. © 2006. The American Astronomical Society. All rights reserved.
• Prescott, M. K., Impey, C. D., Cool, R. J., & Scoville, N. Z. (2006). Quasars in the COSMOS field. Astrophysical Journal Letters, 644(1), 100-115.
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  Abstract: We obtained medium-resolution spectra of 336 quasar candidates in the COSMOS HST Treasury field using the MMT 6.5 m telescope and the Hectospec multiobject spectrograph. Candidates were drawn from the Sloan Digital Sky Survey (SDSS) DR1 catalog using quasar flags set by the SDSS multicolor quasar target selection algorithm. In this paper we present our discovery spectra from 1.39 deg2 (69.5% of the COSMOS field) and a discussion of the selection method and yields. We confirmed 95 quasars, including at least two BAL quasars; 80 of these are new quasars that do not appear in previous quasar confirmation follow-up studies. The candidates additionally included 184 compact emission-line galaxies, a third of which are likely type 2 AGNs, and 12 stars. The quasars span a range in magnitude of 18.3 < g < 22.5 and a range in redshift of 0.2 < z < 2.3. Our results are consistent with a lower limit quasar surface density from SDSS color selection of 102 deg -2 down to g = 22.5 over the entire COSMOS field. This work is the first step toward the eventual goal of setting up a grid of quasar absorption line probes of the 2 deg2 field and of conducting a complete census of supermassive black holes in this well-studied survey region. The total quasar count at the conclusion of this study is 139, making COSMOS one of the most densely sampled regions of sky where a grid of quasar sight lines can be used to probe the intervening volume. © 2006. The American Astronomical Society. All rights reserved.
• McIntosh, D. H., Impey, C. D., & Petry, C. E. (2004). Quasars as absorption probes of the J0053+1234 region. Astronomical Journal, 128(2 1784), 544-560.
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  Abstract: We present UBRI photometry and spectra for 60 quasars found within 1 deg 2 centered on the J0053+1234 region, which has been the subject of the Caltech Faint Galaxy Redshift Survey. Candidate quasars were selected by their ultraviolet excess with respect to the stellar locus and confirmed spectroscopically. The quasars span a wide range in brightness (17.5 < B < 21.6) and redshift (0.43 < z < 2.38). These new quasars comprise a grid of absorption probes that can be used to study large-scale structure, as well as the correlation between luminous galaxies, nonluminous halos, and Lyα absorbers in the direction of the deep pencil-beam galaxy survey. Spectra of 14 emission-line galaxies found using the same technique are also presented.
• Schinnerer, E., Carilli, C. L., Scoville, N. Z., Bondi, M., Ciliegi, P., Vettolani, P., Févre, O. L., Koekemoer, A. M., Bertoldi, F., & Impey, C. D. (2004). The VLA-cosmos survey. I. Radio identifications from the pilot project. Astronomical Journal, 128(5), 1974-1989.
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  Abstract: We present a catalog of 246 radio sources found in the central 1° of the COSMOS field at 1.4 GHz. The VLA pilot project data have a resolution of 1″.9 × 1″.6 and an rms noise limit of ∼25-100 μJy beam -1 covering 0.837 deg 2. About 20 radio sources are clearly extended, and most of them appear to be double-lobed radio galaxies. We find evidence for a cluster of seven radio galaxies with an extent of ∼10′ southeast of the COSMOS field center. This VLA pilot project was undertaken to demonstrate the feasibility of wide-field mosaicking at 2″ resolution at 1.4 GHz using the VLA in its A configuration. The seven-point mosaic data were used to develop the techniques necessary for reduction and analysis. These data will provide the initial astrometric frame for the optical (ground-and space-based) data of the COSMOS 2 deg 2 survey. In addition, they demonstrate the feasibility of obtaining deep (rms ∼ a few microjanskys) radio imaging of this field at 1.4 GHz, since the brightest radio-emission peak detected within the area covered has a flux density of 13 mJy beam -1 and no strong sidelobes from sources surrounding the COSMOS field were detected. Comparison of the number counts with other deep radio surveys shows that the COSMOS field is a representative deep field in the radio domain.
• Kuraszkiewicz, J. K., Wilkes, B. J., Hooper, E. J., McLeod, K. K., Wood, K., Bjorkman, J., Delain, K. M., Hughes, D. H., Elvis, M. S., Impey, C. D., Lonsdale, C. J., Malkan, M. A., McDowell, J. C., & Whitney, B. (2003). The far-infrared spectral energy distributions of X-ray-selected active galaxies. Astrophysical Journal Letters, 590(1 I), 128-148.
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  Abstract: Hard X-ray selection is, arguably, the optimal method for defining a representative sample of active galactic nuclei (AGNs). Hard X-rays are unbiased by the effects of obscuration and reprocessing along the line of sight intrinsic/external to the AGN, which result in unknown fractions of the population being missed from traditional optical/soft X-ray samples. We present the far-infrared (far-IR) observations of 21 hard X-ray-selected AGNs from the HEAO 1 A2 sample observed with Infrared Space Observatory (ISO). We characterize the far-IR continua of these X-ray-selected AGNs and compare them with those of various radio and optically selected AGN samples and with models for an AGN-heated, dusty disk. The X-ray-selected AGNs show broad, warm IR continua covering a wide temperature range (∼20-1000 K in a thermal emission scenario). Where a far-IR turnover is clearly observed, the slopes are less than 2.5 in all but three cases so that nonthermal emission remains a possibility, although the presence of cooler dust resulting in a turnover at wavelengths longward of the ISO range is considered more likely. The sample also shows a wider range of optical/UV shapes than the optical/radio-selected samples, extending to redder near-IR colors. The bluer objects are type 1 Seyfert galaxies, while the redder AGNs are mostly intermediate or type 2 Seyfert galaxies. This is consistent with a modified unification model in which obscuration increases as we move from a face-on toward a more edge-on line of sight. However, this relation does not extend to the mid-infrared as the 25/60 μm ratios are similar in Seyfert galaxies with differing type and optical/UV reddening. The resulting limits on the column density of obscuring material through which we are viewing the redder AGNs (NH ∼ 10 22 cm-2) are inconsistent with standard optically thick torus models (NH ∼ 1024 cm-2) and simple unification models. Instead our results support more complex models in which the amount of obscuring material increases with viewing angle and may be clumpy. Such a scenario, already suggested by differing optical/near-IR spectroscopic and X-ray AGN classifications, allows for different amounts of obscuration of the continuum emission in different wave bands and of the broad emission line region, which, in turn, results in a mixture of behaviors for AGNs with similar optical emission-line classifications. The resulting decrease in the optical depth of the obscuring material also allows the AGN to heat more dust at larger radial distances. We show that an AGN-heated, flared, dusty disk with mass of ∼109 M⊙ and size of approximately a few hundred parsecs is able to generate optical-far-IR spectral energy distributions (SEDs) that reproduce the wide range of SEDs present in our sample with no need for an additional starburst component to generate the long-wavelength, cooler part of the IR continuum.
• Hawking, S., & Impey, C. D. (2002). The universe in a nutshell. Physics Today, 55(4), 77-.
• Impey, C. D., Petry, C. E., Foltz, C. B., Hewett, P. C., & Chaffee, F. H. (2002). LBQS 0015+0239: A binary quasar with small angular separation. Astrophysical Journal Letters, 574(2 I), 623-633.
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  Abstract: We present spectroscopic observations with the Keck Low-Resolution Imaging Spectrometer of LBQS 0015+0239, a pair of quasars at z = 2.45 with a separation of Δθ = ″2.2 (projected linear distance of 17.8 h 70-1 kpc, for Ωm = 0.3, A = 0.7). Lensing is an unlikely interpretation for the images since the spectra show significant differences in the N V and C IV emission-line profiles and there is no luminous galaxy at the anticipated lens position. Rather, we interpret this pair as the highest redshift known example of a binary quasar. The redshift difference of 661 ± 173 km s-1 between the two components is inconsistent with the lensing expectation but is consistent with the line-of-sight velocity difference of a bound pair of galaxies. The spectra show associated narrow metal absorption in highly ionized species of C and N, with a systemic velocity difference of 492 ± 6 km s-1. It is likely that this absorption arises in a group or cluster of galaxies surrounding the quasar pair. After a thorough search, the Large Bright Quasar Survey is known to contain one gravitational lens and four probable binary pairs. The existence of close binaries is evidence that quasar activity can be triggered by galaxy interactions. We speculate that the close pair LBQS 0015+0239 is a rare example of the premerger state of two supermassive black holes.
• Lee, J. C., Salzer, J. J., Impey, C., Thuan, T. X., & Gronwall, C. (2002). H I properties of low-luminosity star-forming galaxies in the KPNO international spectroscopic survey. Astronomical Journal, 124(6 1764), 3088-3117.
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  Abstract: New H I observations are presented for a complete sample of 109 low-luminosity star-forming galaxies taken from the KPNO International Spectroscopic Survey (KISS), the first CCD-based wide-field objective-prism survey for emission-line galaxies. This sample consists of all star-forming galaxies with MB >-18.0 and cz < 11,000 km s-1 from the first Hα-selected survey list. The galaxies in this list lie within a 1°.3-wide strip centered on δ(B1950) = 29°30′, which spans the range α(B1950) = 12h15m to α(B1950) = 17h0m. Overall, 97 out of 109 galaxies have been detected in H I. We confirm the weak trend of increasing gas richness with decreasing luminosity found by previous authors. Gas richness is also shown to be weakly anticorrelated with metallicity. The dependence of star formation rates (SFRs) and H I gas depletion timescales on metallicity is examined. The median solar-metallicity-based SFR and gas depletion timescale are 0.16 M ⊙ yr-1 and 5 Gyr, respectively. Corrections for variations in metallicity decreases SFRs by ∼0.5 dex and increases gas depletion timescales by an average of ∼8 Gyr. The majority of galaxies in this sample still have large reservoirs of H I gas and, despite their large current star formation rates, could have formed stars in a quasi-continuous manner for a Hubble time. Finally, we present the first H I mass function for low-luminosity star-forming galaxies and show that this subpopulation contributes 10%-15% of the overall H I density in the local universe. We conclude that if the H I mass function of the universe does indeed have a steeply rising low-mass slope, as suggested by previous authors, it is not due to the population of low-luminosity star-forming galaxies. Comparison with the H I mass function number densities from H I blind surveys in the range 10 8 M⊙ < MHI < 109 M ⊙ implies that 25%-50% of galaxies in this mass regime are currently undergoing a strong episode of star formation.
• Lehár, J., Falco, E. E., Kochanek, C. S., McLeod, B. A., Muñoz, J., Impey, C. D., Rix, H. -., Keeton, C. R., & Peng, D. C. (2002). Erratum: Hubble space telescope observations of 10 two-image gravitational lenses (Astrophysical Journal (2002) 536 (584)). Astrophysical Journal Letters, 571(2 I), 1021-.
• Peng, C. Y., Ho, L. C., Impey, C. D., & Rix, H. (2002). Detailed structural decomposition of galaxy images. Astronomical Journal, 124(1 1759), 266-293.
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  Abstract: We present a two-dimensional fitting algorithm (GALFIT) designed to extract structural components from galaxy images, with emphasis on closely modeling light profiles of spatially well-resolved, nearby galaxies observed with the Hubble Space Telescope. Our algorithm improves on previous techniques in two areas: by being able to simultaneously fit a galaxy with an arbitrary number of components and with optimization in computation speed, suited for working on large galaxy images. We use two-dimensional models such as the " Nuker " law, the Sérsic (de Vaucouleurs) profile, an exponential disk, and Gaussian or Moffat functions. The azimuthal shapes are generalized ellipses that can fit disky and boxy components. Some potential applications of our program include: standard modeling of global galaxy profiles; extracting bars, stellar disks, double nuclei, and compact nuclear sources; and measuring absolute dust extinction or surface brightness fluctuations after removing the galaxy model. When examined in detail, we find that even simple looking galaxies generally require at least three components to be modeled accurately, rather than the one or two components more often employed. Many galaxies with complex isophotes, ellipticity changes, and position angle twists can be modeled accurately in two dimensions. We illustrate this by way of 11 case studies, which include regular and barred spiral galaxies, highly disky lenticular galaxies, and elliptical galaxies displaying various levels of complexities. A useful extension of this algorithm is to accurately extract nuclear point sources in galaxies. We compare two-dimensional and one-dimensional extraction techniques on simulated images of galaxies having nuclear slopes with different degrees of cuspiness, and we then illustrate the application of the program to several examples of nearby galaxies with weak nuclei.
• Petry, C. E., Impey, C. D., Katz, N., Weinberg, D. H., & Hernquist, L. E. (2002). Comparing simulations and observations of the Lyα forest. I. Methodology. Astrophysical Journal Letters, 566(1 I), 30-50.
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  Abstract: We describe techniques for comparing spectra extracted from cosmological simulations and observational data, using the same methodology to link Lyα properties derived from the simulations with properties derived from observational data. The eventual goal is to measure the coherence or clustering properties of Lyα absorbers using observations of quasar pairs and groups. We quantify the systematic underestimate in opacity that is inherent in the continuum-fitting process of observed spectra over a range of resolution and signal-to-noise ratio (S/N). We present an automated process for detecting and selecting absorption features over the range of resolution and S/N of typical observational data on the Lyα forest. Using these techniques, we detect coherence over transverse scales out to 500 h50-1 kpc in spectra extracted from a cosmological simulation at z = 2.
• Winn, J. N., Kochanek, C. S., McLeod, B. A., Falco, E. E., Impey, C. D., & Rix, H. (2002). PKS 1830-211: A face-on spiral galaxy lens. Astrophysical Journal Letters, 575(1 I), 103-110.
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  Abstract: We present new Hubble Space Telescope images of the gravitational lens PKS 1830-211, which allow us to characterize the lens galaxy and update the determination of the Hubble constant (H0) from this system. The I-band image shows that the lens galaxy is a face-on spiral galaxy with clearly delineated spiral arms. The southwestern image of the background quasar passes through one of the spiral arms, explaining the previous detections of large quantities of molecular gas and dust in front of this image. The lens galaxy photometry is consistent with the Tully-Fisher relation, suggesting the lens galaxy is a typical spiral galaxy for its redshift. The lens galaxy position, which was the main source of uncertainty in previous attempts to determine H0, is now known precisely. Given the current time delay measurement and assuming the lens galaxy has an isothermal mass distribution, we compute H0 = 44 ± 9 km s-1 Mpc-1 for an Ωm = 0.3 flat cosmological model. We describe some possible systematic errors and how to reduce them. We also discuss the possibility raised by Courbin et al. (2002), that what we have identified as a single lens galaxy is actually a foreground star and two separate galaxies.
• Becker, R. H., White, R. L., Gregg, M. D., Laurent-Muehleisen, S. A., Brotherton, M. S., Impey, C. D., Chaffee, F. H., Richards, G. T., Helfand, D. J., Lacy, M., Courbin, F., & Proctor, D. D. (2001). The First Bright Quasar Survey. III. The South Galactic cap. Astrophysical Journal, Supplement Series, 135(2), 227-262.
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  Abstract: We present the results of an extension of the FIRST Bright Quasar Survey (FBQS) to the South Galactic cap, and to a fainter optical magnitude limit. Radio source counterparts with SERC R magnitudes brighter than 18.9 which meet the other FBQS criteria are included. We supplement this list with a modest number of additional objects to test our completeness for quasars with extended radio morphologies. The survey covers 589 deg2 in two equatorial strips in the southern cap. We have obtained spectra for 86% of the 522 candidates and find 321 radio-selected quasars of which 264 are reported here for the first time. A comparison of this fainter sample with the FBQS sample shows the two to be generally similar. Fourteen new broad absorption line (BAL) quasars are included in this sample. When combined with the previously identified BAL quasars in our earlier papers, we can discern a break in the frequency of BAL quasars with radio loudness, namely that the relative number of high-ionization BAL quasars drops by a factor of 4 for quasars with a radio-loudness parameter R* > 100.
• Burkholder, V., Impey, C., & Sprayberry, D. (2001). High and low surface brightness galaxies in the local universe. V. Optical and H I properties. Astronomical Journal, 122(5), 2318-2340.
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  Abstract: Using optical spectra of the nuclei of 250 galaxies chosen from the Automated Plate Measuring Facility (APM) survey of UK Schmidt plates, we derive synthetic B - V and V - R colors, estimates of reddening from Balmer decrements, metallicity via oxygen abundance, and star formation rate from the Hα luminosity. We also present mass-to-light ratios and gas mass fractions from 21 cm H I measurements of 238 APM survey galaxies, 101 of which also have optical spectra. This sample of galaxies spans a large range in surface brightness, from high surface brightness irregular galaxies down to the kind of low surface brightness (LSB) galaxies that are missing from most magnitude-limited catalogs. The generally blue global colors are best explained by a combination of low metallicity and active star formation, primarily because of the tremendous scatter in metallicity versus B - V. Optical spectra show the presence of G and K main-sequence stars in the nuclei of these galaxies, and the stellar mass-to-light ratios and gas mass fractions show the galaxies as a whole to be gas-rich. LSB galaxies are most likely to experience sporadic low levels of star formation over gigayear timescales.
• Finn, R. A., Impey, C. D., & Hooper, E. J. (2001). WFPC2 imaging of quasar environments: A comparison of large bright quasar survey and Hubble Space Telescope archive quasars 1. Astrophysical Journal Letters, 557(2 PART 1), 578-593.
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  Abstract: We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) data on the large-scale environments of 16 0.39 < z < 0.51 quasars from the Large Bright Quasar Survey (LBQS). The LBQS quasars are representative of the radio-quiet population, and this is one of the first looks at their large-scale environments. We compare the LBQS environments with the environments of 27 0.15 < z < 0.55 quasars selected from the HST archive. The majority of the Archive quasars are from the PG and PKS surveys, and these quasars are more luminous on average than the LBQS. By comparing the LBQS and Archive environments, we investigate whether previous quasar environment studies have been biased as a result of studying unusually radio or optically luminous quasars. We compare observed galaxy number counts with expected counts predicted from the CNOC2 field galaxy luminosity function in order to look for statistical excesses of galaxies around the quasars. We detect a significant excess around the Archive quasars but find no such excess around the LBQS quasars. We calculate the amplitude of the spatial correlation function and find that the LBQS environments are consistent with that of the typical galaxy while the Archive environments are slightly less rich than Abell O clusters. We find no difference between the environments of radio-loud and radio-quiet quasars in either sample. However, comparison with previously published work shows that the LBQS radio-loud quasars are in sparse environments when compared with other radio-loud quasars, and the Archive radio-quiet quasars are in dense environments compared to other radio-quiet quasars. The richer environments of the Archive radio-quiet quasars cannot be explained by their higher optical luminosities. We find a positive correlation (95%) between radio luminosity and environment for the radio-loud quasars. This may explain why the LBQS radio-loud quasars, which are less radio luminous, are in sparser environments.
• Impey, C., & Petry, C. (2001). Radio-quiet quasars in the direction of the northern Hubble Deep Field. Astrophysical Journal Letters, 547(1 PART 1), 117-123.
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  Abstract: We match quasars discovered in a multicolor survey centered on the northern Hubble Deep Field (HDF) with radio sources from an ultradeep radio survey. Although three out of 12 quasars are detected at a level below 0.2 mJy at 1.4 GHz, all of the quasars in the search area are radio quiet by the criterion Lr < 1025 hso W Hz-1. We combine this information with other radio surveys of quasars so as to break the degeneracy between redshift and luminosity. In the redshift range 0.02 < z < 3.64, the radio-loud fraction increases with increasing optical luminosity, consistent with some degree of correlation between the nonthermal optical and radio emissions. More tentatively, for low-luminosity quasars in the range -22.5 < MB < -25, the radio-loud fraction decreases with increasing redshift. We can infer from this that the radio luminosity function evolves more slowly than the optical luminosity function. The mechanism that leads to strong radio emission in only a small fraction of quasars at any epoch is still unknown.
• Impey, C., Burkholder, V., & Sprayberry, D. (2001). High and low surface brightness galaxies in the local universe. IV. Optical and 21 centimeter spectroscopy. Astronomical Journal, 122(5), 2341-2380.
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  Abstract: We present flux-calibrated spectra in the range 3500 to 8000 A for the nuclear regions of 250 galaxies and H I 21 cm line profiles for 238 galaxies selected from the catalog of low surface brightness galaxies identified in Automated Plate Measuring Facility (APM) scans of UK Schmidt Telescope survey plates. For the optical spectra, common nebular absorption-and emission-line equivalent widths and emission-line fluxes were measured with typical errors of about 20%. H I mass and velocity widths were measured from the 21 cm line emission, with 15% accuracy. Galaxies in this study span over 6 mag in surface brightness. Only about 20% of these galaxies show spectroscopic evidence of active galactic nuclei. Derived quantities such as synthetic colors, metallicity, star formation rate, the ratio of neutral hydrogen mass to blue light, and gas mass fraction are presented in a companion paper.
• Leighly, K. M., Halpern, J. P., Helfand, D. J., Becker, R. H., & Impey, C. D. (2001). First observations of the second-brightest quasar. Astronomical Journal, 121(6), 2889-2894.
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  Abstract: We report the discovery of a new bright quasar, PHL 1811, as part of a follow-up program to identify bright quasars in the FIRST radio survey. With B = 13.9, R = 13.9, and z = 0.192, this quasar is now the second-brightest quasar (in apparent magnitude) known beyond z = 0.1. Optically classified as a narrow-line Seyfert 1 galaxy (NLS1), PHL 1811 is unusual for an object in this class in that it was not detected in X-rays in the ROSAT All-Sky Survey or in any previous X-ray survey. A follow-up BeppoSAX observation confirms that it is deficient in X-rays compared with other quasars, but poor signal-to-noise ratio prevents a definitive characterization of the X-ray spectrum. We consider three alternative hypotheses to explain why PHL 1811 is a weak X-ray source: (1) it is a broad absorption line quasi-stellar object and suffers X-ray absorption; (2) like several other luminous NLS1s, it exhibits high-amplitude X-ray variability and has been observed only when it is in an X-ray-quiescent state; and (3) it is intrinsically weak because it simply lacks an X-ray-emitting region.
• Muñoz, J., Falco, E. E., Kochanek, C. S., Lehár, J., McLeod, B. A., McNamara, B. R., Vikhlinin, A. A., Impey, C. D., Rix, H. -., Keeton, C. R., Peng, C. Y., & Mullis, C. R. (2001). Multifrequency analysis of the new wide-separation gravitational lens candidate RX J0921 + 4529. Astrophysical Journal Letters, 546(2 PART 1), 769-774.
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  Abstract: We report the discovery of a new two-image gravitational lens candidate. The system RX J0921 + 4529 contains two zs = 1.66 quasars separated by 6″.93 with an H-band magnitude difference of Δm = 1.39. The HST NIC2 H-band images reveal an H = 18.2 spiral galaxy between the quasar images, which is probably a member of a zl = 0.32 X-ray cluster centered on the field. We detect an extended source near the fainter quasar image but not in the brighter image. If this extended source is the host galaxy of the fainter quasar, then the system is a binary quasar rather than a gravitational lens. VLA observations at 3.6 cm reveal emission from the lens galaxy at the flux level of 1 mJy and a marginal detection of the brighter quasar.
• Norman, D. J., & Impey, C. D. (2001). Quasar-galaxy correlations: A detection of magnification bias. Astronomical Journal, 121(5), 2392-2404.
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  Abstract: Weak gravitational lensing by clusters and superclusters can change our perception of the distribution of bright background quasars through the phenomenon of magnification bias. It has been suggested that this lensing will lead to an association of moderate-redshift quasars (z ∼ 1.0-2.0) and foreground galaxies (z ∼ 0.2-0.3). We report on the analysis of a sample of 90 CCD fields centered on moderate-redshift quasars that are bright at both radio (S5 GHz > 1 Jy) and optical (V < 21.0) wavelengths, along with 12 control fields. The quasars presented here are chosen to have the highest potential for evidence of this weak lensing effect. We find no evidence for the presence of rich galaxy clusters (Abell richness class 1) in the foreground of these fields. However, using a Spearman rank-order correlation test, we detect a correlation between galaxy number density and angular distance from bright quasars on scales of ∼15′ with a significance of 96%. The observed correlation is unlikely the result of Galactic dust and is consistent with weak gravitational lensing of the quasars by foreground matter. We also determine the two-point angular correlation function between these background quasars and foreground galaxies. We find a correlation amplitude of 0.056 ± 0.020 over scales of ∼5′. This angular correlation is consistent with the semianalytic weak gravitational lensing models of Dolag & Bartelmann (published in 1997), which incorporate nonlinear evolution of mass clustering.
• Rusin, D., Kochanek, C. S., Norbury, M., Falco, E. E., Impey, C. D., Lehár, J., Mcleod, B. A., Rix, H. -., Keeton, C. R., Muñoz, J., & Peng, C. Y. (2001). B1359 + 154: A six-image lens produced by a z ≃ 1 compact group of galaxies. Astrophysical Journal Letters, 557(2 PART 1), 594-604.
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  Abstract: Hubble Space Telescope (HST) V-and I-band observations show that the gravitational lens B1359 + 154 consists of six images of a single zs = 3.235 radio source and its star-forming host galaxy, produced by a compact group of galaxies at zl ≃ 1. Very Long Baseline Array (VLBA) observations at 1.7 GHz strongly support this conclusion, showing six compact cores with similar low-frequency radio spectra. B1359 + 154 is the first example of galaxy-scale gravitational lensing in which more than four images are observed of the same background source. The configuration is due to the unique lensing mass distribution: three primary lens galaxies lying on the vertices of a triangle separated by 0″.7 ≃ 4 h-1 kpc, inside the 1″.7 diameter Einstein ring defined by the radio images. The gravitational potential has additional extrema within this triangle, creating a pair of central images that supplement the "standard" four-image geometry of the outer components. Simple mass models, consisting of three lens galaxies constrained by HST and VLBA astrometry, naturally reproduce the observed image positions but must be finely tuned to fit the flux densities.
• Young, P. A., Impey, C. D., & Foltz, C. B. (2001). Observations of Lyα absorption in a triple quasar system. Astrophysical Journal Letters, 549(1 PART 1), 76-89.
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  Abstract: We have obtained follow-up observations of the quasar pair LBQS 0107-025A and B and new observations of the nearby quasar LBQS 0107-0232 with the Hubble Space Telescope Faint Object Spectrograph. Extended wavelength coverage of LBQS 0107-025A and B using the G270H grating was also obtained. This triple system is unique in providing sensitivity to coherent Lyα absorption on transverse scales of approximately 1 Mpc at z < 1. Monte Carlo simulations were used to establish the confidence level for matches between absorption features in different lines of sight as a function of velocity separation. Pairwise, there are 8, 9, and 12 lines that match between spectra. Three instances of matches between all three lines of sight were found with velocity separations of less than 550 km s-1. Two of the pairings have coincident lines within | Δv | < 200 km s-1 that would occur with less than 10% probability by chance. Taking into account the equivalent widths of the lines, one of these triple coincidences is significant at the 99.99% confidence level based on Monte Carlo simulations with random line placements. Matches with strong lines preferentially have small velocity separations. These same simulations are used to demonstrate that the distribution of matches for a population of absorbers randomly distributed in velocity space is peaked toward | Δv | = 0, which has implications for the statistical significance of matches. One of the triple coincidences appears to be a strong absorber with a sheetlike, but inhomogeneous, geometry and a coherence length approaching or exceeding 1 Mpc.
• Impey, C. D., Bychkov, V., Tapia, S., Gnedin, Y., & Pustilnik, S. (2000). Rapid polarization variability in the BL Lacertae object S5 0716 + 714. Astronomical Journal, 119(4), 1542-1561.
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  Abstract: We present optical polarimetry of the violently variable BL Lacertae object S5 0716 + 714, obtained over a three year period using the 6 m telescope at the Special Astrophysical Observatory in Russia. The degree of polarization is high and variable throughout the observations. With a minimum time resolution of 1 minute, significant variations on a timescale of 10-15 minutes are observed. The variations are complex and not fully sampled by this data, but they are characterized by large amplitude changes on a timescale of about 1 day superposed on smooth variations with a timescale of about 1 week. Possible periods of 12.5, 2.5, and 0.14 days have been discovered. S5 0716 + 714 is a highly compact radio source with nonthermal emission observed across the electromagnetic spectrum out to gamma rays. The optical polarization variations are consistent with relativistically beamed synchrotron emission viewed at a very small angle to the line of sight.
• Keeton, C. R., Falco, E. E., Impey, C. D., Kochanek, C. S., Lehár, J., McLeod, B. A., Rix, H. -., Muñoz, J., & Peng, C. Y. (2000). The host galaxy of the lensed quasar Q0957 + 561. Astrophysical Journal Letters, 542(1 PART 1), 74-93.
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  Abstract: Infrared images of the Q0957 + 561 gravitational lens obtained with the Hubble Space Telescope show two large (∼5″) lensed images of the zs = 1.41 quasar host galaxy. Parts of the host galaxy are doubly imaged, like the quasar, while other parts are quadruply imaged. The distortions of the host galaxy offer the best probe yet of the global structure of the lensing potential, which is essential for determining the Hubble constant from the measured time delay. The distortions are inconsistent with the predictions of previously published lens models, which invalidates those models and their implications for H0. New models show that the distortions finally break the long-standing degeneracy between the shape of the lens galaxy and the tidal shear contributed by the cluster containing the lens galaxy. The shape of the lens galaxy's mass distribution must be remarkably similar to the shape of its luminosity distribution, and most models that produce reasonable values for the Hubble constant roughly match the observed ellipticity gradient and isophote twist of the lens galaxy. In addition, the cluster must be nonspherical and produce a relatively small tidal shear. Although there are still degeneracies in the lens models that lead to a 25% uncertainty in the derived value of the Hubble constant, there are also strong prospects for new observations to further improve the constraints and reduce the uncertainties.
• Kochanek, C. S., Falco, E. E., Impey, C. D., Lehar, J., Mcleod, B. A., Rix, H. -., Keeton, C. R., Muñoz, J., & Peng, C. Y. (2000). The fundamental plane of gravitational lens galaxies and the evolution of early-type galaxies in low-density environments. Astrophysical Journal Letters, 543(1 PART 1), 131-148.
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  Abstract: Most gravitational lenses are early-type galaxies in relatively low density environments - a "field" rather than a "cluster" population. Their average properties are the mass-averaged properties of all early-type galaxies. We show that field early-type galaxies with 0 < z < 1, as represented by the lens galaxies, lie on the same fundamental plane as those in rich clusters at similar redshifts. We then use the fundamental plane to measure the combined evolutionary and X-corrections for early-type galaxies in the V, I, and H bands. Only for passively evolving stellar populations formed at zf ≳ 2 (H0 = 65 km s-1 Mpc-1, Ω0 = 0.3, λ0 = 0.7) can the lens galaxies be matched to the local fundamental plane. The high formation epoch and the lack of significant differences between the field and cluster populations contradict many current models of the formation history of early-type galaxies. Lens galaxy colors and the fundamental plane provide good photometric redshift estimates with an empirical accuracy of 〈zFP - zl〉 = -0.04 ± 0.09 for the 20 lenses with known redshifts. A mass model dominated by dark matter is more consistent with the data than either an isotropic or radially anisotropic constant M/L mass model, and a radially anisotropic model is better than an isotropic model.
• Kochanek, C. S., Falco, E. E., Impey, C. D., Lehár, J., McLeod, B. A., Rix, H. -., Keeton, C. R., Muñoz, J., & Peng, C. Y. (2000). The infrared Einstein ring in the gravitational lens MG J1131 + 0456 and the death of the dusty lens hypothesis. Astrophysical Journal Letters, 535(2 PART 1), 692-705.
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  Abstract: We have obtained and modeled NICMOS images of the Einstein ring lens system MG J1131 + 0456, which show that its lens galaxy is an H = 18.6 mag, transparent, early-type galaxy at a redshift of zl ≃ 0.84; it has a major axis effective radius Re = 0″.7 ± 0″.1, projected axis ratio b/a = 0.6 ± 0.1, and major axis P.A. = 55° ± 9°. The lens is the brightest member of a group of at least seven galaxies with similar R-I and I-H colors, and the two closest group members produce sufficient tidal perturbations to explain the shape of the ring. The host galaxy of the MG J1131 + 0456 source is a zs ≳ 2 extremely red object (ERO) that is lensed into optical and infrared rings of dramatically different morphologies. These differences imply a strongly wavelength-dependent source morphology that could be explained by embedding the host in a larger, dusty disk. At 1.6 μm (H), the ring is spectacularly luminous, with a total observed flux of H = 17.4 mag and a demagnified flux of 19.3 mag, corresponding to a 1-2 L* galaxy at the probable source redshift of zs ≳ 2. Thus, it is primarily the stellar emission of the radio source host galaxy that produces the overall colors of two of the reddest radio lenses, MG J1131 + 0456 and JVAS B1938 + 666, aided by the suppression of optical active galactic nucleus emission by dust in the source galaxy. The dusty lens hypothesis - that many massive early-type galaxies with 0 ≲ zl ≲ 1 have large, uniform dust opacities - is ruled out.
• Lehár, J., Falco, E. E., Kochanek, C. S., Mcleod, B. A., Muñoz, J., Impey, C. D., Rix, H. -., Keeton, C. R., & Peng, C. Y. (2000). Hubble Space Telescope observations of 10 two-image gravitational lenses. Astrophysical Journal Letters, 536(2 PART 1), 584-605.
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  Abstract: We report on a program to obtain Hubble Space Telescope (HST) observations of galaxy-mass gravitational lens systems at optical and infrared wavelengths. Here we discuss the properties of 10 two-image gravitational lens systems (Q0142-100 = UM 673, B0218+357, SBS 0909+532, BRI 0952-0115, LBQS 1009-0252, Q1017-207 = J03.13, B1030+074, HE 1104-1805, Q1208+1011, and PKS 1830-211). We grouped these 10 systems because they have limited lens model constraints and often show poor contrast between the images and the lens galaxy. Of the 10 lens galaxies, seven are probably early-type galaxies, two are probably late-type galaxies (B0218+357 and PKS 1830-211), and one was not detected (Q1208+1011). We detect the host galaxies of the zs = 4.50 lensed quasar in BRI 0952-0115, the zs = 2.32 lensed quasar in HE 1104-1805, and the unlensed z = 1.63 quasar near LBQS 1009-0252. We fit a set of four standard lens models to each lens that had sufficient constraints to compare isothermal dark matter and constant mass-to-light lens models, and to explore the effects of local tidal shears.
• O'Neil, K., Bothun, G. D., & Impey, C. D. (2000). Structural characteristics of faint galaxies serendipitously discovered with the Hubble Space Telescope WFPC2. Astrophysical Journal, Supplement Series, 128(1), 99-123.
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  Abstract: Utilizing the F814W and F300W filters, Hubble Space Telescope Wide Field Planetary Camera-2 (WFPC2) images were taken of four low surface brightness galaxies in the direction of the Virgo Cluster - V7L3, V2L8, V1L4, and Malin 1. The high resolution of the WFPC2 combined with the extremely diffuse nature of the four galaxies makes them essentially transparent, allowing for the serendipitous discovery of 139 background galaxies visible through both the disks and nuclei of the foreground galaxies. Surface photometry was done on the newly discovered galaxies through the F814W (I-band) filter. The detected galaxies have both r1/4 and exponential-type profiles with radii (to the μF814W = 25.0 mag arcsec-2 limit) less than 5″.0. Their total magnitudes range from 18.9 through the survey cutoff at 25.0 in the F814W filter. The median central surface brightness of those galaxies with exponential profiles is approximately 1 mag brighter than the background F814W "sky." Thus, with this data set we recover Freeman's law and hence know that we do not have a representative sample of distant galaxies (and neither does anyone else). When possible, the B, V, and I colors of these galaxies were determined using ground-based images, which show the galaxies to be fairly red. Coupled with their small angular size, we estimate the redshifts to be 0.5 ≤ z ≤ 1.5. Classification of the galaxies was done strictly in structural terms, based only on the form of the derived luminosity profile. No morphological considerations were made during the classification process. Twenty-three percent of the galaxies we detected have the r1/4 profile typical of early-type galaxies, matching most previous studies of both the Hubble Deep Field and the Medium Deep Survey, which typically find 15%-40% E/S0 galaxies. In addition, we have attempted to perform bulge/disk deconvolutions. While we find that most of the sample cannot be easily deconvolved into a classic bulge + disk, seven objects could be fitted in this way. For these seven objects we find (1) a large range in bulge-to-total luminosity and (2) some disks that have a large bulge-to-disk ratio. We also present one object, 283-10, which is an excellent example of the structural ambiguity that exists in the luminosity profiles of distant galaxies. In agreement with other studies, we also found a significant percentage of galaxies that have disturbed luminosity profiles indicative of probable galaxy-galaxy interactions or mergers. Indirect indicators suggest that the volume over which r1/4 objects are selected is significantly larger than the volume over which disk galaxies are selected. This implies a relatively low space density of r1/4 at all redshifts out to z ∼~ 2.5 and is consistent with the general idea that r1/4 galaxies are largely confined to galaxy clusters.
• White, R. L., Becker, R. H., Gregg, M. D., Laurent-Muehleisen, S. A., Brotherton, M. S., Impey, C. D., Petry, C. E., Foltz, C. B., Chaffee, F. H., Richards, G. T., Oegerle, W. R., Helfand, D. J., McMahon, R. G., & Cabanela, J. E. (2000). The FIRST Bright Quasar Survey. II. 60 nights and 1200 spectra later. Astrophysical Journal, Supplement Series, 126(2), 133-207.
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  Abstract: We have used the Very Large Array (VLA) FIRST survey and the Automated Plate Measuring Facility (APM) catalog of the Palomar Observatory Sky Survey I (POSS-I) plates as the basis for constructing a new radio-selected sample of optically bright quasars. This is the first radio-selected sample that is competitive in size with current optically selected quasar surveys. Using only two basic criteria, radio-optical positional coincidence and optical morphology, quasars and BL Lac objects can be identified with 60% selection efficiency; the efficiency increases to 70% for objects fainter than 17 mag. We show that a more sophisticated selection scheme can predict with better than 85% reliability which candidates will turn out to be quasars. This paper presents the second installment of the FIRST Bright Quasar Survey (FBQS), with a catalog of 636 quasars distributed over 2682 deg2. The quasar sample is characterized and all spectra are displayed. The FBQS detects both radio-loud and radio-quiet quasars out to redshift z > 3. We find a large population of objects of intermediate radio loudness; there is no evidence in our sample for a bimodal distribution of radio characteristics. The sample includes ∼29 broad absorption line quasars, both high and low ionization, and a number of new objects with remarkable optical spectra.
• Winn, J. N., Hewitt, J. N., Schechter, P. L., Dressler, A., Falco, E. E., Impey, C. D., Kochanek, C. S., Lehár, J., Lovell, J. E., McLeod, B. A., Morgan, N. D., Muñoz, J., Rix, H. -., & Ruiz, M. T. (2000). PMN J1838-3427: A new gravitationally lensed quasar. Astronomical Journal, 120(6), 2868-2878.
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  Abstract: We report the discovery of a new double-image quasar that was found during a search for gravitational lenses in the southern sky. Radio source PMN J1838-3427 is composed of two flat-spectrum components with separation I″.0, flux density ratio 14:1, and matching spectral indices in VLA and VLBA images. Ground-based BRI images show the optical counterpart (total I = 18.6) is also double, with the same separation and position angle as the radio components. An HST/WFPC2 image reveals the lens galaxy. The optical flux ratio (27:1) is higher than the radio value, probably because of differential extinction of the components by the lens galaxy. An optical spectrum of the bright component contains quasar emission lines at z = 2.78 and several absorption features, including prominent Lyα absorption. The lens galaxy redshift could not be measured, but it is estimated to be z = 0.36 ± 0.08. The image configuration is consistent with the simplest plausible models for the lens potential. The flat radio spectrum and observed variability of PMN J1838 - 3427 suggest that the time delay between flux variations of the components is measurable and could thus provide an independent measurement of H0.
• Campos, A., Yahil, A., Windhorst, R. A., Richards, E. A., Pascarelle, S., Impey, C., & Petry, C. (1999). A cluster or filament of galaxies at redshift z = 2.5?. Astrophysical Journal Letters, 511(1 PART 2), L1-L4.
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  Abstract: We report the discovery of 56 new Lyα-emitting candidates (LECs) at redshift z ≈ 2.5 in a field of 8′ × 14′ around two previously known weak radio QSOs and a cosmic microwave background decrement (CMBD) that is plausibly due to the Sunyaev-Zeldovich effect. Broadband and medium-band imaging at the redshifted Lyα wavelength have allowed us to identify the LECs at the redshift of the QSOs. Three of the brightest LECs have been confirmed spectroscopically, with redshifts between z = 2.501 and z = 2.557; one of them is another QSO. Excluding the third QSO, the four spectroscopically confirmed objects form a 3′ filament with a rest-frame velocity dispersion of 1000 km s-1 lying adjacent to the CMBD, and there is a significant concentration of LECs at the northwest end of the filament around the brightest QSO. If confirmed, a velocity dispersion ~1000 km s-1 on a proper scale of ~1 Mpc at redshift z = 2.5 would, in and of itself, constrain the cosmological model to low Ω.
• Falco, E. E., Impey, C. D., Kochanek, C. S., Lehár, J., McLeod, B. A., Rix, H. -., Keeton, C. R., Muñoz, J., & Peng, C. Y. (1999). Dust and extinction curves in galaxies with z > 0: The interstellar medium of gravitational lens galaxies. Astrophysical Journal Letters, 523(2 PART 1), 617-632.
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  Abstract: We determine 37 differential extinctions in 23 gravitational lens galaxies over the range 0 ≲ zl ≲ 1. Only seven of the 23 systems have spectral differences consistent with no differential extinction. The median differential extinction for the optically selected (radio-selected) subsample is ΔE(B-V) = 0.04 (0.06) mag. The extinction is patchy and shows no correlation with impact parameter. The median total extinction of the bluest images is E(B-V) = 0.08 mag, although the total extinction distribution is dominated by the uncertainties in the intrinsic colors of quasars. The directly measured extinction distributions are consistent with the mean extinction estimated by comparing the statistics of quasar and radio lens surveys, thereby confirming the need for extinction corrections when using the statistics of lensed quasars to estimate the cosmological model. A disjoint subsample of two face-on, radio-selected spiral lenses shows both high differential and total extinctions, but standard dust-to-gas ratios combined with the observed molecular gas column densities overpredict the amount of extinction by factors of 2-5. For several systems we can estimate the extinction law, ranging from Rv = 1.5 ± 0.2 for a zl = 0.96 elliptical, to Rv = 7.2 ± 0.1 for a zl = 0.68 spiral. For the four radio lenses where we can construct nonparametric extinction curves, we find no evidence for gray dust over the IR-UV wavelength range. The dust can be used to estimate lens redshifts with reasonable accuracy, although we sometimes find two degenerate redshift solutions.
• Galama, T. J., Briggs, M. S., Wijers, R. A., Vreeswijk, P. M., Rol, E., Band, D., Paradijs, J. V., Kouveliotou, C., Preece, R. D., Bremer, M., Smith, I. A., Tilanus, R. P., Bruyn, A. D., Strom, R. G., Pooley, G., Castro-Tirado, A., Tanvir, N., Robinson, C., Hurley, K., , Heise, J., et al. (1999). The effect of magnetic fields on γ-ray bursts inferred from multi- wavelength observations of the burst of 23 January 1999. Nature, 398(6726), 394-399.
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  Abstract: Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature of which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of γ-ray, optical, infrared, submillimetre, millimetre and radio observations of the burst GRB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.
• Impey, C. D., Petry, C. E., & Flint, K. P. (1999). A study of Lyα quasar absorbers in the nearby universe. Astrophysical Journal Letters, 524(2 PART 1), 536-565.
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  Abstract: Spectroscopy of 10 quasars obtained with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) is presented. In the 10 sight lines, we detect 357 absorption lines above a significance level of 3 σ and 272 lines above a significance level of 4.5 σ. Automated software is used to detect and identify the lines, almost all of which are unresolved at the GHRS G140L resolution of 200 km s-1. After identifying Galactic lines, intervening metal lines, and higher order Lyman lines, we are left with 139 Lyα absorbers in the redshift range 0 < z < 0.22 (lines within 900 km s-1 of geocoronal Lyα are not selected). These diffuse hydrogen absorbers have column densities that are mostly in the range 1013-1015 cm-2 for an assumed Doppler parameter of 30 km s-1. The number density of lines above a rest equivalent width of 0.24 Å, dN/dz = 38.3 ± 5.3, agrees well with the measurement from the Quasar Absorption-Line Key Project. There is marginal evidence for cosmic variance in the number of absorbers detected among the 10 sight lines. A clustering analysis reveals an excess of nearest-neighbor line pairs on velocity scales of 250-750 km s-1 at a 95%-98% confidence level. The hypothesis that the absorbers are randomly distributed in velocity space can be ruled out at the 99.8% confidence level. No two-point correlation power is detected (ξ < 1 with 95% confidence). Lyα absorbers have correlation amplitudes on scales of 250-500 km s-1, at least 4-5 times smaller than the correlation amplitude of bright galaxies. A detailed comparison between absorbers in nearby galaxies is carried out on a limited subset of 11 Lyα absorbers where the galaxy sample in a large contiguous volume is complete to MB = -16. Absorbers lie preferentially in regions of intermediate galaxy density, but it is often not possible to assign a unique galaxy counterpart to an absorber. This sample provides no explicit support for the hypothesis that absorbers are preferentially associated with the halos of luminous galaxies. We have made a preliminary comparison of the absorption-line properties and environments with the results of hydrodynamic simulations. The results suggest that the Lyα absorbers represent diffuse or shocked gas in the intergalactic medium that traces the cosmic web of large-scale structure.
• Lui, C. T., Petry, C. E., Impey, C. D., & Foltz, C. B. (1999). Quasars as absorption probes of the hubble deep field 1. Astronomical Journal, 118(5), 1912-1921.
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  Abstract: We present a catalog of 30 quasars (QSOs) and their spectra, in the square degree of sky centered on the northern Hubble Deep Field. These QSOs were selected by multicolor photometry and subsequently confirmed with spectroscopy. They range in magnitude from 17.6 < B < 21.0 and in redshift from 0.44 < z < 2.98. We also include in the catalog an active galactic nucleus with redshift z = 0.135. Together, these objects comprise a new grid of absorption probes that can be used to study the correlation between luminous galaxies, nonluminous halos, and Lyα absorbers along the line of sight toward the Hubble Deep Field.
• Norman, D. J., & Impey, C. D. (1999). Quasar-galaxy correlations: A search for amplification bias. Astronomical Journal, 118(2), 613-624.
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  Abstract: Weak gravitational lensing by clusters and superclusters can change our perception of the distribution of bright background quasars through a phenomenon known as amplification bias. It has been suggested that this lensing will lead to an association of moderate redshift quasars (z ∼ 1.0-2.0) and foreground galaxies (z ∼ 0.2-0.3). We report on the analysis of a homogeneous sample of 32 CCD fields centered on quasars that are bright both at radio (S5 GHz > 1 Jy) and optical (V < 18.5) wavelengths, along with 11 control fields. These 69′ × 69′ fields are a small subset of our larger survey of the fields around approximately 100 quasars and approximately 20 control fields. The quasars presented here are chosen to have the highest potential for evidence of this weak lensing effect. We find no evidence for the presence of rich galaxy clusters (Abell richness class = 1) in the foreground of these fields. However, using a Spearman rank-order correlation test, we have detected a correlation between galaxy number density and angular distance from bright quasars on scales less than 30′ with a significance of 99%. This angular correlation is consistent with the recent semianalytic models of Dolag & Bartelmann that incorporate nonlinear evolution of matter.
• O'Neil, K., Bothun, G. D., & Impey, C. D. (1999). Hubble Space Telescope WFPC2 imaging of three low surface brightness dwarf elliptical galaxies in the Virgo Cluster. Astronomical Journal, 118(4), 1618-1634.
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  Abstract: Utilizing the F814W and F300W filters, Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images were taken of three low surface brightness dwarf elliptical galaxies in the Virgo Cluster. The intent of the observations was to determine the small-scale structure in these enigmatic galaxies and to attempt to learn something about the nature of their giant branches through the detection of lumi-nosity fluctuations. In two of the three studied galaxies, V7L3 and V1L4, the luminosity fluctuations in the inner, constant surface brightness regions were unambiguously detected. At the nominal distance of the Virgo Cluster, the measured luminosity fluctuations in the F814W band yields a density of 2-10 red giants per pixel. In the most extreme of these two cases, V7L3, we derive a surface density of giant stars of ∼3 per 10 pc2. Using the observed B-V and V-I colors as a constraint, we could find no model that would reproduce the observed fluctuation signal and blue colors if there were a significant population of M giants in these systems. Overall, our results are consistent with a mean spectral type of K0-K2, which implies a relatively metal-poor population. The third system, V2L8, did not have a detectable fluctuation signal, which possibly implies that it is not in the Virgo Cluster. Interestingly, this system is highly nucleated. Our observations have resolved this nucleus, and if V2L8 is in Virgo, then we have discovered what is likely the smallest bulge measured to date, having an effective radius of only 50 pc. This bulge is quite red (as red as giant ellipticals), and it is entirely possible that this nucleated dwarf elliptical galaxy, in fact, is a very large galaxy located in the background. As such, it is highly reminiscent of the manner in which Malin 1 was discovered. Optical spectroscopy of this nucleus is required to confirm this. Finally, we find no evidence for small-scale clumping of stars in any of the studied systems at this much-improved spatial resolution. This implies that these systems are dynamically well relaxed and that the physical cause of their observed low surface brightnesses is their low density. When imaged at the high spatial resolution of the WFPC2 (∼6 pc pixel-1), the galaxies are easy to look right through without even knowing that they are present in the very middle of the WFPC2 frame. They appear only as elevated " sky noise. ".
• Peng, C. Y., Impey, C. D., Falco, E. E., Kochanek, C. S., Lehár, J., Mcleod, B. A., Rix, H. -., Keeton, C. R., & Muñoz, J. (1999). The quasar pair Q1634+267A,B and the binary QSO versus dark lens hypotheses. Astrophysical Journal Letters, 524(2 PART 1), 572-581.
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  Abstract: Deep HST/NICMOS H-band (F160W) observations of the z = 1.96 quasar pair Q1634+267A and B reveal no signs of a lens galaxy to a 1 σ threshold of ≃22.5 mag. The minimum luminosity for a normal lens galaxy would be a 6L* galaxy at z ≃ 0.5, which is 650 times greater than our detection threshold. Our observation constrains the infrared mass-to-light ratio (M/L) of any putative, early-type, lens galaxy to (M/L)H ≳ 690 h65 (1200 h65) for Ω0 = 0.1 (1.0) and H0 = 65 h65 km s-1 Mpc-1. We would expect to detect a galaxy somewhere in the field because of the very strong Mg II absorption lines at z = 1.1262 in the Q1634+267A spectrum, but the HST H-band, I-band (F785LP), and V-band (F555W) images require that any associated galaxy be very underluminous ≲0.1L*H (1.0L*I) if it lies within ≲40 h-165 (100 h-165) kpc from Q1634+267A and B. While the large image separation (3″.85) and the lack of a lens galaxy strongly favor interpreting Q1634+267A and B as a binary quasar system, the spectral similarity remains a puzzle. We estimate that, at most, 0.06% of randomly selected quasar pairs would have spectra as similar to each other as the spectra of Q1634+267A and B. Moreover, spectral similarities observed for the 14 known quasar pairs are significantly greater than would be expected for an equivalent sample of randomly selected field quasars. Depending on how strictly we define similarity, we estimate that only 0.01%-3% of randomly drawn samples of 14 quasar pairs would have as many similar pairs as the observational sample.
• Pickering, T. E., Gorkom, J. V., Impey, C. D., & Quillen, A. C. (1999). Kinematics and neutral hydrogen properties of the giant low surface brightness galaxy UGC 2936. Astronomical Journal, 118(2), 765-776.
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  Abstract: We present high-sensitivity, high-velocity resolution VLA H I observations of the giant low surface brightness (LSB) galaxy, UGC 2936. Like the giant LSBs presented in Pickering et al., UGC 2936 is a large and massive galaxy. Its H I mass is nearly 1010 M⊙ h-275, it has detectable H I extending beyond 30 kpc h-175, and it is a fast rotator (Vmax ≃ 250 km s-1) with a slowly rising rotation curve. This galaxy also exhibits warping in the outermost isophotes of the optical images that appears to be visible in the H I distribution and kinematics as well. This galaxy's high inclination and relatively large amount of Hα emission provides a unique opportunity to compare high-quality H I and optical rotation curves in the same LSB galaxy. The optical and H I data show good agreement as long as the effects of beam smearing on the H I rotation curve are taken into account. A large part of the disk of UGC 2936 lies above the critical density for star formation as described by Kennicutt. This is consistent with the relatively large amount of star formation occurring within the disk of this galaxy and perhaps brings into question whether this galaxy should be considered a true LSB galaxy.
• Dinshaw, N., Foltz, C. B., Impey, C. D., & Weymann, R. J. (1998). Ultraviolet spectroscopy of the quasar pair LB 9605, LB 9612 with the Hubble Space Telescope: Evolution in the size of the Lyα absorbers?. Astrophysical Journal Letters, 494(2 PART I), 567-580.
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  Abstract: Ultraviolet spectroscopy has been obtained with the faint object spectrograph of the Hubble Space Telescope in the Lyα forest of the quasar pair, LB 9605 (zem = 1.834) and LB 9612 (zem = 1.898), in order to measure the size of the Lyα absorbers. The quasars are separated by 1′.65 on the sky corresponding to a proper separation of 412 h-1 kpc at z = 1.83 (where h ≡ H0/100 km s-1 Mpc-1; q0 = 0.5). We detect five Lyα absorption lines common to both spectra within a velocity difference of 400 km s-1 in the redshift range 1.06 < z < 1.69, and 20 lines which are seen in the spectrum of one quasar but not the other. The number of coincidences expected for randomly distributed absorbers in this redshift interval is 3.2 ± 1.8, implying a less than 2 σ significance for the observed coincidences. If none of the observed coincidences are real, then we can place an upper limit on the absorber radius with 95% confidence of ∼285 h-1 kpc for redshifts 1.06 < z < 1.69. If, on the other hand, all of the observed coincidences are real, then a maximum likelihood estimate of the characteristic absorber radius in the context of identical, spherical clouds give a most probable radius of 380 h-1 kpc with 95% confidence that the characteristic radius lies in the range 305 < R < 595 h-1 kpc. Taken together with a low redshift estimate and new ground-based estimates, the upper limit on the absorber size provides tantalizing evidence for evolution in the radius of the Lyα absorbers with cosmic time, in the sense that the characteristic size of the Lyα absorbers increases with decreasing redshift. © 1998. The American Astronomical Society. All rights reserved.
• Impey, C. D., Falco, E. E., Kochanek, C. S., Lehár, J., McLeod, B. A., Rix, H. -., Peng, C. Y., & Keeton, C. R. (1998). An infrared Einstein ring in the gravitational lens PG 1115+080. Astrophysical Journal Letters, 509(2 PART I), 551-560.
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  Abstract: Hubble Space Telescope observations of the gravitational lens PG 1115+080 in the infrared show the known zt = 0.310 lens galaxy and reveal the zs = 1.722 quasar host galaxy. The main lens galaxy G is a nearly circular (ellipticity ε < 0.07) elliptical galaxy with a de Vaucouleurs profile and an effective radius of Re = 0″.59 ± 0″.06 (1.7 ± 0.2 h-1 kpc for Ω0 = 1 and h = H0/100 km s-1 Mpc-1). G is part of a group of galaxies that is a required component of all successful lens models. The new quasar and lens positions (3 mas uncertainty) yield constraints for these models that are statistically degenerate, but several conclusions are firmly established. (1) The principal lens galaxy is an elliptical galaxy with normal structural properties, lying close to the fundamental plane for its redshift. (2) The potential of the main lens galaxy is nearly round, even when not constrained by the small ellipticity of the light of this galaxy. (3) All models involving two mass distributions place the group component near the luminosity-weighted centroid of the brightest nearby group members. (4) All models predict a time delay ratio rABC ≃ 1.3. (5) Our lens models predict H0 = 44 ± 4 km s-1 Mpc-1 if the lens galaxy contains dark matter and has a flat rotation curve and H0 = 65 ± 5 km s-1 Mpc-1 if it has a constant mass-to-light ratio. (6) Any dark halo of the main lens galaxy must be truncated near 1″.5 (4 h-1 kpc) before the inferred H0 rises above ∼60 km s-1 Mpc-1. (7) The quasar host galaxy is lensed into an Einstein ring connecting the four quasar images, whose shape is reproduced by the models. Improved near-infrared camera multiobject spectrograph (NICMOS) imaging of the ring could be used to break the degeneracy of the lens models. © 1993 The American Astronomical Society. All rights reserved.
• Muñoz, J., Falco, E. E., Kochanek, C. S., Lehár, J., Mcleod, B. A., Impey, C. D., Rix, H. -., & Peng, C. Y. (1998). The castles project. Astrophysics and Space Science, 263(1-4), 51-54.
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  Abstract: We describe the goals of the CASTLES (CfA-Arizona-Space-Telescope-LEns-Survey) 1 project including a sample of NICMOS images of gravitational lenses and a brief list of the preliminary findings.
• O'Neil, K., Bothun, G. D., Impey, C. D., & McGaugh, S. (1998). Hubble Space Telescope Wide Field Planetary Camera 2 imaging of UGC 12695: A remarkably unevolved galaxy at low redshift. Astronomical Journal, 116(2), 657-672.
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  Abstract: Utilizing the F814W and F300W filters, short-exposure Hubble Space Telescope Wide Field Planetary Camera 2 images were taken of UGC 12695, a nearby (z ∼ 0.021) low surface brightness disk galaxy. UGC 12695 has an unusual morphology, consisting of a Y-shaped nucleus surrounded by a faint spiral arm with a number of bright H II regions interspersed throughout the galaxy. Surface photometry indicates that the majority of recent star formation in this galaxy occurred in these very localized regions, most of which have a radius of ≲2″. This uneven stellar distribution, combined with the galaxy's overall extremely blue color and low metallicity, indicates that UGC 12695 is an unevolved galaxy. Some of the structural peculiarities of this galaxy arise because a number of background galaxies, previously thought to be morphological components of this galaxy, are showing through both the outer nucleus and spiral arms of UGC 12695. Surface photometry of these galaxies shows them to be fairly small (α < 1″.8) disk galaxies with total magnitudes ranging from 19.6 through 24.2 mag and central surface brightnesses in the range 20.2 mag arcsec-2 ≤ μ(0) ≤ 23.1 mag arcsec-2. When possible, the U, B, V, and I colors of these galaxies were measured using ground-based images, which show the galaxies to be fairly red, indicating that they are likely at redshifts z ≥ 0.5. Inclusion of them in the photometry of UGC 12695 makes the galaxy appear significantly redder. When these galaxies are masked out, the resultant U-I color of UGC 12695 is -0.2 ± 0.1, making it perhaps the bluest galaxy ever measured in this color system and confirming its nature as a very unevolved galaxy at low redshift. Spectroscopy of these background galaxies, through the transparent disk of UGC 12695, may help to clarify its chemical evolution and current heavy-element content.
• Petry, C. E., Impey, C. D., & Foltz, C. B. (1998). Small-scale structure in the Lyα forest at high redshift. Astrophysical Journal Letters, 494(1 PART I), 60-89.
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  Abstract: We have obtained spectra of the four components of the system B1422+231 with HST's Faint Object Spectrograph in order to study spatial inhomogeneities and velocity variations in the absorbers which are pierced by the four lines of sight. The four images are well resolved, and the spectra clearly show the system is a lensed quasar; a new redshift based on the Lyα and N v emission lines is measured to be 3.6261. The following are results from the analysis of the four individual lines of sight that probe redshifts 2.9 ≤ z ≤ 3.5 and transverse dimensions 0-0.14 h100-1 kpc: (1) In the region between Lyβ and Lyα (less 6000 km s-1 to allow for the proximity effect: 4745-5511 Å), the limiting rest equivalent width (1σlim) is measured to be 0.059, 0.052, 0.034, and 0.22 Å for components A, B, C, and D, respectively. (2) The total number of 5σW lines found in this region are 83, 85, 102, and 44 for A-D, respectively; for A-C 11, 9, and 13 are identified with metal line systems and the remaining 72, 76, and 89 are assumed to be Lyα lines. Between paired lines of sight A and B to C, and A paired to B, there are 62, 68, and 62 coincidences and 3, 1, and 1, anticoincidences for the Lyα lines. For the metal lines there are 8, 9, and 8 coincidences for AC, BC, and AB, respectively, and each has one anticoincidence. (3) Scatter plots of the equivalent widths for the coincident lines for both Lyα and metals are highly correlated and all pairs match within 100 and 75 km s-1, respectively. (4) Maximum-likelihood analysis limits spatial inhomogeneity for Lyα absorbers in components A, B, and C to no more than ∼15% of the equivalent width of an average (W = 2 Å) line. This corresponds to a limit on the mean variation in column density, assuming unsaturated lines, of log (ΔNH) = 13.1 cm-2. (5) A method of detecting spatial inhomogeneities independent of continuum-fitting and line-profile measurement is developed yielding an upper limit on the column density difference for unsaturated lines of log (δNH) = 12.8. This is approximately a factor of 2 smaller than the limits obtained from the line pairs. (6) The distribution of the velocity differences for AC, BC, and AB for the Lyα lines and the metal line pairs have a mean close to zero and rms ∼40 km s-1. Maximum-likelihood analysis gives an upper limit on the variations in velocity of ∼40 km s-1 for Lyα absorbers in paired lines of sight. The overall result of this work is a set of stringent limits on the inhomogeneity of low column density hydrogen absorbers on subkiloparsec scales at high redshift. © 1998. The American Astronomical Society. All rights reserved.
• Bothun, G., Impey, C., & McGaugh, S. (1997). Low-surface-brightness galaxies: Hidden galaxies revealed. Publications of the Astronomical Society of the Pacific, 109(737), 745-758.
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  Abstract: In 20 years, low-surface-brightness (LSB) galaxies have evolved from being an idiosyncratic notion to being one of the major baryonic repositories in the Universe. The story of their discovery and the characterization of their properties is told here. Their recovery from the noise of the night-sky background is a strong testament to the severity of surface-brightness selection effects. LSB galaxies have a number of remarkable properties which distinguish them from the more familiar Hubble sequence of spirals. The two most important are (1) they evolve at a significantly slower rate and may well experience star formation outside of the molecular-cloud environment, (2) they are embedded in dark-matter halos which are of lower density and more extended than the halos around high-surface-brightness (HSB) disk galaxies. Compared to HSB disks, LSB disks are strongly dark-matter dominated at all radii and show a systematic increase in M/L with decreasing central surface brightness. In addition, the recognition that large numbers of LSB galaxies actually exist has changed the form of the galaxy luminosity function and has clearly increased the space density of galaxies at z = 0. Recent CCD surveys have uncovered a population of red LSB disks that may be related to the excess of faint blue galaxies detected at moderate redshifts. LSB galaxies offer us a new window into galaxy evolution and formation which is every bit as important as those processes which have produced easy-to-detect galaxies. Indeed, the apparent youth of some LSB galaxies suggest that galaxy formation is a greatly extended process. While the discovery of LSB galaxies has led to new insights, it remains unwise to presume that we now have a representative sample which encompasses all galaxy types and forms.
• Dinshaw, N., Weymann, R. J., Impey, C. D., Foltz, C. B., Morris, S. L., & Ake, T. (1997). Additional observations and analysis of the Lyman-α absorption lines toward the QSO Pair Q0107-025A 1,2. Astrophysical Journal Letters, 491(1 PART I), 45-68.
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  Abstract: We present further analysis and discussion of the properties of the absorption lines in the QSO pair Q0107-025A,B (zem = 0.956, 0.952; angular separation 1′.129) based upon spectroscopy obtained with the Faint Object Spectrograph (FOS) of the Hubble Space Telescope (HST). We also present observations of the pair at shorter wavelengths taken with the HST Goddard High Resolution Spectrograph lowresolution grating, as well as Multiple Mirror Telescope spectra obtained with the intent of looking for metal-line counterparts to the Lyman-α absorption systems. The most interesting feature revealed by the GHRS spectra is a weak Lyman limit system with a redshift of ZLLS = 0.3997. The Lyman limit system itself is seen only in Q0107-025B, but corresponding Lyα lines can be seen in the FOS spectra of both Q0107-025A and B indicating that lower column density gas extends out to greater distances than higher column density gas. From a sample of 5 σ lines with W0 > 0.32 Å detected in the FOS spectra, we count five systems (including a probable system) common to both spectra with velocity differences less than 150 km s-1, and six systems that are not in common to both spectra in the redshift range 0.48 < z < 0.89. From the presence of common absorption, we obtain lower limits on the radius of the absorbers of 140-160 h-1 kpc [h = H0/(100 km s-1 Mpc-1); q0 = 0.5]. Using a simple maximum-likelihood analysis, we estimate a characteristic radius of 505 h-1 kpc assuming spherical absorbers, with 95% confidence lower and upper limits of 345 < R < 1520 h-1 kpc. For disklike absorbers, we derive a most probable radius of 715 h-1 kpc and 95% confidence limits of 470 < R < 2310 h-1 kpc. We also present a new statistical technique to test the relative likelihood of three geometric models. In particular, we consider spherical absorbers, with and without a distribution in size, as well as filamentary and disklike absorbers. Spherical absorbers with uniform radius can be ruled out since they cannot simultaneously reproduce the large equivalent width coincidences and anticoincidences observed. The model that best reproduce the equivalent width distribution of the observed coincident and anticoincident systems corresponds to randomly inclined disks with characteristic radius 915 h-1 kpc and 95% confidence interval 560 < R < 1270 h-1 kpc. Our results are in agreement with recent cosmological simulations that produce Lyα forest absorbers in the form of filaments and sheets with coherence lengths as great as 1 Mpc. © 1997. The American Astronomical Society. All rights reserved.
• Hooper, E. J., Impey, C. D., & Foltz, C. B. (1997). Bubble space telescope imaging of z ≥ 0.4 quasar host galaxies selected by quasar radio and optical properties. Astrophysical Journal Letters, 480(2 PART II), L95-L98.
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  Abstract: A sample of 16 quasars selected from the Large Bright Quasar Survey in the redshift range 0.4 < 2 < 0.5 has been imaged in the R band with the Planetary Camera on the WFPC2 instrument of the Hubble Space Telescope. The host galaxy magnitudes are mostly similar to or brighter than L*, and the host luminosity is positively correlated with the luminosity of the quasar nuclear component. There is no distinction in host galaxy magnitude between radio-loud and radio-quiet quasars, assuming they are all of the same galaxy type. Many of the host galaxies in the sample have small axial ratios, which may indicate that they are inclined disk systems. Alternatively, this elongated appearance may be due to bars or other distinctive morphological features which are visible while the bulk of the underlying lower surface brightness components of the host galaxy are not. © 1997. The American Astronomical Society. All rights reserved.
• Impey, C., & Bothun, G. (1997). Low surface brightness galaxies. Annual Review of Astronomy and Astrophysics, 35(1), 267-307.
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  Abstract: The properties of galaxies that are lower in surface brightness than the dark night sky are reviewed. There are substantial selection effects against the discovery of galaxies that are unevolved or diffuse; these systems are missing from most wide field catalogs. Low surface brightness galaxies make up a significant amount of the luminosity density of the local universe. They contribute substantial but poorly determined amounts to the census of baryons and dark matter. Low surface brightness galaxies are also relevant to the interpretation of quasar absorption lines and to the understanding of rapidly evolving galaxy populations in the more distant universe. Theories of galaxy formation and evolution must accomodate the properties of these diffuse stellar systems.
• O'Neil, K., Bothun, G. D., Schombert, J., Cornell, M. E., & Impey, C. D. (1997). A wide field ccd survey for low surface brightness galaxies. II. Color distributions, stellar populations, and missing baryons. Astronomical Journal, 114(6), 2448-2462.
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  Abstract: We have performed a digital survey for low surface brightness (LSB) galaxies in the spiral-rich Cancer and Pegasus clusters as well as the low density regime defined by the Great Wall. A total of 127 galaxies of angular diameter larger than 15 arcseconds were found with μB(0)≥ 22.0 mag/arcsec2, 119 of which were previously unidentified. Structural parameters (μ B(0),α ,r25, etc) and colors (Johnson/Cousins U, B, V, I, & R, when possible) were determined for all galaxies. Paper I of this series described structural parameters; here we focus the discussion on the stellar populations of these newly discovered galaxies as implied by their observed broad-band colors. The colors of the survey galaxies range continuously from very blue (U-B=-0.56, B-V=0.31) to very red (U-B=0.65, V-I=2.2), and include a group of old galaxies which show evidence for recent star formation. This survey is also the first to discover a significant population of LSB galaxies which have red colors. Since galaxies must fade and redden, the absence of red LSB disks in previous surveys has been puzzling. Their recovery in this survey suggests that photographic selection from blue plates, which forms the basis of previous LSB surveys, carries with it strong selection effects. The continuous range of colors, from very blue to very red, that we observe for this sample clearly shows that LSB galaxies at the present epoch define a wide range of evolutionary states. Consistent with other surveys we find a significant number of galaxies with μB(0)≥ 23.0 mag/arcsec2 which suggests that the space density of galaxies as a function of μB(0) is not strongly peaked. To more rigorously test this hypothesis we compare the actual surface brightness distribution from our survey with that from two different types of Monte-Carlo based sky images, one with an underlying flat surface brightness distribution and one with an underlying Gaussian distribution (falling off at 24.0 mag arcsec-2). We show that there is no way of distinguishing between the flat distribution and the Gaussian one as the proper description of the underlying surface brightness distribution for this survey beyond 24.0 mag arcsec-2, in spite of the fact that our survey limit is actually 26.0 mag arcsec-2. This demonstrates that an even deeper and more difficult to obtain isophotal limit is required to more accurately determine the space density of very LSB galaxies as well as to define the faintest possible central surface brightnesses that galactic disks can have. © 1997 American Astronomical Society.
• Pickering, T. E., Impey, C. D., Gorkom, J. V., & Bothun, G. D. (1997). Neutral hydrogen distributions and kinematics of giant low surface brightness disk galaxies. Astronomical Journal, 114(5), 1858-1882.
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  Abstract: We present high sensitivity VLA H I observations of four giant low surface brightness (LSB) disk galaxies. As in the optical, the H I surface brightnesses of these galaxies are low, though their total H I masses are quite high (≃ 1010 M⊙h-275). The H I extents are similar to the optical disk sizes, in some cases as large as 130 kpc h-175. The rotation curves rise slowly and are consistent with negligible disk contribution, similar to many previously observed dwarf galaxies. However, the peak rotation velocities of these galaxies are high (> 200 km s-1) and infer high dynamical masses. These galaxies provide the first examples of galaxies that are both massive and dark matter dominated. Therefore, their rotation curves provide an important constraint on the structure of dark matter halos. Due to the small contribution of the baryonic component, interpreting the shapes of these curves is not strongly dependent on assumptions of stellar M/L. Also, the large masses of these LSB halos makes it difficult for large-scale baryonic blow-outs to affect the structure of the halos as can occur in dwarf galaxies. At large radii even fairly low surface densities of gas can rise above the critical density threshold described by Kennicutt (1989, ApJ, 344, 685). This is observed in two of these galaxies, however the regions with gas surface density above the threshold do not have attendant star formation. This implies that σgas may be greater than 10 km s-1 in these cases. In one of these systems, F568-6, we find gas moving at high speed with respect to the regularly rotating disk and coincident with a region of active star formation. Most likely these are the remains of a dwarf fallen through the disk and having set off star formation in the process. This provides a possible clue as to how stars ever formed in these low volume density systems. © 1997 American Astronomical Society.
• Sprayberry, D., Impey, C. D., Irwin, M. J., & Bothun, G. D. (1997). Low surface brightness galaxies in the local universe. III. Implications for the field galaxy luminosity function. Astrophysical Journal Letters, 482(1 PART I), 104-113.
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  Abstract: We present a luminosity function for low surface brightness (LSB) galaxies identified in the Automatic Plate Measuring Facility survey of Impey et al. These galaxies have central surface brightnesses [μ(0)] in B in the range 22.0 ≤ μ(0) ≤ 25.0. Using standard maximum likelihood estimators, we determine that the best-fit Schechter function parameters for this luminosity function (LF) are α = -1.42, M* = -18.34, and φ* = 0.0036, assuming H0 = 100 h100 km s-1 Mpc-1. We compare the luminosity and number densities derived from this luminosity function with those obtained from other recent field galaxy studies and find that surveys that do not take account of the observational selection bias imposed by surface brightness are missing a substantial fraction of the galaxies in the local universe. Under our most conservative estimates, our derivation of the LF for LSB galaxies suggests that the CfA Redshift Survey has missed at least one-third of the local galaxy population. This overlooked fraction is not enough by itself to explain the large number of faint blue galaxies observed at moderate redshift under no-evolution models, but it does help to close the gap between local and moderate-redshift galaxy counts. © 1997. The American Astronomical Society. All rights reserved.
• Dinshaw, N., & Impey, C. D. (1996). Two-point velocity and spatial correlation functions of the C IV absorption systems toward the tololo quasar group: Evidence for superclustering at z ≃ 2¹. Astrophysical Journal Letters, 458(1 PART I), 73-99.
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  Abstract: The large number of apparently correlated C IV absorption systems observed in the lines of sight to the wide QSO pair Tol 1037-2704 (zcm = 2.193) and Tol 1038-2712 (zcm = 2.331) between the redshifts 1.88 ≤ z ≤ 2.15 is thought to be produced by an intervening supercluster. We present high-resolution echelle spectra of Tol 1037-2704 and Toi 1038-2712, as well as two neighboring QSOs with the aim of searching for absorption associated with the putative supercluster. The lines of sight toward the original pair are separated by 17′.9 corresponding to a proper separation of D⊥ ≃ 4.4 h-1 Mpc (h = Ho/100 km s-1 Mpc-1; qo = 0.5) at z = 2. We confirm the existence of the C IV absorption complexes already reported, and find additional complexes. At a resolution of ∼30 km s-1 FWHM, many of the C IV complexes break up into multiple discrete components with a velocity spread of Δv ≃ 50-1000 km s-1. This brings the total number of C IV absorption systems with secure identifications to 22 in Tol 1037 - 2704 and 11 in Toi 1038 - 2712 in the redshift range 1.48 ≤ z ≤ 2.15, well above the expectation from Poisson statistics. The two neighboring QSOs, Tol 1035-2737 (zcm = 2.159) and Tol 1029-2654 (zcm = 2.586), are located roughly 40′ (D⊥. ≃ 10 h-1 Mpc) southwest and 115′ (D⊥ ≃ 30 h-1 Mpc) northwest of the Tololo pair, respectively. We found five C IV systems in the line of sight to Tol 1035-2737, and six toward Toi 1029-2654. Most of these systems appear to match systems in the original QSO pair within a velocity separation of less than 5000 km s-1, though the significance of the matches is not strong. We find a marginal excess of C IV absorption systems in the line of sight to Tol 1029-2654, whereas the number of systems in the line of sight to Tol 1035 -2737 agrees with expectation. The inferred proper dimensions of the proposed supercluster are at least 30 h-1 Mpc on the plane of the sky and approximately 80 h-1 Mpc along the line of sight. We examined the clustering properties of the absorbers toward the Tololo QSOs using the two-point velocity and spatial correlation functions. The velocity correlation function of the complete sample of 44 C IV systems shows strong clustering for velocity separations less than 1000 km s-1, which probe clouds in galactic halos as well as individual galaxies in clusters, and significant clustering signal out to scales of 7000 km s-1. In a subsample of 16 systems (Wo ≥ 0.15 Å), where the power associated with virialized clusters on scales less than 1000 km s-1 has been removed, significant correlation signal persists for velocity separations of 4000-7000 km s-1, which correspond to comoving spatial scales of ∼30-40 h-1 Mpc. The spatial correlation function of the same subsample, derived by pairing absorbers between different lines of sight, shows a marginally significant peak on comoving scales of less than 18 Mpc. The clustering amplitude on these scales is larger than predicted by current theories of the formation of large-scale structure. © 1996. The American Astronomical Society. All rights reserved.
• Flint, K. P., & Impey, C. D. (1996). Large-scale structure in the direction of the virgo cluster. Astronomical Journal, 112(3), 865-870.
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  Abstract: We explore the distribution of galaxies behind the Virgo cluster out to z ≤ 0.1, using the CfA redshift catalog, ZCAT, in conjunction with Abell clusters and pencil beam surveys as other tracers of large-scale structure. The field galaxies of ZCAT are found to trace the same overdensity as indicated by a supercluster defined by Abell clusters in the catalog of Einasto et al. [MNRAS, 269, 351 (1994)]. On even larger scales, galaxy peaks in three different pencil beam lines of sight toward the Virgo cluster are all found to fall at z ≈ 0.08. We show this similarity in redshift to have a small probability of being caused by a random alignment, suggesting a sheet-like structure at that redshift, spanning ≳ 150h-175 Mpc. © 1996 American Astronomical Society.
• Hooper, E. J., Impey, C. D., Foltz, C. B., & Hewett, P. C. (1996). The radio properties of optically selected quasars. III. Comparison between optical and x-ray selected samples. Astrophysical Journal Letters, 473(2 PART I), 746-759.
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  Abstract: A sample of 103 quasars from the Large Bright Quasar Survey (LBQS) has been observed with the VLA at 8.4 GHz to study the evolution of the radio luminosity distribution and its dependence on absolute magnitude. Radio data from pointed observations are now available for 359 of the 1055 LBQS quasars. The radio-loud fraction is constant at ≈10% over the absolute magnitude range -28 ≲ MB ≲ -23, and it rises to ∼20% (log R8.4 > 1) or ∼35% (log L8.4 > 25) at the brightest absolute magnitudes in the sample. This nearly flat distribution differs markedly from those of the optically selected Palomar-Green (PG) Bright Quasar Survey and the X-ray selected Extended Medium Sensitivity Survey (EMSS), both of which have lower radio-loud fractions for absolute magnitudes fainter than MB = -24 and higher fractions at brighter magnitudes. The reason for the high radio-loud fraction at bright absolute magnitudes in the PG, compared to the LBQS and other optically selected quasar surveys, is unknown. The trend of increasing radio-loud fraction with absolute magnitude in the EMSS is due at least in part to a correlation between X-ray and radio luminosity. Combining the LBQS data with radio studies of high-redshift quasars leads to the conclusion that the radio-loud fraction in optically selected quasars does not appear to evolve significantly, aside from a modest increase at z ∼ 1, from z = 0.2 to redshifts approaching 5, a result contrary to previous studies that found a decrease in radio-loud fraction with increasing redshift by comparing the low-z fraction in the PG to higher redshift samples. © 1996. The American Astronomical Society. All rights reserved.
• Impey, C. (1996). Quasars, blazars, and the gamma-ray sky. Astronomical Journal, 112(6), 2667-2683.
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  Abstract: The statistical properties of gamma-ray emitting AGN are discussed, based on radio sources stronger than 1 Jy at 5 GHz that have been detected by the EGRET experiment on the Compton Gamma Ray Observatory. Most strong gamma-ray sources are radio quasars; radio galaxies are a small component of the population. Among the 1 Jy quasars, gamma-ray detections have stronger than average radio flux, but are not unusually compact or flat spectrum. The redshift distributions of gamma-ray detections and nondetections are indistinguishable. The energy budget of radio quasars detected by EGRET is dominated by gamma-rays. A model is constructed where radio and gamma-ray flux densities are weakly correlated, as observed, but with a large scatter much of which may be due to variability. Using this model to fit the EGRET detection rate in the 1 Jy sample, it is predicted that the gamma-ray detections arise from the high Sγ/Sr tail of the radio source population, and that many radio sources must emit gamma-ray fluxes not far below the EGRET sensitivity limit. Radio quasars contribute ∼50% of the diffuse extragalactic background; optically selected QSOs must contribute 5% or less. Most of the flux and variance due to unresolved point sources is contributed by radio sources in the range 0.05rad
• Impey, C. D., Foltz, C. B., Petry, C. E., Browne, I. W., & Patnaik, A. R. (1996). Bubble space telescope observations of the gravitational lens system B1422+231. Astrophysical Journal Letters, 462(2 PART II), L53-L57.
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  Abstract: Hubble Space Telescope observations of the gravitational lens system B1422+231 are presented. The spectra of the four components are virtually identical, although there are differences among the Lyα emission-line equivalent widths that might be due to microlensing. There are also significant differences between the component flux density ratios measured at radio and optical wavelengths. High-resolution imaging reveals the lens close to the faintest quasar image. The lens has photometric properties consistent with an elliptical galaxy at z ∼ 0.4. The addition of new constraints to simple lensing models affirms the B1422+231 system as a suitable tool for an eventual direct measure of the Hubble constant. © 1996. The American Astronomical Society. All rights reserved.
• Impey, C. D., Petry, C. E., Malkan, M. A., & Webb, W. (1996). Spectroscopy of bright quasars with the Hubble Space Telescope and lyman-alpha absorption lines in the redshift range 0.5 < z < 1.7. Astrophysical Journal Letters, 463(2 PART I), 473-488.
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  Abstract: We report ultraviolet spectroscopy of three bright quasars obtained with the Faint Object Spectrograph of the Hubble Space Telescope. The good quality spectra covering the range 1800-3300 A result from spectropolarimetry acquired for these targets, the interpretation of which has been published elsewhere. Objective algorithms were used to select absorption lines whose strength exceeded 4 times the rms noise in the nearby continuum, resulting in 109 significant lines for PG 1222 + 228, 91 significant lines for PG 1634 + 706, and 19 significant lines for PG 2302 + 029. Most of the spectral range covers the region with a high density of lines due to intervening absorbers, blueward of the Lyman-α emission line. In PG 1222 + 228, we identify about 35% of the lines as being associated with the seven metal line systems already known in this quasar. Three have seven or more metal lines identified. An additional 12% are either galactic lines or Lyman-α, Lyman-β pairs with no associated metals. In PG 1634 + 706, nearly 42% of the absorption lines are identified with metal systems. Some are associated with the two metal line systems previously known in this quasar, others are associated with two newly identified C IV systems at z = 0.6540 and z = 0.9057. Another 19% are galactic lines or Lyman-α, Lyman-β pairs with no associated metals. Six galactic lines are identified in the spectrum of PG 2302 + 029; no lines due to intervening absorbers could be identified. The data for PG 1222 + 228 and PG 1634 + 706 can be used to estimate the number density of Lyman-α absorbers in the redshift range 0.5 < z < 1.7. Above an effective rest equivalent width of 0.4 Å there are 25 Lyman-α lines in PG 1222 + 228 in the wavelength range 2300-3300 Å, and 11 Lyman-α lines in PG 1634 + 706 in the wavelength range 1865-2650 Å. We have been able to demonstrate that the identification procedure and the method of fitting lines in blended regions is unlikely to contribute systematic errors beyond the Poisson error to these numbers. The deduced number density is consistent with the number density of Lyman-α absorbers at zero redshift, using published data from the HST Quasar Absorption Line Key Project, indicating little or no evolution over 55% (q0 = 0) to 70% (q0 = 0.5) of the age of the universe. It is also consistent with an extrapolation to lower redshift of the rapid increase in number density that is seen before z ≈ 2, implying an inflection at z = 1-1.5. The rapid decline in dN/dz might be caused by an ionization effect caused by the changing comoving space density of quasars, because the decline follows a similar form to the decline in the integrated UV intensity of quasars at the hydrogen Lyman edge. The evolution in dN/dz over the range 1 < z < 3 also mirrors the change in the star formation rate in disk galaxies in the universe. It is not clear whether transition in Lyman-α evolution at z = 1-1.5 is due to a change in ionization state, or whether it marks a fundamental change in the intervening absorber population. © 1996. The American Astronomical Society. All rights reserved.
• Marchã, M., Browne, I. W., Impey, C. D., & Smith, P. S. (1996). Optical spectroscopy and polarization of a new sample of optically bright flat radio spectrum sources. Monthly Notices of the Royal Astronomical Society, 281(2), 425-448.
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  Abstract: A new sample of bright flat radio spectrum sources selected at 8.4 GHz and consisting of objects brighter than V= 17 is discussed. The sample was selected with three purposes in mind: (i) to find low-luminosity BL Lacertae (BL Lac) objects with radio luminosities comparable to those of BL Lacs selected at X-ray frequencies; (ii) to investigate the differences between BL Lacs and other flat radio spectrum sources; and (iii) to define a sample of nearby radio-loud objects, the host galaxies of which are easy to study. Using information on four observational parameters, radio polarization, optical percentage polarization, break contrast and equivalent width of the strongest emission line, we compare the properties of BL Lacs with those of other types of active galactic nuclei (AGN) found in the sample. We find that most of the objects have weak emission lines although some sources with Seyfert-type spectra were also found. With only a few exceptions, the two types of sources appear well separated in their observational properties. Among the objects studied we report 10 new BL Lacs and BL Lac candidates, and we define a 'complete' sample of bright flat radio spectrum sources that consists of those objects with redshift ≤ 0.1.
• Perlman, E. S., Stocke, J. T., Schachter, J. F., Elvis, M., Ellingson, E., Urry, C. M., Potter, M., Impey, C. D., & Kolchinsky, P. (1996). The Einstein Slew Survey sample of BL Lacertae objects. Astrophysical Journal, Supplement Series, 104(2), 251-285.
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  Abstract: We have used an efficient method of finding new BL Lacertae objects in the Einstein Slew Survey suggested by Stocke et al. By exploiting the unique broadband (radio/optical/X-ray) spectra of X-ray-selected BL Lac objects, we have identified 29 new BL Lac objects out of 32 candidates observed, effectively doubling the size of the Slew Survey BL Lac sample, which now includes 62 objects. Six more BL Lac candidates have yet to be observed, and seven blank field radio sources cannot be ruled out as optically faint BL Lac objects, although this is unlikely. The correct classification of one BL Lac candidate remains uncertain. We present finding charts optical spectroscopy, radio images, and optical polarimetry for these new BL Lac objects. Based upon nearly complete radio imaging and optical spectroscopy observations, we present the northern, high-latitude Slew Survey BL Lac sample, in the area bounded by δ ≥ -40° and \bII| ≥ 15° and excluding the region within 5° of M31. This sample contains 48 BL Lac objects, five BL Lac candidates, and five blank fields which may contain possible BL Lac candidates. This is considerably larger than any previous complete sample of BL Lac objects. We compare this sample with the X-ray-selected Extended Medium-Sensitivity Survey (EMSS) sample of BL Lac objects as well as the radioselected 1 Jy sample.
• Sprayberry, D., Impey, C. D., & Irwin, M. J. (1996). Low surface brightness galaxies in the local universe. II. Selection effects and completeness of the automated plate measuring 1 survey. Astrophysical Journal Letters, 463(2 PART I), 535-542.
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  Abstract: We discuss the techniques employed for identifying low surface brightness galaxies for the Automated Plate Measuring (APM) survey. We present the method of calibrating the photometry performed from the APM scans and discuss the uncertainties associated with the calibrations. We also review the possible sources of incompleteness in the survey, and we present the results of a program of simulations to estimate the completeness as a function of galaxy parameters. Finally, we discuss the implications of the survey results and show, in particular, that the observed distribution of central surface brightnesses among all galaxies in one survey field is almost flat for μB(0) ≥ 23 mag arcsec-2. © 1996. The American Astronomical Society. All rights reserved.
• Dinshaw, N., Foltz, C. B., Impey, C. D., Weymann, R. J., & Morris, S. L. (1995). Large size of Lyman-α gas clouds at intermediate redshifts. Nature, 373(6511), 223-225.
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  Abstract: THE Lyman-α absorption lines observed in the spectra of quasars are thought to be caused by intervening clouds of atomic hydrogen, the origin and physical nature of which are still unknown. These clouds might be structures confined by the pressure of the surround-ing intergalactic medium1, relicts of density fluctuations in the early Universe2, gravitationally bound clouds associated with condensations of cold dark matter3 or the result of shocks owing to galaxy formation4. Here we present ultraviolet spectra of the quasar pair Q0107-025A and B, in which we detect four Lyman-α lines common to both spectra in the redshift range 0.5 < z < 0.9. These common lines indicate that the characteristic radius of the clouds has a lower limit of 160h-1 kpc, where h is the Hubble constant in units of 100 km s-1 Mpc-1. The typical velocity difference between the common absorption lines along the two lines of sight is only about 100 km s-1. The clouds are larger in extent and have smaller internal motions than can be explained by any current model.
• Elowitz, R. M., Green, R. F., & Impey, C. D. (1995). Search for correlations of Lyman-alpha clouds and metal systems on closely spaced lines of sight. Astrophysical Journal Letters, 440(2), 458-465.
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  Abstract: Spectra of four quasars projected within 2°.8 on the sky with emission redshifts near 1.9 were analyzed for correlations of metal-line and Lyα absorbers. The group of bright quasars LB 9601, LB 9605, LB 9612, and LB 9615 probes scales on the sky of clusters and superclusters of galaxies. No significant correlation signal was detected in any of the three pairs of sight lines tested out to a velocity separation of 10,000 km s-1. One common pair of Lyα absorbers was newly detected, with velocities identical to within 10 km s-1. Four new heavy-element absorption systems are reported. Three C IV systems, one along each of three of the lines of sight, agree in velocity to within 4% of the accessible velocity range, which may indicate that the absorbers are associated. No evidence is found that intervening hydrogen clouds are correlated on the scale of superclusters of galaxies.
• Hooper, E. J., Impey, C. D., Foltz, C. B., & Hewett, P. C. (1995). Radio properties of optically selected quasars. Astrophysical Journal Letters, 445(1), 62-79.
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  Abstract: Approximately one-quarter (256 objects) of the Large Bright Quasar Survey (LBQS) has been observed with the VLA at 8.4 GHz, resulting in 44 detections (17%) with a median 3 σ noise limit of 0.29 mJy. Quasars with radio luminosity detectable at this limit are underrepresented at faint absolute blue magnitudes (MB ≥ -24), an effect which cannot be explained by a potential LBQS selection bias against quasars which have large radio luminosities and small optical luminosities. The radio-loud (8 GHz luminosity > 1025 W Hz-1) fraction is observed to change as a function of redshift and MB, for MB < -24, although the causal variable is ambiguous. The description most consistent with the available data is that radio-loud fraction is approximately constant over the range -27.5 ≲ MB ≲ -24 and increases at brighter absolute magnitudes. The radio-loud fraction as a function of redshift reaches a local maximum at z ≈ 1, and, aside from the effects of increased radio-loud fraction at bright MB, remains roughly constant to redshifts approaching 5. The log R8.4 distribution (radio-to-optical luminosity ratio) of the current LBQS sample may be bimodal, but the results of statistical tests are ambiguous, requiring a larger sample size to become definite.
• Impey, C. D. (1995). The search for life in the Universe: A humanistic perspective. Vistas in Astronomy, 39(4), 553-571.
• Impey, C. D., Malkan, M. A., Webb, W., & Petry, C. E. (1995). Ultraviolet spectropolarimetry of high-redshift quasars with the hubble space telescope. Astrophysical Journal Letters, 440(1), 80-90.
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  Abstract: Ultraviolet spectropolarimetry of three bright high-redshift low-polarization quasars (LPQs) was obtained with the Faint Object Spectrograph of the Hubble Space Telescope. Two of the quasars, PG 1634+706 and PG 2302+029, had polarizations p ∼ 0.5%-1.0% throughout the ultraviolet, and showed no significant variation of polarization amplitude or position angle with wavelength. PG 2302+029 was also marginally (2.4 σ) circularly polarized in the optical continuum. For the highest redshift quasar, PG 1222+228 (Ton 1530), the polarization was measured down to rest wavelengths below 800 Å. Although the continuum of PG 1222+228 was weakened by Lyman limit absorption from an intergalactic gas cloud, the polarization increased sharply from 1% to about 4.5%, a change of 4 σ significance. This abrupt rise in polarization does not appear attributable to any known instrumental artifact. These UV polarizations were only slightly less than those previously observed for these same objects in the optical. The polarization spectra were flat with a typical slope of the polarized flux pFv ∝ v-0.8±0.5. Unlike the case of several high luminosity Seyfert 1 nuclei studied previously, polarization caused by scattering from dust grains does not provide the best fit to the polarization spectra of these luminous quasars The hypotheses that the polarization in these quasars is produced by transmission through aligned interstellar grains (in the Milky Way or the host galaxy), or by a synchrotron power-law component, appear to be ruled out. These observed spectra are consistent with a wavelength-independent polarization proportional to the total non-stellar light or, possibly, to the contribution of the blue thermal component. The polarization spectra have insufficient signal-to-noise to locate the scatters with respect to the continuum source and the much larger broad line region. A decrease in amplitude and rotation of the position angle of the polarization vector at the shortest wavelengths, which could result from general relativistic effects near a spinning black hole, was not observed. In fact, in PG 1222+228, the polarization was observed to increase at the shortest wavelengths. The rise in polarization with frequency is so sharp that it cannot be due to any wavelength-independent polarizing mechanism at any radius in an accretion disk. Such a rise could be attributable, for example, to a relative increase in scattering opacity over absorption at higher frequencies.
• Sprayberry, D., Bernstein, G. M., Impey, C. D., & Botoun, G. D. (1995). The mass-to-light ratios of low surface brightness spiral galaxies: Clues from the tully-fisher relation. Astrophysical Journal Letters, 438(1), 72-82.
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  Abstract: We have obtained 21 cm profiles and CCD surface photometry for a subset of field low surface brightness (LSB) spiral galaxies found by a large survey using the Automated Plate Measuring machine. We find that the LSB spirals generally follow the same Tully-Fisher relations defined by a sample of higher surface brightness (HSB) galaxies drawn from the Ursa Major cluster, albeit with a considerably greater scatter. This general trend implies that LSB galaxies of a given total luminosity have mass-to-light ratios (M/L) similar to those of HSB galaxies of comparable total luminosity, despite their differences in luminosity density (i.e., surface brightness). We also find evidence that galaxies with extremely large half-light radii (the "Malin 1 cousins") tend to be excessively luminous for their rotation speeds. We find that, at a given profile width, the luminosity of an LSB galaxy relative to the Tully-Fisher relation seems to be weakly anticorrelated with gas richness, indicating that some of the higher scatter may be associated with the evolutionary status of the LSB galaxies. Finally, we find that the LSB galaxies tend to have higher total atomic gas masses than the Ursa Major comparison galaxies, despite the generally comparable optical luminosities between the two sets.
• Sprayberry, D., Impey, C. D., Bothun, G. D., & Irwin, M. J. (1995). Properties of the class of giant low surface brightness spiral galaxies. Astronomical Journal, 109(2), 558-571.
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  Abstract: We have obtained CCD surface photometry and optical spectroscopy for a sample of eight giant low surface brightness (LSB) spiral galaxies discovered in the course of a large survey for LSB galaxies. We find that these LSB giants have disks of larger scale length and lower central surface brightness than other spiral galaxies, although none have parameters as extreme as the prototype Malin 1. We find that the integrated colors of these LSB giants are redder than the integrated colors of smaller LSB galaxies, and that the LSB galaxies of all sizes follow a relation between redder colors and increasing disk scale length. Two of these eight LSB giants have active nuclei with the broad permitted lines characteristic of a Seyfert 1 nucleus, and one has the narrow lines of a Seyfert 2. The colors and absorption line indices of the bulges of these giants are indistinguishable from those of HSB spirals, suggesting that their bulges have similar stellar populations and evolutionary histories. We also observed four and detected three of these galaxies in the 21 cm line of H I. These LSB giants generally have high total H I masses, although none is as extreme as Malin 1. Finally, the small-scale environments around these galaxies reveal several nearby companions. These LSB giants are at least as likely as smaller LSB galaxies to have close companions, and their average number of neighbors approaches that of HSB galaxies.
• Dinshaw, N., Impey, C. D., Foltz, C. B., Weymann, R. J., & Chaffee, F. H. (1994). Common Lyman-alpha absorption toward the quasar pair Q1343+2640A, B: Evidence for large and quiescent clouds. Astrophysical Journal Letters, 437(2 PART 2), L87-L90.
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  Abstract: We present observations of the Lyα forest of the close quasar pair Q1343+2640A (zem = 2.029) and B (zem = 2.031). We detect eight absorption lines of Lyα common to both spectra and not attributable to metalline systems in the redshift range 1.7 < z < 2.1 and four lines which are seen in one spectrum but not the other. At the 9″.5 separation of the two quasars, this implies a firm lower limit on the characteristic size of the Lyα clouds of 40 h-1100 kpc (where h100 ≡ H0/100 km s-1 Mpc-1, q0 = 0.5) at a redshift z ≃ 1.8. The upper limit on the cloud size is much more uncertain owing to the small number of observed lines, but taking the observed fraction of common lines at face value in the context of a simple model, the absorbers are shown to have radii smaller than about 310 h-1100 kpc. Significant velocity and equivalent width variations are seen with an rms velocity difference of ∼65 km s-1 between the common absorption lines along the two lines of sight.
• Dolan, J. F., Boyd, P. T., Wolinski, K. G., Smith, P. S., Impey, C. D., Bless, R. C., Nelson, M. J., Percival, J. W., Taylor, M. J., Elliot, J. L., Robinson, E. L., & Citters, G. V. (1994). The linear polarization of 3C 345 in the ultraviolet. Astrophysical Journal Letters, 432(2), 560-566.
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  Abstract: The linear polarization of 3C 345, a superluminal radio source and OVV quasar, was observed in two bandpasses in the ultraviolet (centered at 2160 Å and 2770 Å) in 1993 April using the High Speed Photometer on the Hubble Space Telescope. The quasar is significantly polarized in the UV (p > 5%). Ground-based polarimetry was obtained 11 days later, but a difference in the position angle between the observations in the visible and those in the UV indicate that the magnitude of the polarization of 3C 345 may have changed over that time. If the two observation sets represent the same state of spectral polarization, then the large UV flux implies that either the polarization of the synchrotron continuum must stop decreasing in the UV, or that there is an additional source of polarized flux in the ultraviolet. Only if the UV observations represent a spectral polarization state with the same position angle in the visible seen previously in 3C 345 can the polarized flux be represented by a single power law consistent with the three-component model of Smith et al. (1986, 1988). This model consists of a polarized synchrotron component, an unpolarized component from the broad-line region, and an unpolarized component attributed to thermal radiation from an optically thick accretion disk. Additional simultaneous polarimetry in the UV and visible will be required to further constrain models of the continuum emission processes in 3C 345 and determine if the UV polarized flux is synchrotron in origin.
• Pickering, T. E., Impey, C. D., & Foltz, C. B. (1994). ROSAT observations of z>3 quasars. Astronomical Journal, 108(5), 1542-1547.
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  Abstract: Successful pointed observations using the ROSAT PSPC were made of seven z>3 optically selected quasars from the Large Bright Quasar Survey (LBQS). Four detections and three 3σ upper limits resulted. Combining these data with the heterogeneous sample of Avni & Tananbaum [ApJ, 305, 83 (1986)] confirms their conclusion that the ratio of x-ray to optical luminosity is correlated with optical luminosity and probably not correlated with redshift. This suggests that x-ray luminosity evolves more slowly than optical luminosity. These results are then used in conjunction with the LBQS database to estimate the contribution to the 2 keV x-ray background of bright, optically selected quasars with mBJ < 18.85; the result is about 5%.
• Bothun, G. D., Schombert, J. M., Impey, C. D., Sprayberry, D., & Mcgaugh, S. S. (1993). The small scale environment of low surface brightness disk galaxies. Astronomical Journal, 106(2), 530-547.
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  Abstract: We use a sample of ≈340 low surface brightness (LSB) disk galaxies with measured redshifts in combination with the Center for Astrophysics redshift survey to test the hypothesis that LSB galaxies have a deficit of nearby companion galaxies compared to high surface brightness (HSB) disk galaxies. We find a very strong statistical deficit of galaxies located within a projected radius of 0.5 Mpc and within a velocity of 500 km s-1 around LSB disk galaxies compared to HSB ones. Further, comparing LSB and HSB disk galaxies which are located in the same portion of the sky indicates that the average distance to the nearest neighbor is 1.7 times farther for LSB disk galaxies. A Komologorov-Smirnoff test rules out, at greater than the 99% confidence level, the hypothesis that the distribution of nearest-neighbor distances is the same for HSB and LSB disk galaxies. We speculate that LSB disk galaxies have relatively long formation time scales and therefore must form in relative isolation. In addition, the lack of tidal interactions over a Hubble time serves to suppress the overall star-formation rate as no external trigger is available to help clump the gas. The observed low surface densities of H I in combination with the low probability of tidal interactions effectively prevents these disk galaxies from evolving very rapidly.
• Francis, P. J., Hooper, E. J., & Impey, C. D. (1993). The ultraviolet spectra of radio-loud and radio-quiet quasars. Astronomical Journal, 106(2), 417-425.
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  Abstract: The rest-frame ultraviolet spectral properties of matched samples of radio-loud, radio-moderate, and radio-quiet quasars are investigated, using quasars drawn from the Large Bright QSO Survey. We confirm the absence of spectral differences between radio-loud and radio-quiet quasars at rest-frame wavelengths longward of 1600 Å, as reported by previous authors. However, at shorter wavelengths we find a significant difference: radio-loud quasars have narrower (96% confidence), higher equivalent width (97% confidence) Lyman-α and C IV emission lines. We further investigate quasars which are radio quiet, but have radio-to-optical flux ratios at the upper extreme of the radio-quiet population. Broad absorption line quasars are overabundant by a factor of ∼10 in this radio-moderate population; the overabundance is significant at a 99.99% confidence level.
• Impey, C., & Gregorini, L. (1993). Energy distributions of radio galaxies. Astronomical Journal, 105(3), 853-863.
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  Abstract: Far-infrared observations of 140 radio galaxies from the 3CR and B2 catalogs are presented. The galaxies span a range of over four orders of magnitude in radio power, from weak nuclear sources in nearby galaxies, to powerful FR II doubled-lobed sources at moderate redshift. Scanned or pointed observations with IRAS detect 38 of the radio galaxies in at least one waveband; 21 are detected at both 60 and 100 μm. The strength of the far-infrared emission is more closely correlated with core radio emission than with total radio emission, and it is independent of the absolute visual magnitude of the host galaxy. The more luminous 3CR galaxies have warmer far infrared emission than the less luminous B2 galaxies. The relationship between radio and far-infrared luminosity is determined in a way that is insensitive to the large number of infrared nondetections. Radio galaxies have far-infrared emission with similar color temperatures to normal ellipticals; however the inferred amounts of cold interstellar material are typical of gas-rich galaxies. Infrared luminous radio galaxies are rare. Only one in 10 ellipticals with Mv< -17 has a strong enough radio source to have 1011 L⊙ of far-infrared emission. Indirect arguments indicate that far-infrared emission in radio galaxies represents star formation that is more closely tied to the active nucleus than to the global properties of the galaxy. The far-infrared luminosity function shows good continuity between radio galaxies and radio loud quasars.
• Sprayberry, D., Impey, C. D., Irwin, M. J., Mcmahon, R. G., & Bothun, G. D. (1993). Discovery of a third giant low surface brightness disk galaxy. Astrophysical Journal Letters, 417(1), 114-126.
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  Abstract: We report the discovery of a third very large, gas-rich low surface brightness (LSB) disk galaxy, which we designate 1226 + 0105. Its equatorial coordinates are α = 12h26m39s.2 and δ = 1°05′38″ (1950), and it is at a redshift of 23,660 km s-1 It exhibits a number of similarities to the previous described examples of this class, Malin 1 (Bothun et al. 1987; Impey & Bothun 1989) and F568-6 (Bothun et al. 1990), in that it has a large total H I mass of ∼2 × 1010 M⊙, a long disk scale length of ∼14 kpc, a high total luminosity of MB = -21.6, and it harbors an active nucleus. It also shows some striking differences from the previous two. In particular, the newest member of the class shows evidence of pronounced color variations in the disk that are not present in Malin 1 or F568-6. The discovery of this galaxy shows that surveys for LSB galaxies can succeed in finding these giant LSB disks, and the common properties of the three galaxies indicate that they may form a unique class among spirals.
• Webb, W., Malkan, M., Schmidt, G., & Impey, C. (1993). The wavelength dependence of polarization of active galaxies and quasars. Astrophysical Journal Letters, 419(2), 494-514.
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  Abstract: We measured UBV R1 polarizations of 0.5%-2.5% in more than a dozen "normal" quasars and Seyfert 1 galaxies. The position angle is wavelength-independent, suggesting the polarization in a given object originates in a single physical process. In many cases the percentage of polarization increases with frequency. By fitting the multiwavelength spectra with models, we estimate the fractions of the observed flux originating in separate continuum components: infrared, recombination, stellar, and thermal. We compare the polarized fluxes observed in each filter with the predictions of competing models of polarization in AGNs: synchrotron emission, scattering from electrons or different types of dust grains, and electron scattering in an accretion flow. In nine of the sources, the polarization is best understood as scattering off dust grains. Eight of these, 0134 + 329 (3C 48), 0711 + 458 (Mrk 376), 1004 + 130 (4C 13.41), 1426 + 015 (Mrk 1383), 1534 + 580 (Mrk 290), 1535 + 547 (Mrk 486), PG 1552 + 08, and PG 2209 + 185, appear to have a nonspherical distribution of dust with grains best described by the 1977 model of Mathis, Rumpl, and Norsieck. These objects tend to be redder and, in certain cases (e.g., Mrk 486), have broad emission lines which show the same polarization as the continuum. The second most successful fit to the data assumes polarization proportional to the flux of the blue thermal component. This model best characterizes 1208 + 397 (NGC 4151), 2041 - 109 (Mrk 509), and possibly 1415 + 253 (NGC 5548) and 1534 + 580 (Mrk 290). These objects have lower levels of polarization and unpolarized (where measured) emission lines. The modest polarization associated with the thermal ultraviolet component might be due to electron scattering in an accretion disk or torus.
• Visnovsky, K. L., Impey, C. D., Foltz, C. B., Hewett, P. C., Weymann, R. J., & Morris, S. L. (1992). Radio properties of optically selected quasars. Astrophysical Journal Letters, 391(2), 560-568.
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  Abstract: Radio observations are presented for 124 QSOs from the Large Bright Quasar Survey (LBQS). The QSOs were observed at 8.4 GHz with the Very Large Array (VLA) in the C and C/D hybrid array configurations. An average 3 σ noise limit of 0.255 mJy was reached, allowing 24, or 19%, of the QSOs to be detected. The noise statistics are such that we expect about one spurious 3 σ detection to occur in our sample. The radio detection probability is independent of both redshift and B-magnitude, and the distribution of the parameter R8.4 (radio-to-optical luminosity ratio) is independent of both redshift and absolute magnitude. All of the sources are unresolved. The log R8.4 distribution of the LBQS sample covers nearly four orders of magnitude and appears to be bimodal. Comparisons with the Palomar Bright Quasar Survey (BQS) are hampered by a potential selection effect in the BQS data. Using both samples in a region unaffected by this bias, it appears that the radio-loud fraction of optically selected QSOs decreases with increasing redshift and decreases with fainter absolute magnitude.
• Bothun, G. D., Impey, C. D., & Malin, D. F. (1991). Extremely low surface brightness galaxies in the Fornax Cluster: Properties, stability, and luminosity fluctuations. Astrophysical Journal Letters, 376(2), 404-423.
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  Abstract: Using the method of photographic amplification, we have discovered additional examples of very low surface brightness (LSB) galaxies in the Fornax Cluster. In general, the properties of these galaxies are very similar to those discovered in the Virgo Cluster and discussed by Impey, Bothun, & Malin. To avoid redundancy, we focus attention on other aspects of this population, including (1) their morphological appearance, (2) their dynamical stability, and (3) their usefulness as distance indicators via the technique of luminosity fluctuations. In addition, we combine this sample with the Virgo sample to reinvestigate the faint-end slope of the galaxy luminosity function (LF). We find that the failure to account properly for surface brightness selection biases leads to artificially flatter slopes in the case where the light profile of dwarf galaxies is exponential in nature. Under this assumption, we derive a faint-end slope of ≈ -1.55. Most of these LSB objects exhibit a ragged morphological appearance, consisting of numerous surface brightness enhancements (i.e., blobs) embedded in an overall LSB envelope. The colors of these blobs are the same as the envelope and thus they are probably not stellar clusters but instead are random piles of stars which reflect a stellar system with chaotic orbits. Crude dynamical arguments involving questions of disk stability and resistance to disruption by the mean tidal field of the cluster provide circumstantial evidence for an M/L ratio which is significantly higher than that implied by their observed blue colors (mean B-V ≈ 0.6, mean V-I ≈1.0). When coupled with the increase in the faint-end slope of the LF, this implies a nonnegligible mass contribution from these objects. The origin of this population, however, continues to be a mystery, since there is no relation between surface brightness and color which would be expected from any fading scenario. Finally, a subset of these LSB galaxies have regions of constant surface brightness that encompass several hundred pixels. As such, they become amenable to the measurement of luminosity fluctuations for purposes of deriving distances. From a sample of five galaxies which meet this criteria, we derive a distance modulus to Fornax of (m - M) = 31.3 ± 0.3. The typical fluctuation in these LSB galaxies is ≈ 10% (e.g., 100 red giants per pixel). Our derived distance modulus should not be taken seriously, since we do not yet understand the dominant stellar spectral type which is responsible for the fluctuations in these systems. However, the fact that we have derived a reasonable distance suggests that this method may ultimately work for these galaxies. More precise distance measurements would require darker backgrounds (especially at I) than can be obtained from the ground.
• Impey, C. D., Lawrence, C. R., & Tapia, S. (1991). Optical polarization of a complete sample of radio sources. Astrophysical Journal Letters, 375(1), 46-68.
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  Abstract: Polarimetry of 50 strong radio sources is presented and combined with VLBI observations and optical spectroscopy in a study of the radio and optical properties of a complete 5 GHz sample. The sample is almost equally divided into radio galaxies and quasars. High polarization (p > 3%) is strongly correlated with the fraction of the total 5 GHz flux density found in a milliarcsecond core (Fc). The fraction of sources with high polarization rises from 13% for sources with Fc < 0.1 to 45% for sources with Fc > 0.1. Few radio galaxies contain optical cores with high polarization, but the detection rate of such cores is consistent with the hypothesis that all radio sources contain optically polarized cores, with strength proportional to the core radio flux density Two-thirds of the known superluminal sources in the sample have high polarization. High optical polarization optical power-law fraction, line-to-continuum ratio, emission lines of small equivalent width, and large amplitude flux variability are all strongly correlated with the fraction of the 5 GHz radio flux density that is unresolved on VLBI scales. The distributions of these optical properties are well matched by a model where the radio and optical radiation have the same beaming geometry, and with Doppler boosting of a factor of 10-100 in the optical. We confirm in this complete sample a striking alignment of the position angle of the VLBI structure axis and the position angle of optical polarization in highly polarized sources. We also find that the highly polarized sources have a large amount of misalignment between the VLBI structure axis and the axis of large-scale radio structure. The properties of weak-lined objects are consistent with the hypothesis that such objects are of two kinds: sources with high isotropic radio and line luminosity and large γ, viewed at an angle to the line of sight much less than 1/γ; and sources of low isotropic radio and line luminosity and moderate γ, viewed at ∼ 1/γ to the line of sight. Various properties of core-dominated sources require a beaming model with a range of Lorentz factors.
• Bothun, G. D., Schombert, J. M., Impey, C. D., & Schneider, S. E. (1990). Discovery of a second giant low surface brightness galaxy: Further confirmation of slowly evolving disk galaxies. Astrophysical Journal, 360(2), 427-435.
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  Abstract: We report the discovery of another very large, gas-rich, low surface brightness (LSB) disk galaxy. Its overall properties are strikingly similar to those exhibited by Malin 1, which argues that the initial, accidental discovery of Malin 1 by Bothun et al. in 1987 was not a fluke and that the space density of these large, but LSB, galaxies is not negligible. Importantly, these two galaxies are at the top of the disk galaxy luminosity function (owing to their large scale lengths) but have essentially remained uncataloged until now. This again illustrates the importance of selection effects in our understanding of the general galaxy population because even very massive objects (up to 1012 M⊙) can be missed. Overall, the low surface brightnesses of these large disks argues for a relatively quiescent evolution which probably reflects their lower than average H I surface densities. However, some star formation is occurring in this disk as a few kpc size H II regions with luminosities in the Ha line approaching 1041 ergs s-1 have been detected. On average, these H II regons are located at a distance of 40 kpc from the nucleus. This is an unprecedented situation for a noninteracting disk galaxy and is just one of the remarkable properties of this newly discovered galaxy. If the clockrate of disk galaxy evolution is fundamentally driven by the surface density of atomic hydrogen, then the recovery of LSB, gas-rich disk galaxies from the night sky background is important in the context of determining the true range of disk formation efficiencies.
• Impey, C. D., & Tapia, S. (1990). The optical polarization properties of quasars. Astrophysical Journal, 354(1), 124-139.
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  Abstract: Optical polarimetry is presented for 163 radio-selected quasars, including a complete sample of 90 sources stronger than 2 Jy at 5 GHz. Including new and published data, the polarimetry is over 90% complete. Synchrotron components are detected in over 30% of these flat spectrum quasars, with an optical polarization threshold of p > 3%. The fraction of polarized quasars is a strong function of the compactness of the radio emission, as measured on VLBI scales. Including the duty cycle correction, essentially every quasar with Score > Sext at radio wavelengths has a prominent blazar component at optical wavelengths. Virtually all of the radio sources with weak emission lines (or BL Lac objects) are highly polarized. Optical and radio polarization are not correlated, and there is no difference in the αro distributions of high-and low-polarization quasars, or sources with strong and weak emission lines. Redshift information is incomplete, but BL Lac objects appear to have lower redshifts than strong-lined quasars. The distribution of V/Vmax for radio BL Lac objects indicates much stronger evolution than is found for X-ray-selected BL Lac objects. Overall, there is a strong statistical link between compact radio structure, apparent superluminal motion, strong optical polarization, and weak emission lines. This can be understood if both the optical and the compact radio emission are relativistically beamed. However, the Lorentz factors of the material emitting the optical and radio radiation must be different.
• Impey, C. D., Wynn-Williams, C., & Becklin, E. E. (1990). Infrared studies of elliptical galaxies. II. A radio-selected sample. Astrophysical Journal, 356(1), 62-74.
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  Abstract: We have made an infrared survey of radio galaxies in the Bologna B2 catalog. We find that 40% of our sample have infrared luminosities of at least 109 L⊙, as opposed to about 8% of normal ellipticals. The galaxies are inhomogeneous in their infrared properties; some galaxies have active nuclei, while others appear to be powered by star formation. The most infrared-luminous galaxies in our sample are those listed as "peculiar" by Zwicky. Statistically, these galaxies are strikingly different from Seyfert galaxies in their infrared properties in that they show much more radio emission in comparison with their infrared emission than do Seyferts, even when the emission from the extended radio lobes has been discounted. We also reanalyze the available data on the infrared properties of normal elliptical galaxies and find confirmation that the 10-12 μm emission from these galaxies comes either from circumstellar dust shells around evolved stars or from PAH-like (polycyclic aromatic hydrocarbon) grains.
• Impey, C., Bothun, G., Malin, D., & Staveley-Smith, L. (1990). An optical counterpart to the H I cloud in the local supercluster. Astrophysical Journal, 351(2 PART 2), L33-L35.
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  Abstract: We report the detection of an optical counterpart to the large H I cloud recently discovered by Giovanelli and Haynes in the Local Supercluster. The peak of the H I flux corresponds to a low surface brightness, dwarf irregular galaxy. The counterpart was discovered on a photographically amplified image from the UK Schmidt Telescope, and its maximum extent is 180″ at the 27 mag arcsec-2 isotope. We suggest that the H I and optical data can be explained by a pair of LSB dwarf irregular galaxies that have similar velocities. Some star formation appears to be taking place in this system, but the H I column density is below the threshold for extensive star formation. A kinematic study is required to determine the total mass. This galaxy is similar to other LSB dwarfs and dI pairs which have enormous mass-to-light ratios and H I sizes far in excess of the optical scale length.
• Schombert, J. M., Bothun, G. D., Impey, C. D., & Mundy, L. G. (1990). CO deficiency in LSB galaxies. Clues to their star-formation history. Astronomical Journal, 100(5), 1523-1531.
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  Abstract: An optical Hα, CO, and H I survey of six low surface-brightness galaxies from the Schombert and Bothun catalog is presented. These galaxies are all late-type, low H I mass disk systems with unusually low-luminosity densities. Overall, the sample exhibits several paradoxical properties such as no detectable CO emission (a mean upper limit of script M signH2/script M signHI.
• Impey, C. D., & Brand, P. W. (1981). IR photometry of flat spectrum radio sources. Nature, 292(5826), 814-816.
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  Abstract: IR measurements of optically faint, flat spectrum radio sources have implied the possible existence of a new class of quasar, with IR excess more extreme than that of any extragalactic object. In the present preliminary study, 18 Parkes flat spectrum sources have been observed in the IR without regard to optical morphology, and the red sources are found to be just the tail of the normal distribution of quasar colours. The results imply that a considerable fraction of flat spectrum sources from complete samples may have properties similar to the BL Lac objects. © 1981 Nature Publishing Group.

Proceedings Publications

• Impey, C. D., & Wenger, M. C. (2019, January/Spring). Online Astronomy for Formal and Informal Learners. In The European Physical Journal Conferences, 200.
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  An increasing amount of formal and informal education is being delivered online. A majority of college students in the U.S. are now taking one or more courses online or flipped (video lectures online, hands-on activities or labs in the classroom). Meanwhile, massive open online classes, or MOOCs, are transforming the landscape of informal science learning. In contrast to university classes, MOOCs have low completion rates and involve “free choice” learners who are typically adults with jobs, rather than full time students. Based on several years of experience of teaching astronomy online, lessons have been learned on how to engage students in the asynchronous and “disembodied” environment. For non-science students taking an introductory astronomy course, flipped models optimize interactions by putting the lectures online and allowing classroom time to be used entirely for labs, discussion, and small group activities. Research has shown that normalized learning gains are greatest in such learner-centered classes. We have enrolled over 110,000 learners from 150 countries in two astronomy MOOCs: a Udemy course called “Astronomy: State of the Art” and a Coursera offering called “Astronomy: Exploring Time and Space.” The core content is a set of video lectures, augmented by quizzes, activities, and peer writing assignments. We have a large amount of research data on learner demographics and motivations, and on the types of engagement that correlate with completing the courses. In a peer writing assignment, the learners comment on recent discoveries in astronomy. A rubric and a model answer are provided, and each person grades writing of three other learners. Learners who complete either the first activity or the first peer writing assignment are highly engaged in online discussions and social media, completing the course at a rate ten times higher than average.
• Impey, C. D., Buxner, S. R., & Wenger, M. C. (2019, November/Fall). New Practical Research Volumes for Astronomy Education. In Astronomical Society of the Pacific Conference Series, 524, 225.
• Impey, C. D., Formanek, M., Buxner, S. R., & Wenger, M. C. (2019, November/Fall). MOOC Learners’ Motivation and Course Performance: Preliminary Study. In Astronomical Society of the Pacific Conference Series, 524, 163.
• Wenger, M. C., Impey, C. D., & Danehy, A. (2019, November/Fall). Teach Astronomy: Online Resources and New Features. In Astronomical Society of the Pacific Conference Series, 524, 333.
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  Teach Astronomy is an online resource for astronomy instructors, outreach professionals, and amateur astronomers. Our user-friendly website is designed to encourage astronomy students of all levels to learn and explore. In the last year we have updated back-end services, improved availability, and added functionality. In addition to the updated and free online textbook and wiki-map style concept mapping tools, Teach Astronomy offers a Quiz Tool with real-time feedback and improved searches, including new answers to frequently asked astronomy questions from our students and online users.
• Impey, C. D. (2018, Spring). Science Under Threat: Communicating Astronomy in the Age of Misinformation. In National Astronomical Observatory of Japan, 398.
• Impey, C. D., & Danehy, A. (2018, Spring). Innovative Web Site for Astronomy Outreach. In National Astronomical Observatory of Japan, 329.
• Impey, C. D., & Wenger, M. (2018, Spring). Bringing the Universe to the World. In National Astronomical Observatory of Japan, 288.
• Austin, C., Impey, C. D., & Wenger, M. (2014, jan). Exploring the Potential of the Massive, Open, Online Astronomy Course. In American Astronomical Society Meeting Abstracts, 223, #448.06.
• Buxner, S., Impey, C. D., Nieberding, M. N., Romine, J. M., Antonellis, J. C., Llull, J., Tijerino, K., & (CATS), C. (2014, jan). Findings from Five Years Investigating Science Literacy and Where Students Get their Information about Science. In American Astronomical Society Meeting Abstracts, 223, #322.08.
• Impey, C. D. (2014, Fall). The Inspiration of Astronomical Phenomena. In Proceedings of the Seventh Conference on the Inspiration of Astronomical Phenomena, 16, 5-13.
• Prather, E. E., Brissenden, G., Impey, C. D., Lee, K. M., & (CATS), C. (2014, jan). The Collaboration of Astronomy Teaching Scholars (CATS) - Reporting from the Nationion rsquos Largest College-Level, Astronomy Education Research Initiative. In American Astronomical Society Meeting Abstracts, 223, #322.07.
• Romine, J., Buxner, S., Impey, C. D., Nieberding, M. N., Antonellis, J. C., & (CATS), C. (2014, jan). Investigating Science Literacy: Students' Conceptions of Radiation. In American Astronomical Society Meeting Abstracts, 223, #448.05.
• Rudolph, A. L., Impey, C. D., Bieging, J. H., Phillips, C. B., Tieu, J., & Povich, M. S. (2014, jan). The CAMPARE Program:A New Model Promoting Minority Participation in Astronomy Research and Education. In American Astronomical Society Meeting Abstracts, 223, #445.03.
• Austin, C., Impey, C. D., Hardegree-Ullman, K., Patikkal, A., & Ganesan, N. (2013, jan). Teach Astronomy: An Online Resource for Introductory Astronomy Courses and Informal Learners. In American Astronomical Society Meeting Abstracts, 221, #255.03.
• Buxner, S., Impey, C., Tijerino, K., & CATS, . (2013, apr). Investigating Where Students Get Their Information About Science. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 305.
• Buxner, S., Llull, J., Impey, C. D., Tijerino, K., & (CATS), C. (2013, jun). Where Students Get Their Information about Science and Technology and Assessment of That Knowledge. In American Astronomical Society Meeting Abstracts, 222, #120.17.
• Hardegree-Ullman, K. K., Impey, C. D., Patikkal, A., & Austin, C. L. (2013, apr). Using the Teach Astronomy Website to Enrich Introductory Astronomy Classes. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 43.
• Hardegree-Ullman, K., Impey, C. D., Austin, C., Patikkal, A., Paul, M., & Ganesan, N. (2013, jun). TeachAstronomy.com - Digitizing Astronomy Resources. In American Astronomical Society Meeting Abstracts, 222, #120.15.
• Impey, C. (2013, apr). Galaxy Redshifts: From Dozens to Millions. In Origins of the Expanding Universe: 1912-1932, 471, 289.
• Impey, C., Buxner, S., Antonellis, J., & CATS, . (2013, apr). An Ongoing Investigation of Science Literacy: Results of a 22-Year Study Probing Students' Knowledge and Attitude Towards Science. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 189.
• Prather, E. E., Wallace, C. S., Teske, J. K., Impey, C., & Buxner, S. (2013, apr). Uncovering What Our Students Really Think about Science and Society mdashAre We Doomed?. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 97.
• Rudolph, A. L., Impey, C. D., Bieging, J. H., Phillips, C. B., Tieu, J., Prather, E. E., & Povich, M. S. (2013, jan). The California-Arizona Minority Partnership for Astronomy Research and Education (CAMPARE): A New Model for Promoting Minority Participation in Astronomy Research and Education. In American Astronomical Society Meeting Abstracts, 221, #115.03.
• Tijerino, K., Buxner, S., Impey, C., & CATS, . (2013, apr). Investigating What Undergraduate Students Know About Science: Results from Complementary Strategies to Code Open-Ended Responses. In Communicating Science: A National Conference on Science Education and Public Outreach, 473, 301.
• Brissenden, G., Impey, C., Prather, E. E., Lee, K. M., & CATS, C. (2012, jan). The Center for Astronomy Education (CAE) Collaboration of Astronomy Teaching Scholars (CATS) Program: A Year-Four Research Update. In American Astronomical Society Meeting Abstracts #219, 219, #251.01.
• Buxner, S., Impey, C., Antonellis, J., & CATS, C. (2012, jan). A Long-Term Study of Science Literacy and Attitudes Towards Science: Exploring Changes Among College Undergraduate and Public Understanding over Twenty-Two Years. In American Astronomical Society Meeting Abstracts #219, 219, #134.02.
• Buxner, S., Impey, C., Tijerino, K., Antonellis, J., & CATS, C. (2012, jan). A Long-Term Study of Science Literacy and Attitudes Towards Science: Comparing Survey Responses of Undergraduates to Scientists. In American Astronomical Society Meeting Abstracts #219, 219, #251.06.
• Donley, J., Koekemoer, A. M., Brusa, M., Capak, P., Cardamone, C. N., Civano, F., Ilbert, O., Impey, C. D., Kartaltepe, J., Miyaji, T., Salvato, M., Sanders, D. B., Trump, J. R., & Zamorani, G. (2012, jan). Identifying Luminous AGN in Deep Surveys: Revised IRAC Selection Criteria. In American Astronomical Society Meeting Abstracts #219, 219, #225.07.
• Hardegree-Ullman, K., Impey, C. D., & Patikkal, A. (2012, may). Teach Astronomy: An Online Textbook for Introductory Astronomy Courses and Resources for Informal Learners. In American Astronomical Society Meeting Abstracts #220, 220, #437.07.
• Hardegree-Ullman, K., Impey, C., Patikkal, A., Srinathan, A., & CATS, C. (2012, jan). Teach Astronomy: An Online Resource for General Education and Informal Learning. In American Astronomical Society Meeting Abstracts #219, 219, #251.09.
• Schneider, E., & Impey, C. (2012, jan). Steps Toward Unveiling the True Population of AGN: Photometric Selection of Broad-Line AGN. In American Astronomical Society Meeting Abstracts #219, 219, #435.17.

Presentations

• Buxner, S. R., Impey, C. D., Nieberding, M., Romine, J., Antonellis, J., Llull, J. C., & Tijerino, K. (2014, January). Findings from five years investigating science literacy and where students get their information about science. 223rd Meeting of the American Astronomical Society. Washington, DC.
• Buxner, S. R., Impey, C. D., Nieberding, M., Romine, J., Buxner, S. R., Impey, C. D., Nieberding, M., & Romine, J. (2014, November). Where Non-Science Majors Get Information about Science and How They Rate that Information. 46th Meeting of the Division of Planetary Sciences. Tucson, AZ.
• Impey, C. D., & Buxner, S. R. (2014, August). Studying students’ science literacy: Nonscientific beliefs and science literacy measures. Astronomical Society of the Pacific: Celebrating Science: Putting Education Best Practices to Work. Burlingame, CA.
• Romine, J., Buxner, S. R., Impey, C. D., Nieberding, M., & Antonellis, J. C. (2014, November). Investigating Undergraduate Students' Conceptions of Radiation. 46th Meeting of the Division of Planetary Sciences. Tucson, AZ.

Poster Presentations

• Buxner, S. R., Romine, J., Impey, C. D., & Nieberding, M. (2014, August). Investigating the relationship between students’ science knowledge and their sources of information. Astronomical Society of the Pacific: Celebrating Science: Putting Education Best Practices to Work. Burlingame.
• Impey, C. D., Buxner, S. R., Romine, J., & Nieberding, M. (2014, August). Investigating undergraduate students’ science literacy: Responses related to radiation and DNA. Astronomical Society of the Pacific: Celebrating Science: Putting Education Best Practices to Work. Burlingame.
• Nieberding, M., Impey, C. D., Buxner, S. R., & Romine, J. (2014, November). Undergraduate Conceptions About What it Means to Study Something Scientifically. 46th Division of Planetary Science Meeting. Tucson, AZ.
• Romine, J., Buxner, S. R., Impey, C. D., Nieberding, M. N., & Antonellis, J. C. (2014, January). Investigating science literacy: Students' conceptions of radiation. 223rd Meeting of the American Astronomical Society. Washington, DC.

Others

• Impey, C. D. (2020, August). Coronavirus: social distancing is delaying vital scientific research. The Conversation. https://theconversation.com/coronavirus-social-distancing-is-delaying-vital-scientific-research-133689
• Impey, C. D. (2020, August). How technology can combat the rising tide of fake science. The Conversation. https://theconversation.com/how-technology-can-combat-the-rising-tide-of-fake-science-132158
• Impey, C. D. (2020, August). Massive open online classes see exponential growth during COVID-19 pandemic. The Conversation. https://theconversation.com/massive-online-open-courses-see-exponential-growth-during-covid-19-pandemic-141859
• Impey, C. D. (2020, October). I’m an astronomer and I think aliens might be out there — but UFO sightings aren’t persuasive. The Conversation. https://theconversation.com/im-an-astronomer-and-i-think-aliens-may-be-out-there-but-ufo-sightings-arent-persuasive-150498
• Impey, C. D. (2020, October). The Scariest things in the universe are black holes —and here are 3 reasons. The Conversation. https://theconversation.com/the-scariest-things-in-the-universe-are-black-holes-and-here-are-3-reasons-148615
• Impey, C. D. (2020, September). 5 ways families can enjoy astronomy during the pandemic. The Conversation. https://theconversation.com/5-ways-families-can-enjoy-astronomy-during-the-pandemic-144647
• Impey, C. D. (2020, September). Mad, Bad, and…Black Holes. New Scientist.
  More info

  May 23 issue, page 46.
• Impey, C. D. (2020, September). What Buddhism and science can teach each other — and us — about the universe. The Conversation. https://theconversation.com/what-buddhism-and-science-can-teach-each-other-and-us-about-the-universe-134322
• Impey, C. D. (2017, January). Buddhism is Not Just Compatible with Modern Cosmology, It Welcomes It. Nautilus Magazine. http://cosmos.nautil.us/short/93/buddhism-is-not-just-compatible-with-modern-cosmology-it-welcomes-it