Edward W Olszewski

Interests

No activities entered.

Courses

Scholarly Contributions

Journals/Publications

• Bennet, P., Sand, D., Crnojevi{'c}, D., Weisz, D., Caldwell, N., Guhathakurta, P., Hargis, J., Karunakaran, A., Mutlu-Pakdil, B. .., Olszewski, E., Salzer, J., Seth, A., Simon, J., Spekkens, K., Stark, D., Strader, J., Tollerud, E., Toloba, E., & Willman, B. (2022). "Hubble Space Telescope Imaging of Isolated Local Volume Dwarfs GALFA Dw3 and Dw4". apj, 924(2), 98.
• Buttry, R., Pace, A. B., Koposov, S. E., Walker, M. G., Caldwell, N., Kirby, E. N., Martin, N. F., Mateo, M., Olszewski, E. W., Starkenburg, E., Badenes, C., & Mazzola, D. C. (2021). "Stellar kinematics of dwarf galaxies from multi-epoch spectroscopy: application to Triangulum II". arXiv e-prints, arXiv:2108.10867.
• Mutlu-Pakdil, B. .., Sand, D., Walker, M., Caldwell, N., Carlin, J., Collins, M., Crnojevic, D., Mateo, M., Olszewski, E., Seth, A., Strader, J., Willman, B., & Zaritsky, D. (2021). "VizieR Online Data Catalog: Spectroscopy & HST photometry of galaxy Leo V (Mutlu-Pakdil+, 2019)". VizieR Online Data Catalog, J/ApJ/885/53.
• Pace, A. B., Walker, M. G., Koposov, S. E., Caldwell, N., Mateo, M., Olszewski, E. W., Bailey, I., & Wang, M. (2021). "Spectroscopic Confirmation of the Sixth Globular Cluster in the Fornax Dwarf Spheroidal Galaxy". apj, 923(1), 77.
• Paterson, K., Lundquist, M., Rastinejad, J., Fong, W., Sand, D., Andrews, J., Amaro, R., Eskandari, O., Wyatt, S., Daly, P., Bradley, H., Zhou-Wright, S. .., Valenti, S., Yang, S., Christensen, E., Gibbs, A., Shelly, F., Bilinski, C., Chomiuk, L., , Corsi, A., et al. (2021). "Searches after Gravitational Waves Using ARizona Observatories (SAGUARO): Observations and Analysis from Advanced LIGO/Virgo's Third Observing Run". apj, 912(2), 128.
• Song, Y., Mateo, M., Bailey, I., Walker, M. G., Roederer, I. U., Olszewski, E. W., Reiter, M., & Kremin, A. (2021). "Dynamical masses and mass-to-light ratios of resolved massive star clusters - II. Results for 26 star clusters in the Magellanic Clouds". mnras, 504(3), 4160-4191.
• Soraisam, M., Saha, A., Matheson, T., Lee, C. -., Narayan, G., Vivas, A., Scheidegger, C., Oppermann, N., Olszewski, E., Sinha, S., & Desantis, S. (2021). "VizieR Online Data Catalog: CTIO/DECam LCs for Galactic bulge variable stars (Soraisam+, 2020)". VizieR Online Data Catalog, J/ApJ/892/112.
• Calamida, A., Matheson, T., Saha, A., Olszewski, E., Narayan, G., Claver, J., Shanahan, C., Holberg, J., Axelrod, T., Bohlin, R., Stubbs, C., Deustua, S., Hubeny, I., MacKenty, J., Points, S., Rest, A., & Sabbi, E. (2020). VizieR Online Data Catalog: Candidate spectrophotometric standard DA WDs (Calamida+, 2019). VizieR Online Data Catalog, J/ApJ/872/199.
• Johnson, C. I., Dupree, A. K., Mateo, M., Bailey, I., Olszewski, E. W., & Walker, M. G. (2020). The Most Metal-poor Stars in Omega Centauri (NGC 5139). \aj, 159(6), 254.
• Johnson, C., Dupree, A., Mateo, M., Bailey, J., Olszewski, E., & Walker, M. (2020). VizieR Online Data Catalog: Parameters for 453 metal-poor stars in NGC5139 (Johnson+, 2020). VizieR Online Data Catalog, J/AJ/159/254.
• Mutlu-Pakdil, B., Sand, D. J., Crnojevi{\'c}, D., Olszewski, E. W., Zaritsky, D., Strader, J., Collins, M. L., Seth, A. C., & Willman, B. (2020). The Elusive Distance Gradient in the Ultrafaint Dwarf Galaxy Hercules: A Combined Hubble Space Telescope and Gaia View. \apj, 902(2), 106.
• Saha, A., Vivas, A., Olszewski, E., Smith, V., Olsen, K., Blum, R., Valdes, F., Claver, J., Calamida, A., Walker, A., Matheson, T., Narayan, G., Soraisam, M., Cunha, K., Axelrod, T., Bloom, J., Cenko, S., Frye, B., Juric, M., , Kaleida, C., et al. (2020). VizieR Online Data Catalog: RRab stars toward Baade's window with Blanco/DECam (Saha+, 2019). VizieR Online Data Catalog, J/ApJ/874/30.
• Soraisam}, M. D., Saha, A., Matheson, T., Lee, C., Narayan, G., Vivas, A. K., Scheidegger, C., Oppermann, N., Olszewski, E. W., Sinha, S., Desantis, S. R., & Collaboration, {. (2020). A Classification Algorithm for Time-domain Novelties in Preparation for LSST Alerts. Application to Variable Stars and Transients Detected with DECam in the Galactic Bulge. \apj, 892(2), 112.
• Tucker, E., Walker, M. G., Mateo, M., Olszewski, E. W., Geringer-Sameth, A., & Miller, C. J. (2020). Galaxy Cluster Mass Estimates in the Presence of Substructure. \apj, 888(2), 106.
• Calamida, A., Matheson, T., Saha, A., Olszewski, E., Narayan, G., Claver, J., Shanahan, C., Holberg, J., Axelrod, T., Bohlin, R., Stubbs, C. W., Deustua, S., Hubeny, I., Mackenty, J., Points, S., Rest, A., & Sabbi, E. (2019). Photometry and Spectroscopy of Faint Candidate Spectrophotometric Standard DA White Dwarfs. \apj, 872(2), 199.
• James, B., Koposov, S., Stark, D., Belokurov, V., Pettini, M., Olszewski, E., & McQuinn, K. (2019). VizieR Online Data Catalog: Blue diffuse dwarf galaxies (James+, 2017). VizieR Online Data Catalog, J/MNRAS/465/3977.
• Lundquist, M., Paterson, K., Fong, W., Sand, D., Andrews, J., Shivaei, I., Daly, P., Valenti, S., Yang, S., Christensen, E., Gibbs, A., Shelly, F., Wyatt, S., Eskandari, O., Kuhn, O., Amaro, R., Arcavi, I., Behroozi, P., Butler, N., , Chomiuk, L., et al. (2019). Searches after Gravitational Waves Using ARizona Observatories (SAGUARO): System Overview and First Results from Advanced LIGO/Virgo\textquoterights Third Observing Run. \apjl, 881(2), L26.
• Mutlu-Pakdil, B., Sand, D. J., Walker, M. G., Caldwell, N., Carlin, J. L., Collins, M. L., Crnojevi{\'c}, D., Mateo, M., Olszewski, E. W., Seth, A. C., Strader, J., Willman, B., & Zaritsky, D. (2019). Signatures of Tidal Disruption in Ultra-faint Dwarf Galaxies: A Combined HST, Gaia, and MMT/Hectochelle Study of Leo V. \apj, 885(1), 53.
• Narayan, G., Matheson, T., Saha, A., Axelrod, T., Calamida, A., Olszewski, E., Claver, J., Mandel, K. S., Bohlin, R. C., Holberg, J. B., Deustua, S., Rest, A., Stubbs, C. W., Shanahan, C. E., Vaz, A. L., Zenteno, A., Strampelli, G., Hubeny, I., Points, S., , Sabbi, E., et al. (2019). Subpercent Photometry: Faint DA White Dwarf Spectrophotometric Standards for Astrophysical Observatories. \apjs, 241(2), 20.
• Saha, A., Vivas, A. K., Olszewski, E. W., Smith, V., Olsen, K., Blum, R., Valdes, F., Claver, J., Calamida, A., Walker, A. R., Matheson, T., Narayan, G., Soraisam, M., Cunha, K., Axelrod, T., Bloom, J. S., Cenko, S. B., Frye, B., Juric, M., , Kaleida, C., et al. (2019). Mapping the Interstellar Reddening and Extinction toward Baade\textquoterights Window Using Minimum Light Colors of ab-type RR Lyrae Stars: Revelations from the De-reddened Color-Magnitude Diagrams. \apj, 874(1), 30.
• Song, Y., Mateo, M., Mackey, A., Olszewski, E. W., Roederer, I. U., Walker, M. G., & Bailey, J. I. (2019). Dynamical masses and mass-to-light ratios of resolved massive star clusters - I. NGC 419 and NGC 1846. \mnras, 490(1), 385-407.
• Spencer, M., Mateo, M., Olszewski, E., Walker, M., McConnachie, A., & Kirby, E. (2019). VizieR Online Data Catalog: Spectroscopy of RGB stars in Draco \& Ursa Minor (Spencer+, 2018). VizieR Online Data Catalog, J/AJ/156/257.
• Jiang, L., Wu, J., Bian, F., Chiang, Y., Ho, L., Shen, Y., Zheng, Z., Bailey, J., Blanc, G., Crane, J., Fan, X., Mateo, M., Olszewski, E., Oyarz{'un}, G., Wang, R., & Wu, X. (2018). A giant protocluster of galaxies at redshift 5.7. Nature Astronomy, 2, 962-966.
• Johnson, C., Rich, R., Caldwell, N., Mateo, M., Bailey, J., Olszewski, E., & Walker, M. (2018). Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569. aj, 155, 71.
• Koposov, S., Walker, M., Belokurov, V., Casey, A., Geringer-Sameth, A. .., Mackey, D., Da Costa, G., Erkal, D., Jethwa, P., Mateo, M., Olszewski, E., & Bailey, J. (2018). Snake in the Clouds: a new nearby dwarf galaxy in the Magellanic bridge*. mnras, 479, 5343-5361.
• Narayan, G., Matheson, T., Saha, A., Axelrod, T., Calamida, A., Olszewski, E., Claver, J., Mandel, K., Bohlin, R., Holberg, J., Deustua, S., Rest, A., Stubbs, C., Shanahan, C., Vaz, A., Zenteno, A., Strampelli, G., Hubeny, I., Points, S., , Sabbi, E., et al. (2018). Sub-percent Photometry: Faint DA White Dwarf Spectophotometric Standards for Astrophysical Observatories. arXiv e-prints.
• Spencer, M., Mateo, M., Olszewski, E., Walker, M., McConnachie, A., & Kirby, E. (2018). The Binary Fraction of Stars in Dwarf Galaxies: The Cases of Draco and Ursa Minor. aj, 156, 257.
• Caldwell, N., Walker, M., Mateo, M., Olszewski, E., Koposov, S., Belokurov, V., Torrealba, G., Geringer-Sameth, A. .., & Johnson, C. (2017). Crater 2: An Extremely Cold Dark Matter Halo. \apj, 839, 20.
• Caldwell, N., Walker, M., Mateo, M., Olszewski, E., Koposov, S., Belokurov, V., Torrealba, G., Geringer-Sameth, A. .., & Johnson, C. (2017). Erratum: Crater 2: An Extremely Cold Dark Matter Halo (2017, ApJ, 839, 20). \apj, 841, 66.
• Dupree, A., Dotter, A., Johnson, C., Marino, A., Milone, A., Bailey, J., Crane, J., Mateo, M., & Olszewski, E. (2017). NGC 1866: First Spectroscopic Detection of Fast-rotating Stars in a Young LMC Cluster. \apjl, 846, L1.
• James, B., Koposov, S., Stark, D., Belokurov, V., Pettini, M., Olszewski, E., & McQuinn, K. (2017). Blue diffuse dwarf galaxies: a clearer picture. \mnras, 465, 3977-4015.
• Jiang, L., Shen, Y., Bian, F., Zheng, Z., Wu, J., Oyarz{\'un}, G., Blanc, G., Fan, X., Ho, L., Infante, L., Wang, R., Wu, X., Mateo, M., Bailey, J., Crane, J., Olszewski, E., Shectman, S., Thompson, I., & Walker, M. (2017). A Magellan M2FS Spectroscopic Survey of Galaxies at 5.5 $\lt$ z $\lt$ 6.8: Program Overview and a Sample of the Brightest Ly$\alpha$ Emitters. \apj, 846, 134.
• Johnson, C., Caldwell, N., Rich, R., Mateo, M., Bailey, J., Clarkson, W., Olszewski, E., & Walker, M. (2017). A Chemical Composition Survey of the Iron-complex Globular Cluster NGC 6273 (M19). \apj, 836, 168.
• Johnson, C., Caldwell, N., Rich, R., Mateo, M., Bailey, J., Olszewski, E., & Walker, M. (2017). Chemical Complexity in the Eu-enhanced Monometallic Globular NGC 5986. \apj, 842, 24.
• Nidever, D., Olsen, K., Walker, A., Vivas, A., Blum, R., Kaleida, C., Choi, Y., Conn, B., Gruendl, R., Bell, E., Besla, G., Mu{\~noz}, R., Gallart, C., Martin, N., Olszewski, E., Saha, A., Monachesi, A., Monelli, M., Boer, T., , Johnson, L., et al. (2017). SMASH: Survey of the MAgellanic Stellar History. \aj, 154, 199.
• Spencer, M., Mateo, M., Walker, M., & Olszewski, E. (2017). A Multi-epoch Kinematic Study of the Remote Dwarf Spheroidal Galaxy Leo II. \apj, 836, 202.
• Spencer, M., Mateo, M., Walker, M., Olszewski, E., McConnachie, A., Kirby, E., & Koch, A. (2017). The Binary Fraction of Stars in Dwarf Galaxies: The Case of Leo II. \aj, 153, 254.
• Tucker, E., Walker, M., Mateo, M., Olszewski, E., Bailey, J., Crane, J., & Shectman, S. (2017). Magellan/M2FS Spectroscopy of Galaxy Clusters: Stellar Population Model and Application to Abell 267. \aj, 154, 113.
• Vivas, A., Saha, A., Olsen, K., Blum, R., Olszewski, E., Claver, J., Valdes, F., Axelrod, T., Kaleida, C., Kunder, A., Narayan, G., Matheson, T., & Walker, A. (2017). Absolute Magnitudes and Colors of RR Lyrae Stars in DECam Passbands from Photometry of the Globular Cluster M5. \aj, 154, 85.
• Bailey, J., Mateo, M., White, R., Shectman, S., Crane, J., & Olszewski, E. (2016). Multiplexing Precision RVs: Searching for Close-in Gas Giants in Open Clusters. Astronomical Journal, 152, 9.
• Martin, N., Jungbluth, V., Nidever, D., Bell, E., Besla, G., Blum, R., Cioni, M., Conn, B., Kaleida, C., Gallart, C., Jin, S., Majewski, S., Martinez-Delgado, D. .., Monachesi, A., Mu{\~noz}, R., No{\"el}, N., Olsen, K., Stringfellow, G., Marel, R., , Vivas, A., et al. (2016). SMASH 1: A Very Faint Globular Cluster Disrupting in the Outer Reaches of the LMC?. ApJ Letters, 830, L10.
• Narayan, G., Axelrod, T., Holberg, J., Matheson, T., Saha, A., Olszewski, E., Claver, J., Stubbs, C., Bohlin, R., Deustua, S., & Rest, A. (2016). Toward a Network of Faint DA White Dwarfs as High-precision Spectrophotometric Standards. Astrophysical Journal, 822, 67.
• Oyarz{\'un}, G., Blanc, G., Gonz{\'alez}, V., Mateo, M., Bailey, J., Finkelstein, S., Lira, P., Crane, J., & Olszewski, E. (2016). How Lyman Alpha Emission Depends on Galaxy Stellar Mass. ApJ Letters, 821, L14.
• Piatek, S., Pryor, C., & Olszewski, E. (2016). Proper Motion of the Leo II Dwarf Galaxy Based On Hubble Space Telescope Imaging. Astronomical Journal, 152, 166.
• Roederer, I., Mateo, M., Bailey, J., Song, Y., Bell, E., Crane, J., Loebman, S., Nidever, D., Olszewski, E., Shectman, S., Thompson, I., Valluri, M., & Walker, M. (2016). Detailed Chemical Abundances in the r-process-rich Ultra-faint Dwarf Galaxy Reticulum 2. Astronomical Journal, 151, 82.
• Walker, M., Mateo, M., Olszewski, E., Koposov, S., Belokurov, V., Jethwa, P., Nidever, D., Bonnivard, V., Bailey, J., Bell, E., & Loebman, S. (2016). Magellan/M2FS Spectroscopy of Tucana 2 and Grus 1. Astrophysical Journal, 819, 53.
• Weisz, D., Koposov, S., Dolphin, A., Belokurov, V., Gieles, M., Mateo, M., Olszewski, E., Sills, A., & Walker, M. (2016). A Hubble Space Telescope Study of the Enigmatic Milky Way Halo Globular Cluster Crater*. Astrophysical Journal, 822, 32.
• {Bonnivard}, V., {Combet}, C., {Maurin}, D., {Geringer-Sameth}, A., {Koushiappas}, S., {Walker}, M., {Mateo}, M., {Olszewski}, E., , J. (2015). Dark Matter Annihilation and Decay Profiles for the Reticulum II Dwarf Spheroidal Galaxy. Astrophysical Journal Letters, 808, L36.
• {Brondel}, B., {Saha}, A., , E. (2015). Population and Star Formation Histories from the Outer Limits Survey. IAU General Assembly, 22, 2255509.
• {James}, B., {Koposov}, S., {Stark}, D., {Belokurov}, V., {Pettini}, M., , E. (2015). Uncovering blue diffuse dwarf galaxies. Monthly Notices of the Royal Astronomical society, 448, 2687-2703.
• {Johnson}, C., {McDonald}, I., {Pilachowski}, C., {Mateo}, M., {Bailey}, J., {Cordero}, M., {Zijlstra}, A., {Crane}, J., {Olszewski}, E., {Shectman}, S., , I. (2015). AGB Sodium Abundances in the Globular Cluster 47 Tucanae (NGC 104). Astronomical Journal, 149, 71.
• {Koposov}, S., {Belokurov}, V., {Zucker}, D., {Lewis}, G., {Ibata}, R., {Olszewski}, E., {L{\'o}pez-S{\'a}nchez}, {., , E. (2015). Exposing Sgr tidal debris behind the Galactic disc with M giants selected in WISE{\cap}2MASS. Monthly Notices of the Royal Astronomical Society, 446, 3110-3117.
• {Martin}, N., {Nidever}, D., {Besla}, G., {Olsen}, K., {Walker}, A., {Vivas}, A., {Gruendl}, R., {Kaleida}, C., {Mu{\~n}oz}, R., {Blum}, R., {Saha}, A., {Conn}, B., {Bell}, E., {Chu}, Y., {Cioni}, M., {de Boer}, T., {Gallart}, C., {Jin}, S., {Kunder}, A., , {Majewski}, S., et al. (2015). Hydra II: A Faint and Compact Milky Way Dwarf Galaxy Found in the Survey of the Magellanic Stellar History. Astrophysical Journal Letters, 804, L5.
• {Pryor}, C., {Piatek}, S., , E. (2015). Proper Motion of the Draco Dwarf Galaxy Based On Hubble Space Telescope Imaging. Astronomical Journal, 149, 42.
• {Saha}, A., {Narayan}, G., {Holberg}, J., {Matheson}, T., {Olszewski}, E., {Stubbs}, C., {Bohlin}, R., {Sabbi}, E., {Deustua}, S., {Rest}, A., {Axelrod}, T., {MacKenty}, J., {Camarota}, L., , R. (2015). Establishing a Network of faint DA white dwarfs as Spectrophotometric Standards. IAU General Assembly, 22, 2257862.
• {Sand}, D., {Crnojevi{\'c}}, D., {Bennet}, P., {Willman}, B., {Hargis}, J., {Strader}, J., {Olszewski}, E., {Tollerud}, E., {Simon}, J., {Caldwell}, N., {Guhathakurta}, P., {James}, B., {Koposov}, S., {McLeod}, B., {Morrell}, N., {Peacock}, M., {Salinas}, R., {Seth}, A., {Stark}, D., , E. (2015). A Comprehensive Archival Search for Counterparts to Ultra-compact High-Velocity Clouds: Five Local Volume Dwarf Galaxies. Astrophyiscal Journal, 806, 95.
• {Walker}, M., {Mateo}, M., {Olszewski}, E., {Bailey}, J., {Koposov}, S., {Belokurov}, V., , N. (2015). Magellan/M2FS Spectroscopy of the Reticulum 2 Dwarf Spheroidal Galaxy. Astrophysical Journal, 808, 108.
• {Walker}, M., {Olszewski}, E., , M. (2015). Bayesian analysis of resolved stellar spectra: application to MMT/Hectochelle observations of the Draco dwarf spheroidal. Monthly Notices of the Royal Astronomical Society, 448, 2717-2732.
• Belokurov, V., Koposov, S. E., Evans, N. W., Pe~narrubia, J., Irwin, M. J., Smith, M. C., Lewis, G. F., Gieles, M., Wilkinson, M. I., Gilmore, G., Olszewski, E. W., & Niederste-Ostholt, M. (2014). Precession of the Sagittarius stream. mnras, 437, 116-131.
• Boer, T. J., Tolstoy, E., Lemasle, B., Saha, A., Olszewski, E. W., Mateo, M., Irwin, M. J., & Battaglia, G. (2014). The episodic star formation history of the Carina dwarf spheroidal galaxy. aap, 572, A10.
• James, B. L., Koposov, S., Stark, D. P., Belokurov, V., Pettini, M., & Olszewski, E. W. (2014). Uncovering Blue Diffuse Dwarf Galaxies. ArXiv e-prints.
• Jia, Y., Du, C., Wu, Z., Peng, X., Ma, J., Zhou, X., Fan, X., Fan, Z., Jing, Y., Jiang, Z., Lesser, M., Nie, J., Olszewski, E., Shen, S., Wang, J., Zou, H., Zhang, T., & Zhou, Z. (2014). Estimation of absolute magnitude-dependent Galactic model parameters in intermediate latitude with SDSS and SCUSS. mnras, 441, 503-512.
• Belokurov, V., Koposov, S. E., Evans, N. W., Peñarrubia, J., Irwin, M. J., Smith, M. C., Lewis, G. F., Gieles, M., Wilkinson, M. I., Gilmore, G., Olszewski, E. W., & Niederste-Ostholt, M. (2013). Precession of the sagittarius stream. Monthly Notices of the Royal Astronomical Society, 437(1), 116-131.
  More info

  Abstract: Using a variety of stellar tracers - blue horizontal branch stars, main-sequence turn-off stars and red giants - we follow the path of the Sagittarius (Sgr) stream across the sky in Sloan Digital Sky Survey data. Our study presents new Sgr debris detections, accurate distances and line-of-sight velocities that together help to shed new light on the puzzle of the Sgr tails. For both the leading and the trailing tails, we trace the points of their maximal extent, or apocentric distances, and find that they lie at RL = 47.8 ± 0.5 kpc and RT = 102.5 ± 2.5 kpc, respectively. The angular difference between the apocentres is 93.°2 ± 3.°5, which is smaller than predicted for logarithmic haloes. Such differential orbital precession can be made consistent with models of the Milky Way in which the dark matter density falls more quickly with radius. However, currently, no existing Sgr disruption simulation can explain the entirety of the observational data. Based on its position and radial velocity, we show that the unusually large globular cluster NGC 2419 can be associated with the Sgr trailing stream. We measure the precession of the orbital plane of the Sgr debris in the Milky Way potential and show that, surprisingly, Sgr debris in the primary (brighter) tails evolves differently from the secondary (fainter) tails, both in the north and the south. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
• Boer, T. D., Tolstoy, E., Saha, A., & Olszewski, E. W. (2013). A new study of stellar substructures in the Fornax dwarf spheroidal galaxy. Astronomy and Astrophysics, 551.
  More info

  Abstract: Using deep V,B - V wide-field photometry, we have conducted a new study of stellar over-densities in the Fornax dwarf spheroidal galaxy by determining detailed star formation histories from colour-magnitude diagram analysis. We have concentrated on the relatively young stellar component (
• Jerjen, H., Costa, G. D., Willman, B., Tisserand, P., Arimoto, N., Okamoto, S., Mateo, M., Saviane, I., Walsh, S., Geha, M., Jordán, A., Olszewski, E., Walker, M., Zoccali, M., & Kroupa, P. (2013). Main-sequence star populations in the virgo overdensity region. Astrophysical Journal, 769(1).
  More info

  Abstract: We present deep color-magnitude diagrams (CMDs) for two Subaru Suprime-Cam fields in the Virgo Stellar Stream (VSS)/Virgo Overdensity (VOD) and compare them to a field centered on the highest concentration of Sagittarius (Sgr) Tidal Stream stars in the leading arm, Branch A of the bifurcation. A prominent population of main-sequence stars is detected in all three fields and can be traced as faint as g ≈ 24 mag. Using theoretical isochrone fitting, we derive an age of 9.1+1.0-1.1Gyr, a median abundance of [Fe/H] = -0.70+0.15-0.20dex, and a heliocentric distance of 30.9 ± 3.0 kpc for the main sequence of the Sgr Stream Branch A. The dominant main-sequence populations in the two VSS/VOD fields (Λ⊙ ≈ 265°, B⊙ ≈ 13°) are located at a mean distance of 23.3 ± 1.6 kpc and have an age of ∼8.2 Gyr, and an abundance of [Fe/H] -0.67+0.16-0.12= dex, similar to the Sgr Stream stars. These statistically robust parameters, derived from the photometry of 260 main-sequence stars, are also in good agreement with the age of the main population in the Sgr dwarf galaxy (8.0 ± 1.5 Gyr). They also agree with the peak in the metallicity distribution of 2-3 Gyr old M giants, [Fe/H] ≈-0.6 dex, in the Sgr north leading arm. We then compare the results from the VSS/VOD fields with the Sgr Tidal Stream model by Law & Majewski based on a triaxial Galactic halo shape that is empirically calibrated with Sloan Digital Sky Survey Sgr A-branch and Two Micron All Sky Survey M-giant stars. We find that the most prominent feature in the CMDs, the main-sequence population at 23 kpc, is not explained by the model. Instead the model predicts in these directions a low-density filamentary structure of Sgr debris stars at ∼9 kpc and a slightly higher concentration of Sgr stars spread over a heliocentric distance range of 42-53 kpc. At best there is only marginal evidence for the presence of these populations in our data. Our findings then suggest that while there are probably some Sgr debris stars present, the dominant stellar population in the VOD originates from a different halo structure that has an almost identical age and metallicity as some sections of the Sgr tidal stream. © 2013. The American Astronomical Society. All rights reserved.
• Boer, T. D., Tolstoy, E., Hill, V., Saha, A., Olszewski, E. W., Mateo, M., Starkenburg, E., Battaglia, G., & Walker, M. G. (2012). The star formation and chemical evolution history of the Fornax dwarf spheroidal galaxy. Astronomy and Astrophysics, 544.
  More info

  Abstract: We present deep photometry in the B, V and I filters from CTIO/MOSAIC for about 270 000 stars in the Fornax dwarf spheroidal galaxy, out to a radius of r ell ≈ 0.8 degrees. By combining the accurately calibrated photometry with the spectroscopic metallicity distributions of individual red giant branch stars we obtain the detailed star formation and chemical evolution history of Fornax. Fornax is dominated by intermediate age (1-10 Gyr) stellar populations, but also includes ancient (10-14 Gyr), and young (=1 Gyr) stars. We show that Fornax displays a radial age gradient, with younger, more metal-rich populations dominating the central region. This confirms results from previous works. Within an elliptical radius of 0.8 degrees, or 1.9 kpc from the centre, a total mass in stars of 4.3 × 10 7 M ⊙ was formed, from the earliest times until 250 Myr ago. Using the detailed star formation history, age estimates are determined for individual stars on the upper RGB, for which spectroscopic abundances are available, giving an age-metallicity relation of the Fornax dSph from individual stars. This shows that the average metallicity of Fornax went up rapidly from [Fe/H] ≤-2.5 dex to [Fe/H] =-1.5 dex between 8-12 Gyr ago, after which a more gradual enrichment resulted in a narrow, well-defined sequence which reaches [Fe/H] ≈-0.8 dex, ≈3 Gyr ago. These ages also allow us to measure the build-up of chemical elements as a function of time, and thus determine detailed timescales for the evolution of individual chemical elements. A rapid decrease in [Mg/Fe] is seen for the stars with [Fe/H] ≥-1.5 dex, with a clear trend in age. © 2012 ESO.
• Sand, D. J., Strader, J., Willman, B., Zaritsky, D., McLeod, B., Caldwell, N., Seth, A., & Olszewski, E. (2012). Tidal signatures in the faintest milky way satellites: The detailed properties of Leo V, Pisces II, and Canes Venatici II. Astrophysical Journal, 756(1).
  More info

  Abstract: We present deep wide-field photometry of three recently discovered faint Milky Way (MW) satellites: LeoV, PiscesII, and Canes Venatici II. Our main goals are to study the structure and star formation history of these dwarfs; we also search for signs of tidal disturbance. The three satellites have similar half-light radii (∼60-90 pc) but a wide range of ellipticities. Both LeoV and CVnII show hints of stream-like overdensities at large radii. An analysis of the satellite color-magnitude diagrams shows that all three objects are old (>10Gyr) and metal-poor ([Fe/H] ∼-2), though neither the models nor the data have sufficient precision to assess when the satellites formed with respect to cosmic reionization. The lack of an observed younger stellar population (≲ 10Gyr) possibly sets them apart from the other satellites at Galactocentric distances ≳ 150kpc. We present a new compilation of structural data for all MW satellite galaxies and use it to compare the properties of classical dwarfs to the ultra-faints. The ellipticity distribution of the two groups is consistent at the ∼2σ level. However, the faintest satellites tend to be more aligned toward the Galactic Center, and those satellites with the highest ellipticity (≳ 0.4) have orientations (Δθ GC) in the range 20° ≲ Δθ GC ≲ 40°. This latter observation is in rough agreement with predictions from simulations of dwarf galaxies that have lost a significant fraction of their dark matter halos and are being tidally stripped. © © 2012. The American Astronomical Society. All rights reserved.
• Mainzer, A., Grav, T., Bauer, J., Masiero, J., McMillan, R. S., Cutri, R. M., Walker, R., Wright, E., Eisenhardt, P., Tholen, D. J., Spahr, T., Jedicke, R., Denneau, L., Debaun, E., Elsbury, D., Gautier, T., Gomillion, S., Hand, E., Mo, W., , Watkins, J., et al. (2011). Neowise observations of near-earth objects: Preliminary results. Astrophysical Journal, 743(2).
  More info

  Abstract: With the NEOWISE portion of the Wide-field Infrared Survey Explorer (WISE) project, we have carried out a highly uniform survey of the near-Earth object (NEO) population at thermal infrared wavelengths ranging from 3 to 22 μm, allowing us to refine estimates of their numbers, sizes, and albedos. The NEOWISE survey detected NEOs the same way whether they were previously known or not, subject to the availability of ground-based follow-up observations, resulting in the discovery of more than 130 new NEOs. The survey's uniform sensitivity, observing cadence, and image quality have permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by NEOWISE during the fully cryogenic portion of the WISEmission to the larger population. We find that there are 981 19 NEAs larger than 1km and 20,500 3000 NEAs larger than 100m. We show that the Spaceguard goal of detecting 90% of all 1km NEAs has been met, and that the cumulative size distribution is best represented by a broken power law with a slope of 1.32 0.14 below 1.5km. This power-law slope produces 13, 200 1900 NEAs with D > 140m. Although previous studies predict another break in the cumulative size distribution below D 50-100m, resulting in an increase in the number of NEOs in this size range and smaller, we did not detect enough objects to comment on this increase. The overall number for the NEA population between 100 and 1000m is lower than previous estimates. The numbers of near-Earth comets and potentially hazardous NEOs will be the subject of future work. © 2011. The American Astronomical Society. All rights reserved.
• Pritzl, B. J., Olszewski, E. W., Saha, A., Venn, K. A., & Skillman, E. D. (2011). Probing the M33 halo using rr lyrae stars. Astronomical Journal, 142(6).
  More info

  Abstract: We present the discovery of RR Lyrae (RRL) variable stars in two fields of M33 to the southeast of its center using Gemini North observations. In the outermost field (45′ to the southeast of the M33 center; projected distance of 10.8kpc; deprojected distance of about 19kpc; about 5 V-band disk scale lengths) we detected two RRLs, consisting of one RRab star and one RRc star. An additional variable was found in this field that is a possible Cepheid. The mean g′-band magnitude for the two RRL stars is 25.60 0.04mag. In the other field (25′ to the southeast of the M33 center; projected distance of 5.6kpc; deprojected distance of about 10kpc; about 3 V-band disk scale lengths) we found 12 RRL stars, all of which are RRab stars. We also detected two candidate Cepheid variables in this field. The mean magnitude for the RRL stars in this field is 25.64 ± 0.14mag. We found a distance modulus of 〈(m - M)0〉 = 24.69 ± 0.17mag from the RRab stars in the field 25′ from M33. The mean periods of the RRab stars in both fields (0.630 ± 0.002days, 45′ southeast; 0.628 ± 0.055days, 25′ southeast) are much longer than found for previously detected RRL stars in the inner regions of M33. This finding argues for a low metallicity for the RRab stars in the field 25′ from M33 ([Fe/H] =-1.84 0.30dex). Given these properties, we conclude that the RRLs we have detected belong to the halo of M33 and thus that there is an old component in the outer regions of M33. © 2011. The American Astronomical Society. All rights reserved.
• Belokurov, V., Walker, M. G., Evans, N. W., Gilmore, G., Irwin, M. J., Just, D., Koposov, S., Mateo, M., Olszewski, E., Watkins, L., & Wyrzykowski, L. (2010). Big fish, little fish: Two new ultra-faint satellites of the milky way. Astrophysical Journal Letters, 712(1 PART 2), L103-L106.
  More info

  Abstract: We report the discovery of two new Milky Way satellites in the neighboring constellations of Pisces and Pegasus identified in data from the Sloan Digital Sky Survey. Pisces II, an ultra-faint dwarf galaxy lies at the distance of 180 kpc, some 15°away from the recently detected Pisces I. Segue 3, an ultra-faint star cluster lies at the distance of 16 kpc. We use deep follow-up imaging obtained with the 4-m Mayall Telescope at Kitt Peak National Observatory to derive their structural parameters. Pisces II has a half-light radius of 60 pc, while Segue 3 is 20 times smaller at only 3 pc. © 2010 The American Astronomical Society.
• Pryor, C., Piatek, S., & Olszewski, E. W. (2010). Proper motion of the Sagittarius dwarf galaxy based on Hubble space telescope imaging. Astronomical Journal, 139(3), 839-856.
  More info

  Abstract: We have derived a proper motion of Sagittarius using archival data obtained with the Hubble Space Telescope. The data consist of imaging at three epochs with a time baseline of about four years in three distinct fields. The zero point for the proper motion is based on the foreground Galactic stellar populations along the line of sight. The measured proper motion in the Galactic coordinate system is (μℓ, μb) = (-2.615 ± 0.22, 1.87 ± 0.19) mas yr-1 and in the equatorial coordinate system is (μα, μδ) = (-2.75 ± 0.20, - 1.65 ± 0.22) mas yr-1. Removing the contribution of the motion of the Sun and of the LSR to the measured proper motion produces a Galactic rest-frame proper motion of (μGrfℓ, μGrfb) = (-0.82 ± 0.22, 1.98 0.19) mas yr -1 and (μGrfα, μGrfδ) = (-2.14 ± 0.20, 0.03 ± 0.20) mas yr -1. The implied space velocity with respect to the Galactic center is (Π, Θ, Z) = (141.9 ± 6.9, 117 ± 29, 238 ± 27) km s-1. This velocity implies that the instantaneous orbital inclination is 67°, with a 95% confidence interval of (58°, 79°). We also present photometry and membership probabilities for the stars in our sample, which can be used to generate color-magnitude diagrams for stellar populations selected by proper motion. © 2010 The American Astronomical Society.
• Saha, A., Olszewski, E. W., Brondel, B., Olsen, K., Knezek, P., Harris, J., Smith, C., Subramaniam, A., Claver, J., Rest, A., Seitzer, P., Cook, K. H., Minniti, D., & Suntzeff, N. B. (2010). First results from the noao survey of the outer limits of the magellanic clouds. Astronomical Journal, 140(6), 1719-1738.
  More info

  Abstract: We describe the first results from the Outer Limits Survey, an NOAO survey designed to detect, map, and characterize the extended structure of the Large and SmallMagellanic Clouds (LMC and SMC). The survey consists of deep images of 55 0.° 6 ×0.° 6 fields distributed at distances up to 20° from the Clouds, with 10 fields at larger distances representing controls for contamination by Galactic foreground stars and background galaxies. The field locations probe the outer structure of both the LMC and SMC, as well as exploring areas defined by the Magellanic Stream, the Leading Arm, and the LMC orbit as recently measured from its proper motion. The images were taken with C, M, R, I, and DDO51 filters on the CTIO Blanco 4 m telescope and Mosaic2 camera, with supporting calibration observations taken at the CTIO 0.9mtelescope. The CRI images reach depths below the oldestmain-sequence (MS) turnoffs at the distance of the Clouds, thus yielding numerous probes of structure combined with good ability to measure stellar ages and metallicities. The M and DDO51 images allow for discrimination of LMC and SMC giant stars from foreground dwarfs, allowing us to use giants as additional probes of Cloud structure and populations. From photometry of eight fields located at radii of 7°-19° north of the LMC bar, we find MS stars associated with the LMC out to 16° from the LMC center, while the much rarer giants can only be convincingly detected out to 11°. In one field, designated as a control, we see the unmistakable signature of the Milky Way (MW) globular cluster NGC 1851, which lies several tidal radii away from the field center. The color-magnitude diagrams show that while at 7° radius LMC populations as young as 500 Myr are present, at radii ≥11° only the LMC's underlying old metal-poor ([M/H]∼ -1) population remains, demonstrating the existence of a mean population gradient at these radii. Nevertheless, even at extreme large distances, the dominant age is significantly younger than that of the Galactic globular clusters. The MS star counts follow an exponential decline with distance with a scale length of 1.15 kpc, essentially the same scale length as gleaned for the inner LMC disk from prior studies. While we cannot rule out the existence of undetected tidal features elsewhere in the LMC periphery, the detection of an ordered structure to 12 disk scale lengths is unprecedented and adds to the puzzle of the LMC's interaction history with the SMC and the MW. Our results do not rule out the possible existence of an LMC stellar halo, which we showmay only begin to dominate over the disk at still larger radii than wherewe have detected LMC populations. 2010. The American Astronomical Society. All rights reserved.
• Walker, M. G., Mateo, M., Olszewski, E. W., Pẽarrubia, J., Evans, N. W., & Gilmore, G. (2010). Erratum: A universal mass profile for dwarf spheroidal galaxies? (Astrophysical Journal (2009) 704 (1274)). Astrophysical Journal Letters, 710(1), 886-890.
• Walker, M. G., McGaugh, S. S., Mateo, M., Olszewski, E. W., & Kuzio, R. (2010). Comparing the dark matter halos of spiral, low surface brightness, and dwarf spheroidal galaxies. Astrophysical Journal Letters, 717(2 PART 2), L87-L91.
  More info

  Abstract: We consider dark masses measured from kinematic tracers at discrete radii in galaxies for which baryonic contributions to overall potentials are either subtracted or negligible. Recent work indicates that rotation curves due to dark matter (DM) halos at intermediate radii in spiral galaxies are remarkably similar, with a mean rotation curve given by log10(V c,DM/(km s-1)) = 1.47+0.15-0.19 + 0.5log10(r/kpc). Independent studies show that while estimates of the dark mass of a given dwarf spheroidal (dSph) galaxy are robust only near the half-light radius, data from the Milky Way's (MW's) dSph satellites are consistent with a narrow range of mass profiles. Here we combine published constraints on the dark halo masses of spirals and dSphs and include available measurements of low surface brightness galaxies for additional comparison. We find that most measured MW dSphs lie on the extrapolation of the mean rotation curve due to DM in spirals. The union of MW-dSph and spiral data appears to follow a mass-radius relation of the form MDM(r)/M⊙ = 200+200-120(r/pc)2, or equivalently a constant acceleration g DM = 3+3-2 × 10-9 cm s-2, spanning 0.02 kpc ≲ r ≲ 75 kpc. Evaluation at specific radii immediately generates two results from the recent literature: a common mass for MW dSphs at fixed radius and a constant DM central surface density for galaxies ranging from MW dSphs to spirals. However, recent kinematic measurements indicate that M31's dSph satellites are systematically less massive than MW dSphs of similar size. Such deviations from what is otherwise a surprisingly uniform halo relation presumably hold clues to individual formation and evolutionary histories. © 2010. The American Astronomical Society. All rights reserved..
• Belokurov, V., Walker, M. G., Evans, N. W., Gilmore, G., Irwin, M. J., Mateo, M., Mayer, L., Olszewski, E., Bechtold, J., & Pickering, T. (2009). The discovery of Segue 2: A prototype of the population of satellites of satellites. Monthly Notices of the Royal Astronomical Society, 397(4), 1748-1755.
  More info

  Abstract: We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope (MMT). From this, we derive a luminosity of Mv = -2.5, a half-light radius of 34 pc and a systemic velocity of ∼ -40 km s-1. Our data also provide evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighbouring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km s-1 and the possible stream as ∼7 km s-1. This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites - survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z > 10 and are good candidates for fossils of the reionization epoch. © 2009 The Authors. Journal compilation © 2009 RAS.
• Held, E. V., Tolstoy, E., Rizzi, L., Cesetti, M., Cole, A. A., Battaglia, G., S., G., Gullieuszik, M., Mateo, M., Olszewski, E. W., & Walker, M. G. (2009). The star formation history of the Fornax dwarf spheroidal galaxy. Proceedings of the International Astronomical Union, 5(S262), 353-354.
  More info

  Abstract: We present the first results of a comprehensive HST study of the star-formation history of Fornax dSph, based on WFPC2 imaging of 7 Fornax fields. Our observations reach the oldest main-sequence turnoffs, allowing us to address fundamental questions of dwarf galaxy evolution, such as the spatial variations in the stellar content, and whether the old stellar population is made up of stars formed in a very early burst or the result of a more continuous star formation. © International Astronomical Union 2010.
• Olszewski, E. W., Saha, A., Knezek, P., Subramaniam, A., Boer, T. D., & Seitzer, P. (2009). A 500 parsec halo surrounding the galactic globular NGC 1851. Astronomical Journal, 138(6), 1570-1576.
  More info

  Abstract: Using imaging that shows 4 mag of main-sequence stars, we have discovered that the Galactic globular cluster NGC1851 is surrounded by a halo that is visible from the tidal radius of 700 arcsec (41 pc) to more than 4500 arcsec (>250 pc). This halo is symmetric and falls in density as a power law of r -1.24. It contains approximately 0.1% of the dynamical mass of NGC 1851. There is no evidence for tidal tails. Current models of globular cluster evolution do not explain this feature, although simulations of tidal influences on dwarf spheroidal galaxies qualitatively mimic these results. Given the state of published models, it is not possible to decide between creation of this halo from either isolated cluster evaporation or from tidal or disk shocking, or from destruction of a dwarf galaxy in which this object may have once been embedded. © 2009 The American Astronomical Society. All rights reserved.
• Sand, D. J., Olszewski, E. W., Willman, B., Zaritsky, D., Seth, A., Harris, J., Piatek, S., & Saha, A. (2009). The star formation history and extended structure of the hercules milky way satellite. Astrophysical Journal Letters, 704(2), 898-914.
  More info

  Abstract: We present imaging of the recently discovered Hercules Milky Way satellite and its surrounding regions to study its structure, star formation history and to thoroughly search for signs of disruption. We robustly determine the distance, luminosity, size, and morphology of Hercules utilizing a bootstrap approach to characterize our uncertainties. We derive a distance to Hercules via a comparison to empirical and theoretical isochrones, finding a best match with the isochrone of M92, which yields a distance of 133 ± 6 kpc. As previous studies have found, Hercules is very elongated, with ε = 0.67 ± 0.03 and a half-light radius of rh ≃ 230 pc. Using the color-magnitude-fitting package StarFISH, we determine that Hercules is old (>12 Gyr) and metal-poor ([Fe/H] ∼ -2.0), with a spread in metallicity, in agreement with previous spectroscopic work. This result is robust with respect to slight variations in the distance to Hercules and mismatches between the observed Hercules color-magnitude diagram and theoretical isochrones. We infer a total absolute magnitude of MV = -6.2 ± 0.4. Our innovative search for external Hercules structure both in the plane of the sky and along the line of sight yields some evidence that Hercules is embedded in a larger stream of stars. A clear stellar extension is seen to the northwest with several additional candidate stellar overdensities along the position angle of Hercules out to ∼ 35′ (1.3 kpc). While the association of any of the individual stellar overdensities with Hercules is difficult to determine, we do show that the summed color-magnitude diagram of all three is consistent with Hercules' stellar population. Finally, we estimate that any change in the distance to Hercules across its face is at most ∼ 6 kpc, and the data are consistent with Hercules being at the same distance throughout. © 2009. The American Astronomical Society. All rights reserved.
• Starkenburg, E., Helmi, A., Morrison, H. L., Harding, P., Woerden, H. V., Mateo, M., Olszewski, E. W., Sivarani, T., Norris, J. E., Freeman, K. C., Shectman, S. A., Dohm-Palmer, R., Frey, L., & Oravetz, D. (2009). Mapping the galactic halo. VIII. Quantifying substructure. Astrophysical Journal Letters, 698(1), 567-579.
  More info

  Abstract: We have measured the amount of kinematic substructure in the Galactic halo using the final data set from the Spaghetti project, a pencil-beam high-latitude sky survey. Our sample contains 101 photometrically selected and spectroscopically confirmed giants with accurate distance, radial velocity, and metallicity information. We have developed a new clustering estimator: the "4distance" measure, which when applied to our data set leads to the identification of one group and seven pairs of clumped stars. The group, with six members, can confidently be matched to tidal debris of the Sagittarius dwarf galaxy. Two pairs match the properties of known Virgo structures. Using models of the disruption of Sagittarius in Galactic potentials with different degrees of dark halo flattening, we show that this favors a spherical or prolate halo shape, as demonstrated by Newberg et al. using the Sloan Digital Sky Survey data. One additional pair can be linked to older Sagittarius debris. We find that 20% of the stars in the Spaghetti data set are in substructures. From comparison with random data sets, we derive a very conservative lower limit of 10% to the amount of substructure in the halo. However, comparison to numerical simulations shows that our results are also consistent with a halo entirely built up from disrupted satellites, provided that the dominating features are relatively broad due to early merging or relatively heavy progenitor satellites. © 2009. The American Astronomical Society. All rights reserved.
• Walker, M. G., Belokurov, V., Evans, N. W., Irwin, M. J., Mateo, M., Olszewski, E. W., & Gilmore, G. (2009). LEO V: Spectroscopy of a distant and disturbed satellite. Astrophysical Journal Letters, 694(2), L144-L147.
  More info

  Abstract: We present a spectroscopic study of Leo V, a recently discovered satellite of the Milky Way (MW). From stellar spectra obtained with the MMT/Hectochelle spectrograph we identify seven likely members of Leo V. Five cluster near the Leo V center (R < 3′) and have a velocity dispersion of 2.4 +2.4-1.4 km s-1. The other two likely members lie near each other but far from the center (R 13′ 700 pc) and inflate the global velocity dispersion to 3.7+2.3-1.4 km s -1. Assuming the five central members are bound, we obtain a dynamical mass of M = 3.3+9.1-2.5 × 105M (M/LV = 75+230-58[M/LV ]). From the stacked spectrum of the five central members we estimate a mean metallicity of [Fe/H]=-2.0 0.2 dex. Thus, with respect to dwarf spheroidals of similar luminosity, Leo V is slightly less massive and slightly more metal rich. Since we resolve the central velocity dispersion only marginally, we do not rule out the possibility that Leo V is a diffuse star cluster devoid of dark matter. The wide separation of its two outer members implies Leo V is losing mass; however, its large distance (D 180 kpc) is difficult to reconcile with MW tidal stripping unless the orbit is very radial. © 2009. The American Astronomical Society. All rights reserved..
• Walker, M. G., Mateo, M., & Olszewski, E. W. (2009). Stellar velocities in the Carina, Fornax, Sculptor, and Sextans dsph galaxies: Data from the magellan/MMFS survey. Astronomical Journal, 137(2), 3100-3108.
  More info

  Abstract: We present spectroscopic data for individual stars observed from 2004 March through 2008 August as part of our Michigan/MIKE Fiber System (MMFS) survey of four dwarf spheroidal (dSph) galaxies: Carina, Fornax, Sculptor, and Sextans. Using MMFS at the Magellan/Clay Telescope at Las Campanas Observatory, we have acquired 8855 spectra from 7103 red-giant candidates in these Galactic satellites. We list measurements of each star's line-of-sight velocity (median error 2.1 km s-1) and spectral line indices for iron and magnesium absorption features. We use globular cluster spectra to calibrate the indices onto standard [Fe/H] metallicity scales, but comparison of the resulting metallicities with published values suggests that the MMFS indices are best used as indicators of relative, not absolute, metallicity. The empirical distributions of velocity and spectral indices also allow us to quantify the amount of contamination by foreground stars. In a companion paper, we develop an algorithm that evaluates the membership probability for each star, showing that the present MMFS sample contains more than 5000 dSph members, including 774 Carina members, 2483 Fornax members, 1365 Sculptor members, and 441 Sextans members. © 2009. The American Astronomical Society. All rights reserved.
• Walker, M. G., Mateo, M., Olszewski, E. W., Sen, B., & Woodroofe, M. (2009). Clean kinematic samples in dwarf spheroidals: An algorithm for evaluting membership and estimating distribution parameters when contamination is present. Astronomical Journal, 137(2), 3109-3138.
  More info

  Abstract: We develop an algorithm for estimating parameters of a distribution sampled with contamination. We employ a statistical technique known as "expectation maximization" (EM). Given models for both member and contaminant populations, the EM algorithm iteratively evaluates the membership probability of each discrete data point, then uses those probabilities to update parameter estimates for member and contaminant distributions. The EM approach has wide applicability to the analysis of astronomical data. Here we tailor an EM algorithm to operate on spectroscopic samples obtained with the Michigan-MIKE Fiber System (MMFS) as part of our Magellan survey of stellar radial velocities in nearby dwarf spheroidal (dSph) galaxies. These samples, to be presented in a companion paper, contain discrete measurements of line-of-sight velocity, projected position, and pseudo-equivalent width of the Mg-triplet feature, for 1000-2500 stars per dSph, including some fraction of contamination by foreground Milky Way stars. The EM algorithm uses all of the available data to quantify dSph and contaminant distributions. For distributions (e.g., velocity and Mg-index of dSph stars) assumed to be Gaussian, the EM algorithm returns maximum-likelihood estimates of the mean and variance, as well as the probability that each star is a dSph member. These probabilities can serve as weights in subsequent analyses. Applied to our MMFS data, the EM algorithm identifies more than 5000 stars as probable dSph members. We test the performance of the EM algorithm on simulated data sets that represent a range of sample size, level of contamination, and amount of overlap between dSph and contaminant velocity distributions. The simulations establish that for samples ranging from large (N 3000, characteristic of the MMFS samples) to small (N 30), resembling new samples for extremely faint dSphs), the EM algorithm distinguishes members from contaminants and returns accurate parameter estimates much more reliably than conventional methods of contaminant removal (e.g., sigma clipping). © 2009. The American Astronomical Society. All rights reserved.
• Belokurov, V., Walker, M. G., Evans, N. W., Faria, D. C., Gilmore, G., Irwin, M. J., Koposov, S., Mateo, M., Olszewski, E., & Zucker, D. B. (2008). Leo v: A companion of a companion of the Milky Way galaxy?. Astrophysical Journal, 686(2 PART 2), L83-L86.
  More info

  Abstract: We report the discovery of a new Milky Way satellite in the constellation Leo, identified in data from the Sloan Digital Sky Survey. It lies at a distance of-180 kpc, and is separated by ≤⊙3° from another recent discovery, Leo IV. We present follow-up imaging from the Isaac Newton Telescope and spectroscopy from the Hectochelle fiber spectrograph at the Multiple Mirror Telescope. Leo V's heliocentric velocity is-173.3 ±3.1 km s-1, offset by-40 km s-1 from that of Leo IV. A simple interpretation of the kinematic data is that both objects may lie on the same stream, although the implied orbit is only modestly eccentric (e-0.2) © 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.
• Jong, J. D., Harris, J., Coleman, M. G., Martin, N. F., Bell, E. F., Rix, H. -., Hill, J. M., Skillman, E. D., Sand, D. J., Olszewski, E. W., Zaritsky, D., Thompson, D., Giallongo, E., Ragazzoni, R., DiPaola, A., Farinato, J., Testa, V., & Bechtold, J. (2008). The structural properties and star formation history of Leo T from deep LBT photometry. Astrophysical Journal, 680(2), 1112-1119.
  More info

  Abstract: We present deep, wide-field g and r photometry of the transition-type dwarf galaxy Leo T, obtained with the blue arm of the Large Binocular Telescope. The data confirm the presence of both very young (5 Gyr) stars. We study the structural properties of the old and young stellar populations by preferentially selecting either population on the basis of their color and magnitude. The young population is significantly more concentrated than the old population, with half-light radii of 104 ±8 and 148 ±16 pc, respectively, and their centers are slightly offset. Approximately 10% of the total stellar mass is estimated to be represented by the young stellar population. Comparison of the color-magnitude diagram (CMD) with theoretical isochrones, as well as numerical CMD fitting, suggests that star formation began over 10 Gyr ago and continued in recent times until at least a few hundred Myr ago. The CMD-fitting results are indicative of two distinct star formation bursts, with a quiescent period around 3 Gyr ago, albeit at low significance. The results are consistent with no metallicity evolution and a value of [Fe/H] ∼-1.5 over the entire age of the system. Finally, the data show little, if any, sign of tidal distortion of Leo T. © 2008. The American Astronomical Society. All rights reserved.
• Mateo, M., Olszewski, E. W., & Walker, M. G. (2008). The velocity dispersion profile of the remote dwarf spheroidal galaxy leo I: A tidal hit and run?. Astrophysical Journal, 675(1), 201-233.
  More info

  Abstract: We present new kinematic results for 387 stars near the Milky Way satellite dwarf spheroidal galaxy Leo I. Spectra were obtained with the Hectochelle multiobject echelle spectrograph on the MMT, centered in the optical near 5200 Å. From 297 repeat measurements of 108 stars, we estimate the mean velocity error (1 σ) of our sample to be 2.4 km s-1, with a systematic precision of
• Piatek, S., Pryor, C., & Olszewski, E. W. (2008). Proper motions of the large magellanic cloud and small magellanic Cloud: Re-analysis of hubble space telescope data. Astronomical Journal, 135(3), 1024-1038.
  More info

  Abstract: Kallivayalil et al. have used the Hubble Space Telescope to measure proper motions of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) using images in 21 and five fields, respectively, all centered on known quasi-stellar objects (QSOs). These results are more precise than previous measurements, but have surprising and important physical implications: for example, the LMC and SMC may be approaching the Milky Way for the first time; they might not have been in a binary system; and the origin of the Magellanic Stream needs to be re-examined. Motivated by these implications, we have re-analyzed the original data in order to check the validity of these measurements. Our work has produced a proper motion for the LMC that is in excellent agreement with that of Kallivayalil et al., and for the SMC that is in acceptable agreement. We have detected a dependence between the brightness of stars and their mean measured motion in a majority of the fields in both our reduction and that of Kallivayalil et al. Correcting for this systematic error and for the errors caused by the decreasing charge transfer efficiency of the detector produces better agreement between the measurements from different fields. With our improved reduction, we do not need to exclude any fields from the final averages and, for the first time using proper motions, we are able to detect the rotation of the LMC. The best-fit amplitude of the rotation curve at a radius of 275 arcmin in the disk plane is 120 15 km s-1. This value is larger than the 60-70 km s-1 derived from the radial velocities of HI and carbon stars, but in agreement with the value of 107 km s-1 derived from the radial velocities of red supergiants. Our measured proper motion for the center of mass of the LMC is (μα, μδ) = (195.6 3.6, 43.5 3.6) mas century-1; that for the SMC is (μα, μδ) = (75.4 6.1, - 125.2 5.8) mas century-1. The uncertainties for the latter proper motion are three times smaller than those of Kallivayalil et al. © 2008. The American Astronomical Society. All rights reserved.
• Sales, L. V., Helmi, A., Starkenburg, E., Morrison, H. L., Engle, E., Harding, P., Mateo, M., Olszewski, E. W., & Sivarani, T. (2008). On the genealogy of the Orphan Stream. Monthly Notices of the Royal Astronomical Society, 389(3), 1391-1398.
  More info

  Abstract: We use N-body simulations to explore the origin and a plausible orbit for the Orphan Stream, one of the faintest substructures discovered so far in the outer halo of our Galaxy. We are able to reproduce its position, velocity and distance measurements by appealing to a single wrap of a double-component satellite galaxy. We find that the progenitor of the Orphan Stream could have been an object similar to today's Milky Way dwarfs, such as Carina, Draco, Leo II or Sculptor; and unlikely to be connected to Complex A or Ursa Major II. Our models suggest that such progenitors, if accreted on orbits with apocentres smaller than ∼35 kpc, are likely to give rise to very low surface brightness streams, which may be hiding in the outer halo and remain largely undetected with current techniques. The systematic discovery of these ghostly substructures may well require wide field spectroscopic surveys of the Milky Way's outer stellar halo. © 2008 RAS.
• Walker, M. G., Mateo, M., & Olszewski, E. W. (2008). Systemic proper motions of milky way satellites from stellar redshifts: The carina, fornax, sculptor, and sextans dwarf spheroidals. Astrophysical Journal, 688(2 PART 2), L75-L78.
  More info

  Abstract: The transverse motions of nearby dwarf spheroidal (dSph) galaxies contribute line-of-sight components that increase with angular distance from the dSph centers, inducing detectable gradients in stellar redshift. In the absence of an intrinsic velocity gradient (e.g., due to rotation or streaming), an observed gradient in the heliocentric rest frame (HRF) relates simply to a dSph's systemic proper motion (PM). Kinematic samples for the Milky Way's brightest dSph satellites are now sufficiently large that we can use stellar redshifts to constrain systemic PMs independently of astrometric data. Data from our Michigan/MIKE Fiber System (MMFS) Survey reveal significant HRF velocity gradients in Carina, Fornax, and Sculptor, and no significant gradient in Sextans. Assuming there are no intrinsic gradients, the data provide a relatively tight constraint on the PM of Fornax, (μαHRF,μδ;H RF)=(+48 ± 15,-25 ± 14) mas century-1, that agrees with published HSTastrometric measurements. Smaller data sets yield weaker constraints in the remaining galaxies, but our Carina measurement, (μαHRF, μδHRF)=(+25 ± 36, +16 ± 43) mas century-1, agrees with the published astrometric value. The disagreement of our Sculptor measurement, (μαHRF,μδHRF)=(-40 ± 29,-69 ± 47) mas century-1, with astrometric measurements is expected if Sculptor has a rotational component as reported by Battaglia and coworkers. For Sextans, which at present lacks an astrometric measurement, we measure (μαHRF, μδHRF)=(-26 ± 41, +10 ± 44) mas century-1. © 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.
• Walker, M. G., Mateo, M., & Olszewski, E. W. (2008). Systemic proper motions of milky way satellites from stellar redshifts: the carina, fornax, sculptor, and sextans dwarf spheroidals. Astrophysical Journal, 687(2 PART 2), L75-L78.
  More info

  Abstract: The transverse motions of nearby dwarf spheroidal (dSph) galaxies contribute line-of-sight components that increase with angular distance from the dSph centers, inducing detectable gradients in stellar redshift. In the absence of an intrinsic velocity gradient (e.g., due to rotation or streaming), an observed gradient in the heliocentric rest frame (HRF) relates simply to a dSph's systemic proper motion (PM). Kinematic samples for the Milky Way's brightest dSph satellites are now sufficiently large that we can use stellar redshifts to constrain systemic PMs independently of astrometric data. Data from our Michigan/MIKE Fiber System (MMFS) Survey reveal significant HRF velocity gradients in Carina, Fornax, and Sculptor, and no significant gradient in Sextans. Assuming there are no intrinsic gradients, the data provide a relatively tight constraint on the PM of Fornax, (μHRFα, μHRFdelta;)=(+48 ± 15,-25 ± 14) mas century-1, that agrees with published HST astrometric measurements. Smaller data sets yield weaker constraints in the remaining galaxies, but our Carina measurement, (μHRFα, μHRFdelta;)=(+25 ± 36, +16 ± 43) mas century-1, agrees with the published astrometric value. The disagreement of our Sculptor measurement, (μHRFα, μHRFdelta;)=(-40 ± 29,-69 ± 47) mas century-1, with astrometric measurements is expected if Sculptor has a rotational component as reported by Battaglia and coworkers. For Sextans, which at present lacks an astrometric measurement, we measure (μHRFα, μHRFdelta;)=(-26 ± 41, +10 ± 44) mas century-1. ©2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.
• Piatek, S., Pryor, C., Bristow, P., Olszewski, E. W., Harris, H. C., Mateo, M., Minniti, D., & Tinney, C. G. (2007). Proper motions of dwarf spheroidal galaxies from hubble space telescope imaging. V. Final measurement for fornax. Astronomical Journal, 133(3), 818-844.
  More info

  Abstract: The measured proper motion of Fornax, expressed in the equatorial coordinate system, is (μα, μδ) = (47.6 ± 4.6, -36.0 ± 4.1) mas century-1. This proper motion is a weighted mean of four independent measurements for three distinct fields. Each measurement uses a quasi-stellar object as a reference point. Removing the contribution of the motion of the Sun and of the local standard of rest to the measured proper motion produces a Galactic rest-frame proper motion of (μαGrf, μδGrf) = (24.4 ± 4.6,-14.3 ± 4.1) mas century-1. The implied space velocity with respect to the Galactic center has a radial component of Vr = -31.8 ± 1.7 km s-1 and a tangential component of Vt = 196 ± 29 km s-1. Integrating the motion of Fornax in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 118 (66, 137) and 152 (144, 242) kpc, respectively, where the values in the parentheses represent the 95% confidence intervals derived from Monte Carlo experiments. The eccentricity of the orbit is 0.13 (0.11, 0.38), and the orbital period is 3.2 (2.5, 4.6) Gyr. The orbit is retrograde and inclined by 101° (94°, 107°) to the Galactic plane. Fornax could be a member of a proposed "stream" of galaxies and globular clusters; however, the membership of another proposed galaxy in the stream, Sculptor, has been previously ruled out. Fornax is in the Kroupa-Theis-Boily plane, which contains 11 of the Galactic satellite galaxies, but its orbit will take it out of that plane. © 2007. The American Astronomical Society. All rights reserved.
• Walker, M. G., Mateo, M., Olszewski, E. W., Bernstein, R., Sen, B., & Woodroofe, M. (2007). THE Michigan/MIKE fiber system survey of stellar radial velocities in dwarf spheroidal galaxies: Acquisition and reduction of data. Astrophysical Journal, Supplement Series, 171(2), 389-418.
  More info

  Abstract: We introduce a stellar velocity survey of dwarf spheroidal galaxies, undertaken using the Michigan/MIKE Fiber System (MMFS) at the Magellan/Clay 6.5 m telescope at Las Campanas Observatory. As of 2006 November we have used MMFS to collect 6415 high-resolution (R = 20,000-25,000) spectra from 5180 stars in four dwarf spheroidal galaxies: Carina, Fornax, Sculptor, and Sextans. Spectra sample the range 5140-5180 Å, which includes the prominent magnesium triplet absorption feature. We measure radial velocity (RV) to a median precision of 2.0 km s-1 for stars as faint as V ∼ 20.5. From the spectra we also are able to measure the strength of iron and magnesium absorption features using spectral indices that correlate with effective temperature, surface gravity, and chemical abundance. Measurement of line strength allows us to identify interloping foreground stars independently of velocity and to examine the metallicity distribution among dSph members. Here we present detailed descriptions of MMFS, our target selection and spectroscopic observations, the data reduction procedure, and error analysis. We compare our RV results to previously published measurements for individual stars. In some cases we find evidence for a mild, velocity-dependent offset between the RVs that we measure using the magnesium triplet and previously published RV measurements derived from the infrared calcium triplet. In companion papers we will present the complete data sets and kinematic analyses of these new observations. © 2007. The American Astronomical Society. All rights reserved.
• Battaglia, G., Helmi, A., Morrison, H., Harding, P., Olszewski, E. W., Mateo, M., Freeman, K. C., Norris, J., & Shectman, S. A. (2006). Erratum: The radial velocity dispersion profile of the Galactic halo: constraining the density profile of the dark halo of the Milky Way (Monthly Notices of the Royal Astronomical Society (2005) 364 (433-442)). Monthly Notices of the Royal Astronomical Society, 370(2), 1055-1056.
• Burke, D., Axelrod, T., Claver, C., Frank, J., Geary, J., Gilmorel, K., Ivezic, Z., Krabbendam, V., Monet, D., O'Connor, P., Oliver, J., Olszewski, E., Pinto, P., Saha, A., Smith, C., Stubbs, C., Takacs, P., & Tyson, A. (2006). Calibrations of LSST camera and telescope systems. Proceedings of SPIE - The International Society for Optical Engineering, 6267 I.
  More info

  Abstract: Science studies made by the Large Synoptic Survey Telescope will reach systematic limits in nearly all cases. Requirements for accurate photometric measurements are particularly challenging. Advantage will be taken of the rapid cadence and pace of the LSST survey to use celestial sources to monitor stability and uniformity of photometric data. A new technique using a tunable laser is being developed to calibrate the wavelength dependence of the total telescope and camera system throughput. Spectroscopic measurements of atmospheric extinction and emission will be made continuously to allow the broad-band optical flux observed in the instrument to be corrected to flux at the top of the atmosphere. Calibrations with celestial sources will be compared to instrumental and atmospheric calibrations.
• Levan, A. J., Tanvir, N. R., Fruchter, A. S., Rol, E., Fynbo, J. P., Hjorth, J., Williams, G., Bergeron, E., Bersier, D., Bremer, M., Gray, T., Jakobsson, P., Nilsson, K., Olszewski, E., Priddey, R. S., Rafferty, D., & Rhoads, J. (2006). The faint afterglow and host galaxy of the short-hard GRB 060121. Astrophysical Journal Letters, 648(1 II), L9-L12.
  More info

  Abstract: We present optical and X-ray observations of the afterglow and host galaxy of the short-hard GRB 060121. The faint R-band afterglow is seen to decline as t-0.66±0.09 while the X-ray falls as t -1.18±0.04, indicating the presence of the cooling break between the two frequencies. However, the R-band afterglow is very faint compared to the predicted extrapolation of the X-ray afterglow to the optical regime (specifically, βOX ∼ 0.2), while the K-band is consistent with this extrapolation (βKX ∼ 0.6), demonstrating suppression of the optical flux. Late-time HST observations place stringent limits on the afterglow R-band flux, implying a break in the R-band light curve. They also show that the burst occurred at the edge of a faint red galaxy, presumably the host, which most likely lies at a significantly higher redshift than the previous optically identified short-duration bursts. Several neighboring galaxies also have very red colors that are similarly suggestive of higher redshift. The least extreme explanation for the faintness and color of the burst is that it occurred at moderately high redshift and was significantly obscured; however, it is also possible that it lies at z > 4.5, in which case the faintness of the R-band afterglow could be attributed to the Lyman break. We discuss the implications that either scenario would have for the nature of the progenitors of short bursts. © 2006. The American Astronomical Society. All rights reserved.
• Olszewski, E. W., Mateo, M., Harris, J., Walker, M. G., Coleman, M. G., & Costa, G. S. (2006). The nature of the density clump in the fornax dwarf spheroidal galaxy. Astronomical Journal, 131(2), 912-921.
  More info

  Abstract: We have imaged the recently discovered stellar overdensity located approximately one core radius from the center of the Fornax dwarf spheroidal galaxy using the Magellan Clay 6.5 m telescope with the Magellan Instant Camera. Superb seeing conditions allowed us to probe the stellar populations of this overdensity and of a control field within Fornax to a limiting magnitude of R = 26. The color-magnitude diagram of the overdensity field is virtually identical to that of the control field, with the exception of the presence of a population arising from a very short (less than 300 Myr in duration) burst of star formation 1.4 Gyr ago. Coleman et al. have argued that this overdensity might be related to a shell structure in Fornax that was created when Fornax captured a smaller galaxy. Our results are consistent with this model, but we argue that the metallicity of this young component favors a scenario in which the gas was part of Fornax itself. © 2006. The American Astronomical Society. All rights reserved.
• Piatek, S., Pryor, C., Bristow, P., Olszewski, E. W., Harris, H. C., Mateo, M., Minniti, D., & Tinney, C. G. (2006). Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging. IV. Measurement for sculptor. Astronomical Journal, 131(3), 1445-1460.
  More info

  Abstract: This article presents a measurement of the proper motion of the Sculptor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope using the Space Telescope Imaging Spectrograph in the imaging mode. Each of two distinct fields contains a quasi-stellar object that serves as the "reference point." The measured proper motion of Sculptor, expressed in the equatorial coordinate system, is (μ α , μ δ) = (9 ± 13, 2 ± 13) mas century -1. Removing the contributions from the motion of the Sun and the motion of the local standard of rest produces the proper motion in the Galactic rest frame: (μ αGrf, μ δGrf) = (-23 ± 13, 45 ± 13) mas century -1. The implied space velocity with respect to the Galactic center has a radial component of V r = 79 ± 6 km s -1 and a tangential component of V t = 198 ± 50 km s -1. Integrating the motion of Sculptor in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 68 (31, 83) and 122 (97, 313) kpc, respectively, where the values in the parentheses represent the 95% confidence interval derived from Monte Carlo experiments. The eccentricity of the orbit is 0.29 (0.26, 0.60), and the orbital period is 2.2 (1.5, 4.9) Gyr. Sculptor is on a polar orbit around the Milky Way: the angle of inclination is 86° (83°, 90°). © 2006. The American Astronomical Society. All rights reserved.
• Walker, M. G., Mated, M., Olszewski, E. W., Bernstein, R., Wang, X., & Woodroofe, M. (2006). Erratum: Internal kinematics of the fornax dwarf spheroidal galaxy (Astronomical Journal (2006) 131 (2114)). Astronomical Journal, 132(2), 968-.
• Walker, M. G., Mateo, M., Olszewski, E. W., Bernstein, R., Wang, X., & Woodroofe, M. (2006). Internal kinematics of the Fornax dwarf spheroidal galaxy. Astronomical Journal, 131(4), 2114-2139.
  More info

  Abstract: We present new radial velocity results for 176 stars in the Fornax dwarf spheroidal galaxy, of which at least 156 are probable Fornax members. We combine with previously published data to obtain a radial velocity sample with 206 stars, of which at least 176 are probable Fornax members. We detect the hint of rotation about an axis near Fornax's morphological minor axis, although the significance of the rotation signal in the galactic rest frame is sensitive to the adopted value of Fornax's proper motion. Regardless, the observed stellar kinematics is dominated by random motions, and we do not find kinematic evidence of tidal disruption. The projected velocity dispersion profile of the binned data set remains flat over the sampled region, which reaches a maximum angular radius of 65′. Single-component King models in which mass follows light fail to reproduce the observed flatness of the velocity dispersion profile. Two-component (luminous plus dark matter) models can reproduce the data, provided that the dark component extends sufficiently beyond the luminous component and the central dark matter density is of the same order as the central luminous density. These requirements suggest a more massive, darker Fornax than standard core-fitting analyses have previously concluded, with M/L V over the sampled region reaching 10-40 times the M/L V of the luminous component. We also apply a nonparametric mass estimation technique, introduced in a companion paper. Although it is designed to operate on data sets containing velocities for >1000 stars, the estimation yields preliminary results suggesting M/L V ∼ 15 inside r < 1.5 kpc. © 2006, The American Astronomical Society. All rights reserved.
• Walker, M. G., Mateo, M., Olszewski, E. W., Pal, J. K., Sen, B., & Woodroofe, M. (2006). On kinematic substructure in the Sextans dwarf spheroidal galaxy. Astrophysical Journal Letters, 642(1 II), L41-L44.
  More info

  Abstract: We present multifiber echelle radial velocity results for 551 stars in the Sextans dwarf spheroidal galaxy and identify 294 stars as probable Sextans members. The projected velocity dispersion profile of the binned data remains flat to a maximum angular radius of 30′. We introduce a nonparametric technique for estimating the projected velocity dispersion surface and use this to search for kinematic substructure. Our data do not confirm previous reports of a kinematically distinct stellar population at the Sextans center. Instead we detect a region near the Sextans core radius that is kinematically colder than the overall Sextans sample with 95% confidence. © 2006. The American Astronomical Society. All rights reserved.
• Battaglia, G., Helmi, A., Morrison, H., Harding, P., Olszewski, E. W., Mateo, M., Freeman, K. C., Norris, J., & Shectman, S. A. (2005). The radial velocity dispersion profile of the Galactic halo: Constraining the density profile of the dark halo of the Milky Way. Monthly Notices of the Royal Astronomical Society, 364(2), 433-442.
  More info

  Abstract: We have compiled a new sample of 240 halo objects with accurate distance and radial velocity measurements, including globular clusters, satellite galaxies, field blue horizontal branch (FHB) stars and red giant stars from the Spaghetti survey. The new data lead to a significant increase in the number of known objects for Galactocentric radii beyond 50 kpc, which allows a reliable determination of the radial velocity dispersion profile out to very large distances. The radial velocity dispersion shows an almost constant value of 120 km s-1 out to 30 kpc and then continuously declines down to 50 km s-1 at about 120 kpc. This fall-off puts important constraints on the density profile and total mass of the dark matter halo of the Milky Way. For a constant velocity anisotropy, the isothermal profile is ruled out, while both a dark halo following a truncated flat (TF) model of mass 1.2-0.5+1.8× 1012M⊙ and a Navarro, Frenk & White (NFW) profile of mass 0.8-0.2+1.2 × 1012 M⊙ and c = 18 are consistent with the data. The significant increase in the number of tracers combined with the large extent of the region probed by these has allowed a more precise determination of the Milky Way mass in comparison to previous works. We also show how different assumptions for the velocity anisotropy affect the performance of the mass models.x © 2005 RAS.
• Piatek, S., Pryor, C., Bristow, P., Olszewski, E. W., Harris, H. C., Mateo, M., Minniti, D., & Tinney, C. G. (2005). Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging, III. Measurement for Ursa Minor. Astronomical Journal, 130(1), 95-115.
  More info

  Abstract: This article presents a measurement of the proper motion of the Ursa Minor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope in two distinct fields. Each field contains a quasi-stellar object that serves as the "reference point." The measured proper motion for Ursa Minor, expressed in the equatorial coor-dinate system, is (μ α, μ δ) = (-50 ± 17, 22 ± 16) mas century -1. Removing the contributions of the solar motion and the motion of the local standard of rest yields the proper motion in the Galactic rest frame: (μ αGrf, μ δGrf) = (-8 ± 17, 38 ± 16) mas century -1. The implied space velocity with respect to the Galactic center has a radial component of V r = -75 ± 44 km s -1 and a tangential component of V t, = 144 ± 50 km s -1. Integrating the motion of Ursa Minor in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 40 (10, 76) and 89 (78, 160) kpc, respectively, where the values in the parentheses represent the 95% confidence intervals derived from Monte Carlo experiments. The eccentricity of the orbit is 0.39 (0.09, 0.79), and the orbital period is 1.5 (1.1, 2.7) Gyr. The orbit is retrograde and inclined by 124° (94°, 136°) to the Galactic plane. Ursa Minor is not a likely member of a proposed stream of galaxies on similar orbits around the Milky Way, nor is the plane of its orbit coincident with a recently proposed planar alignment of galaxies around the Milky Way. Comparing the orbits of Ursa Minor and Carina shows no reason for the different star formation histories of these two galaxies. Ursa Minor must contain dark matter to have a high probability of having survived disruption by the Galactic tidal force until the present. © 2005. The American Astronomical Society. All rights reserved.
• Wang, X., Woodroofe, M., Walker, M. G., Mateo, M., & Olszewski, E. (2005). Estimating dark matter distributions. Astrophysical Journal Letters, 626(1 I), 145-158.
  More info

  Abstract: Thanks to instrumental advances, new, very large kinematic data sets for nearby dwarf spheroidal (dSph) galaxies are on the horizon. A key aim of these data sets is to help determine the distribution of dark matter in these galaxies. Past analyses have generally relied on specifie dynamical models or highly restrictive dynamical assumptions. We describe a new, nonparametric analysis of the kinematics of nearby dSph galaxies designed to take full advantage of the future large data sets. The method takes as input the projected positions and radial velocities of stars known to be members of the galaxies but does not use any parametric dynamical model or the assumption that the mass distribution follows that of the visible matter. The problem of estimating the radial mass distribution M(r) (the mass within the true radius r) is converted into a problem of estimating a regression function nonparametrically. From the Jeans equation we show that the unknown regression function is subject to fundamental shape restrictions, which we exploit in our analysis using statistical techniques borrowed from isotonic estimation and spline smoothing. Simulations indicate that M(r) can be estimated to within a factor of 2 or better with samples as small as 1000 stars over almost the entire radial range sampled by the kinematic data. The technique is applied to a sample of 181 stars in the Fornax dSph galaxy. We show that the galaxy contains a significant, extended dark halo some 10 times more massive than its baryonic component. Although applied here to dSph kinematics, this approach can be used in the analysis of any kinematically hot stellar system in which the radial velocity field is discretely sampled. © 2005. The American Astronomical Society. All rights reserved.
• Dolphin, A. E., Saha, A., Olszewski, E. W., Thim, F., Skillman, E. D., Gallagher, J. S., & Hoessel, J. (2004). Short-period variable stars in the M31 halo. Astronomical Journal, 127(2 1778), 875-896.
  More info

  Abstract: We report the findings of a new search for RR Lyrae stars in an M31 halo field located 40′ from the nucleus of the galaxy along the minor axis. We detected 37 variable stars, of which 24 are classified as RR Lyrae and the others are ambiguous. Estimating a completeness fraction of ∼24%, we calculate that there are approximately 100 RR Lyrae stars in the field, which is consistent with what is expected from deep Hubble Space Telescope (HST) color-magnitude diagrams. We calculate a mean magnitude of 〈(g)〉 = 25.15 ± 0.03, which we interpret to mean that the mean metallicity of RR Lyrae stars is significantly lower than that of the M31 halo as a whole. The presence of ancient, metal-poor stars opens the possibility that initially the M31 halo appeared much like the Milky Way halo.
• Piatek, S., Pryor, C., Olszewski, E. W., Harris, H. C., Mated, M., Minniti, D., & Tinney, C. G. (2004). Erratum: Proper motions of dwarf spheroidal galaxies from hubble space telescope imaging. II. Measurement for carina (The Astronomical Journal (2003) 126 (2346)). Astronomical Journal, 128(2 1784), 951-.
• Morrison, H. L., Norris, J., Mateo, M., Harding, P., Olszewski, E. W., Shectman, S. A., Dohm-Palmer, R., Helmi, A., & Freeman, K. C. (2003). Mapping the galactic halo. VI. Spectroscopic measures of luminosity and metallicity. Astronomical Journal, 125(5 1769), 2502-2520.
  More info

  Abstract: We present our calibration of spectroscopic measures of luminosity and metallicity for halo giant candidates and give metallicities and distances for our first sample of spectroscopically confirmed giants. These giants have distances ranging from 15 to 83 kpc. As surveys reach farther into the Galaxy's halo with K giant samples, identification of giants becomes more difficult. This is because the numbers of foreground halo K dwarfs rise for V magnitudes of 19-20, typical for halo giants at ∼ 100 kpc. Our photometric survey uses the strength of the Mg b/H feature near 5170 Å to weed K dwarfs out of the disk and thick disk, but we need spectroscopic measures of the strength of the Ca II K, Ca I λ4227, and Mg b/H features to distinguish between the very metal-poor dwarfs and halo giants. Using a full error analysis of our spectroscopic measures, we show why a signal-to-noise ratio of ∼15 pixel-1 at Ca I λ4227 and ∼10 at Ca II K is needed for reliable luminosity discrimination. We use the Ca II K and Mg b features to measure metallicity in our halo giants, with typical errors (random plus systematic) of 0.3 dex for [Fe/H] values from -0.8 to -3.0.
• Piatek, S., Pryor, C., Olszewski, E. W., Harris, H. C., Mateo, M., Minniti, D., & Tinney, C. G. (2003). Proper motions of dwarf spheroidal galaxies from Hubble space telescope imaging. II. Measurement for Carina. Astronomical Journal, 126(5 1775), 2346-2361.
  More info

  Abstract: This article presents and discusses a measurement of the proper motion for the Carina dwarf spheroidal galaxy (dSph) from images in two distinct fields in the direction of Carina taken with the Hubble Space Telescope, at three epochs. Each field contains a confirmed quasi-stellar object that is the reference point for measuring the proper motion of the dSph. The consecutive epochs are 1-2 yr apart. The components of the measured proper motion for Carina, expressed in the equatorial coordinate system, are μ α = 22 ± 9 mas century -1 and μ δ = 15 ± 9 mas century -1. The quoted proper motion is a weighted mean of two independent measurements and has not been corrected for the motions of the Sun and of the local standard of rest. Given the proper motion and its uncertainty, integrating the family of possible orbits of Carina in a realistic gravitational potential for the Milky Way indicates that Carina is bound gravitationally to the Milky Way and is close to apogalacticon. The best estimate of, and the 95% confidence interval for, the apogalacticon of the orbit is 102 kpc and (102, 113) kpc, for the perigalacticon is 20 kpc and (3.0, 63) kpc, and for the orbital period is 1.4 Gyr and (1.3, 2.0) Gyr. Carina does not seem to be on a polar orbit. The best estimate of the inclination of the orbit with respect to the Galactic plane is 39°, but the 95% confidence interval is so wide, (23°, 102°), that it includes a polar orbit. We are unable to confirm or to rule out the membership of Carina in a "stream" of galaxies in the Galactic halo because the difference between the observed and predicted directions of the proper motion is 1.6 times the uncertainty of the difference. Carina must contain dark matter to have survived the tidal interaction with the Milky Way until the present. The triggering of star formation by perigalacticon passages and crossings of the Galactic disk do not explain the history of star formation in Carina.
• Stanek, K. Z., Matheson, T., Garnavich, P. M., Martini, P., Berlind, P., Caldwell, N., Challis, P., Brown, W. R., Schild, R., Krisciunas, K., Calkins, M. L., Lee, J. C., Hathi, N., Jansen, R. A., Windhorst, R., Echevarria, L., Eisenstein, D. J., Pindor, B., Olszewski, E. W., , Harding, P., et al. (2003). Spectroscopic discovery of the supernova 2003dh associated with GRB 030329. Astrophysical Journal Letters, 591(1 II), L17-L20.
  More info

  Abstract: We present early observations of the afterglow of GRB 030329 and the spectroscopic discovery of its associatedsupernova SN 2003dh. We obtained spectra of the afterglow of GRB 030329 each night from March 30.12 (0.6 days after the burst) to April 8.13 (UT) (9.6 days after the burst). The spectra cover a wavelength range of 350-850 nm. The early spectra consist of a power-law continuum (Fv ∝ v-0.9) with narrow emission lines originating from H II regions in the host galaxy, indicating a low redshift of z = 0.1687. However, our spectra taken after 2003 April 5 show broad peaks in flux characteristic of a supernova. Correcting for the afterglow emission, we find that the spectrum of the supernova is remarkably similar to the Type Ic "hypernova" SN 1998bw. While the presence of supernovae has been inferred from the light curves and colors of gamma-ray burst afterglows in the past, this is the first direct, spectroscopic confirmation that a subset of classical gamma-ray bursts originate from supernovae.
• Piatek, S., Pryor, C., Armandroff, T. E., & Olszewski, E. W. (2002). Structure of the Draco dwarf spheroidal galaxy. Astronomical Journal, 123(5 1757), 2511-2524.
  More info

  Abstract: This article studies the structure of the Draco dwarf spheroidal galaxy, with an emphasis on the question of whether the spatial distribution of its stars has been affected by the tidal interaction with the Milky Way, using R- and V-band CCD photometry for 11 fields. The article reports coordinates for the center, a position angle of the major axis, and the ellipticity. It also reports the results of searches for asymmetries in the structure of Draco. These results and searches for a "break" in the radial profile and for the presence of principal sequences of Draco in a color-magnitude diagram for regions more than 50′ from the center yield no evidence that tidal forces from the Milky Way have affected the structure of Draco.
• Piatek, S., Pryor, C., Olszewski, E. W., Harris, H. C., Mateo, M., Minniti, D., Monet, D. G., Morrison, H., & Tinney, C. G. (2002). Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging. I. Method and a preliminary measurement for fornax. Astronomical Journal, 124(6 1764), 3198-3221.
  More info

  Abstract: This article presents and discusses a method for measuring the proper motions of the Galactic dwarf spheroidal galaxies using images taken with the Hubble Space Telescope. The method involves fitting an effective point-spread function to the image of a star or quasi-stellar object to determine its centroid with an accuracy of about 0.005 pixels (0.25 mas) - an accuracy sufficient to measure the proper motion of a dwarf spheroidal galaxy, using images separated by just a few years. The data consist of images, dithered to reduce the effects of undersampling, taken at multiple epochs with the Space Telescope Imaging Spectrograph or the Wide Field Planetary Camera. The science fields are in the directions of the Carina, Fornax, Sculptor, and Ursa Minor dwarf spheroidal galaxies, and each has at least one quasi-stellar object whose identity has been established by other studies. The rate of change with time of the centroids of the stars of the dwarf spheroidal with respect to the centroid of the quasi-stellar object is the proper motion. Four independent preliminary measurements of the proper motion of Fornax for three fields agree within their uncertainties. The weighted average of these measurements is μ α = 49 ± 13 mas century-1 and μ δ = -59 ± 13 mas century-1. The Galactocentric velocity derived from the proper motion implies that Fornax is near perigalacticon, may not be bound to the Milky Way, and is not a member of any of the proposed streams of galaxies and globular clusters in the Galactic halo. If Fornax is bound, the Milky Way must have a mass of at least (1.6 ± 0.8) × 1012M⊙.
• Dohm-Palmer, R. C., Helmi, A., Morrison, H., Mateo, M., Olszewski, E. W., Harding, P., Freeman, K. C., Norris, J., & Shectman, S. A. (2001). Mapping the galactic halo. V. Sagittarius dwarf spheroidal tidal debris 60° from the main body. Astrophysical Journal Letters, 555(1 PART 2), L37-L40.
  More info

  Abstract: As part of the Spaghetti Project Survey, we have detected a concentration of giant stars well above expectations for a smooth halo model. The position (l ∼ 350°, b ∼ 50°) and distance (∼50 kpc) of this concentration match those of the northern overdensity detected by the Sloan Digital Sky Survey. We find additional evidence for structure at ∼80 kpc in the same direction. We present radial velocities for many of these stars, including the first published results from the 6.5 m Magellan telescope. The radial velocities for stars in these structures are in excellent agreement with models of the dynamical evolution of the Sagittarius dwarf tidal debris, whose center is 60° away. The metallicity of stars in these streams is lower than that of the main body of the Sgr dwarf, which may indicate a radial metallicity gradient prior to disruption.
• Dolphin, A. E., Walker, A. R., Hodge, P. W., Mateo, M., Olszewski, E. W., Schommer, R. A., & Suntzeff, N. B. (2001). Old stellar populations of the small magellanic cloud. Astrophysical Journal Letters, 562(1 PART I), 303-313.
  More info

  Abstract: We present WFPC2 and ground-based VI photometry of NGC 121 and a nearby field in the outer SMC. For NGC 121, we measure a true distance modulus of μ0 = 18.96 ± 0.04 (distance of 61.9 ± 1.1 kpc), age of 10.6 ± 0.5 Gyr, metallicity of [Fe/H] = -1.03 ± 0.06, and initial mass of 4.1 ± 0.4 × 105 M⊙, assuming a Salpeter IMF with lower cutoff at 0.1 M⊙. In the outer SMC field, we find evidence of stars covering a wide range of ages - from 2 Gyr to at least 9-12 Gyr old. We have measured the distance, extinction, and star formation history (past star formation rates and enrichment history) using a CMD-fitting algorithm. The distance modulus of the SMC is measured to be μ0 = 18.88 ± 0.08, corresponding to a distance of 59.7 ± 2.2 kpc. The overall star formation rate appears to have been relatively constant over this period, although there may be small gaps in the star-forming activity too small to be resolved. The lack of current star-forming activity is a selection effect, as the field was intentionally chosen to avoid recent activity. The mean metallicity of this field has increased from an average of [Fe/ H] = -1.3 ± 0.3 for stars older than 8 Gyr to [Fe/H] = -0.7 ± 0.2 in the past 3 Gyr. © 2001. The American Astronomical Society. All rights reserved.
• Harding, P., Morrison, H. L., Olszewski, E. W., Arabadjis, J., Mateo, M., Dohm-Palmer, R., Freeman, K. C., & Norris, J. E. (2001). Mapping the Galactic halo. III. Simulated observations of tidal streams. Astronomical Journal, 122(3), 1397-1419.
  More info

  Abstract: We have simulated the evolution of tidal debris in the Galactic halo in order to guide our ongoing survey to determine the fraction of halo mass accreted via satellite infall. Contrary to naive expectations that the satellite debris will produce a single narrow velocity peak on a smooth distribution, there are many different signatures of substructure, including multiple peaks and broad but asymmetrical velocity distributions. Observations of the simulations show that there is a high probability of detecting the presence of tidal debris with a pencil-beam survey of 100 deg2. In the limiting case of a single 107 M⊙ satellite contributing 1% of the luminous halo mass the detection probability is a few percent using just the velocities of 100 halo stars in a single 1 deg2 field. The detection probabilities scale with the accreted fraction of the halo and the number of fields surveyed. There is also surprisingly little dependence of the detection probabilities on the time since the satellite became tidally disrupted, or on the initial orbit of the satellite, except for the time spent in the survey volume.
• Morrison, H. L., Olszewski, E. W., Mateo, M., Norris, J. E., Harding, P., Dohm-Palmer, R., & Freeman, K. C. (2001). Mapping the galactic halo. IV. Finding distant giants reliably with the Washington system. Astronomical Journal, 121(1), 283-294.
  More info

  Abstract: We critically examine the use of the Washington photometric system (with the DDO51 filter) for identifying distant halo giants. While this is the most powerful photometric technique for isolating G and K giant stars, spectroscopic follow-up of giant candidates is vital. There are two situations in which interlopers outnumber genuine giants in the diagnostic M - 51/M -T2 plot and are indistinguishable photometrically from the giants. (1) In deep surveys covering tens of square degrees, very metal-poor halo dwarfs are a significant contaminant. An example is our survey of the outer halo, where these metal-poor dwarfs dominate the number of photometric giant candidates at magnitudes fainter than V = 18 and cannot be isolated photometrically. (2) In deep surveys of smaller areas with low photometric precision, most objects in i:he giant region of the color-color plot are dwarfs whose photometric errors have moved them there. Color errors in M - 51 and M - T2 need to be smaller than 0.03 mag to avoid this problem. An example of a survey whose photometric errors place the giant identifications under question is the survey for extratidal giants around the Carina dwarf spheroidal galaxy of Majewski et al. Accurate photometry and spectroscopic follow-up of giant candidates are essential when using the Washington system to identify the rare outer halo giants.
• Piatek, S., Pryor, C., Armandroff, T. E., & Olszewski, E. W. (2001). Stars of the Draco dwarf spheroidal galaxy beyond its measured tidal boundary. Astronomical Journal, 121(2), 841-860.
  More info

  Abstract: We report R-and V-band photometry derived from CCD imaging for objects in nine fields in and around the Draco dwarf spheroidal galaxy. The most distant fields are about 1°.3 from the center. We use these data to search for Draco stars outside of its measured tidal boundary. The search involves three methods: (1) plotting color-magnitude diagrams for individual fields, for sections of fields, and for combined fields and sections - a colour-magnitude diagram can reveal a population of Draco stars by the presence of the expected principal sequences; (2) measuring field-to-field fluctuations in the surface density of objects located near the Draco principal sequences in the color-magnitude diagram; (3) measuring intrafield fluctuations in the surface density of those objects. We find evidence for the presence of Draco stars immediately beyond the measured tidal boundary of Draco and place an upper limit on the number of such stars in more distant fields that lie close to the extension of its major axis. The best evidence is the presence of the Draco principal sequences in the color-magnitude diagram for some combined fields and sections of fields. The measurements of the field-to-field fluctuations in the stellar surface density confirm this result.
• Côté, P., Mateo, M., Sargent, W. L., & Olszewski, E. W. (2000). An improved mass determination for M31 from its satellite galaxies. Astrophysical Journal Letters, 537(2 PART 2), L91-L94.
  More info

  Abstract: The High-Resolution Echelle Spectrometer on the Keck I telescope has been used to measure the first radial velocities for stars belonging to the Andromeda I and III dwarf spheroidal galaxies. Our radial velocity for And III matches that reported by Blitz & Robishaw for an adjacent H I gas complex, supporting the association of this galaxy with a high-velocity cloud. New and previously published radial velocities for a sample of confirmed or suspected M31 satellites are combined with a homogeneous set of distance estimates to calculate the total mass of M31. Assuming the satellite orbits are isotropic, we find a median mass of MA ≃ (7.9 ± 0.5) × 1011 M⊙ from eight candidate satellites having deprojected distances from M31 in excess of RA ≃ 100 kpc. If the orbits are radial, the inferred mass increases to MA ≃ (21.5 ± 3.8) × 1011 M⊙ for circular orbits, the enclosed mass is MA ≃ (3.7 ± 0.4) × 1011 M⊙. These masses are somewhat lower than those found for luminous spiral galaxies based on the ensemble dynamics of the their satellites, although the estimates are nevertheless consistent given the large uncertainties.
• Dohm-Palmer, R., Mateo, M., Olszewski, E., Morrison, H., Harding, P., Freeman, K. C., & Norris, J. (2000). Mapping the Galactic halo. II. Photometric survey. Astronomical Journal, 120(5), 2496-2512.
  More info

  Abstract: We present imaging results from a high Galactic latitude survey designed to examine the structure of the Galactic halo. The objective of the survey is to identify candidate halo stars which can be observed spectroscopically to obtain radial velocities and confirm halo membership. The Washington filter system is used for its ability to distinguish between dwarfs and giants, as well as provide a metallicity indicator. Our most successful imaging run used the BTC camera on the CTIO 4 m telescope in 1999 April. Photometric conditions during these observations provided superb photometry, with average errors for a star at M = 18.5 of 0.009, 0.008, 0.011, and 0.009 for C, M, DDO51, and T2, respectively. We use these data as a template to describe the details of our photometric reduction process. It is designed to perform CCD reductions and stellar photometry automatically during the observation run, without the aid of external packages, such as IRAF and IDL. We describe necessary deviations from this procedure for other instruments used in the survey up to 2000 June. Preliminary results from spectroscopic observations indicate a 97% efficiency in eliminating normal dwarfs from halo giant candidates for M < 18.5. Unfortunately, low-metallicity subdwarfs cannot be photometrically distinguished from giants using the Washington filters. These major contaminants unavoidably reduced the overall giant identification efficiency to 66% for M < 18.5. Our improved knowledge of these stars will increase this efficiency for future spectroscopic observations.
• Morrison, H. L., Mateo, M., Olszewski, E. W., Harding, P., Dohm-Palmer, R., Freeman, K. C., Norris, J. E., & Morita, M. (2000). Mapping the galactic halo. I. The "spaghetti" survey. Astronomical Journal, 119(5), 2254-2273.
  More info

  Abstract: We describe a major survey of the Milky Way halo designed to test for kinematic substructure caused by destruction of accreted satellites. We use the Washington photometric system to identify halo stars efficiently for spectroscopic follow-up. Tracers include halo giants (detectable out to more than 100 kpc), blue horizontal-branch (BHB) stars, halo stars near the main-sequence turnoff, and the "blue metal-poor stars" of Preston, Beers, & Shectman. We demonstrate the success of our survey by showing spectra of stars we have identified in all these categories, including giants as distant as 75 kpc. We discuss the problem of identifying the most distant halo giants. In particular, extremely metal-poor halo K dwarfs are present in approximately equal numbers to the distant giants for V > 18, and we show that our method will distinguish reliably between these two groups of metal-poor stars. We plan to survey 100 deg2 at high Galactic latitude and expect to increase the numbers of known halo giants, BHB stars, and turnoff stars by more than an order of magnitude. In addition to the strong test that this large sample will provide for the question, Was the Milky Way halo accreted from satellite galaxies? we will improve the accuracy of mass measurements of the Milky Way beyond 50 kpc via the kinematics of the many distant giants and BHB stars we find. We show that one of our first data sets constrains the halo density law over Galactocentric radii of 5-20 kpc and z-heights of 2-15 kpc. The data support a flattened power-law halo with b/a of 0.6 and exponent -3.0. More complex models with a varying axial ratio may be needed with a larger data set.
• Cook, K. H., Mateo, M., Olszewski, E. W., Vogt, S. S., Stubbs, C., & Diercks, A. (1999). The systemic velocity and internal kinematics of the dwarf galaxy LGS 3: An optical foray beyond the milky way. Publications of the Astronomical Society of the Pacific, 111(757), 306-312.
  More info

  Abstract: We have obtained radial velocities of three K giants and one faint carbon star in LGS 3, a dwarf companion of M31, based on 12 individual spectra obtained with the HIRES spectrograph on the Keck I telescope. The mean precision of these measurements is 3.8 km s-1. The mean systemic velocity of LGS 3 is -282.2 ± 3.5 km s-1. Monte Carlo simulations that take into account the individual velocity uncertainties and the maximum observed velocity difference reveal that the central velocity dispersion of LGS 3 is in the range 2.6-30.5 km s-1, with 95% confidence; the most likely value for the central dispersion is 7.9+5.3-2.9 km s-1. These results agree with the kinematics of H I gas in LGS 3. This contrasts with the tendency for the gas and stars in other low-luminosity Local Group dwarfs to exhibit distinct spatial and kinematic properties. Taking into account the relative youth of LGS 3, we conclude that the " asymptotic " M/L ratio - the value the galaxy would exhibit if it were composed only of ancient stars - is M/Lv,LGS3 ≥ 11 (at a 97.5% confidence level), with a most probable value of 95+175-56. These values are consistent with the M/Lv ratios observed in other well-studied early-type dwarfs of the Local Group. We have also estimated the mass of LGS 3 using modified Newtonian dynamics. These data represent the first moderately high precision optical spectra of giants in a dwarf system beyond the Galactic halo. We suggest future studies that are now feasible to study the dynamics of dwarf galaxies throughout the Local Group and beyond.
• Côté, P., Mateo, M., Olszewski, E. W., & Cook, K. H. (1999). Internal kinematics of the Andromeda II dwarf spheroidal galaxy. Astrophysical Journal Letters, 526(1 PART 1), 147-151.
  More info

  Abstract: The High-Resolution Echelle Spectrometer on the Keck I telescope has been used to measure nine radial velocities having a median precision of ≃2 km s-1 for seven red giants belonging to the isolated dwarf spheroidal galaxy Andromeda II (And II). We find a weighted mean radial velocity of v̄r = -188 ± 3 km s-1 and a central velocity dispersion of σ0 = 9.3+2.7-2.6 km s-1. There may be evidence for a radial velocity gradient across the face of the galaxy, although the significance of this result is low because of the small number of stars having measured velocities. Our best estimate for the global mass-to-light ratio of And II is M/Lv = 20.9+13.9-10.1 M ⊙/Lv, ⊙. This value is similar to those of several Galactic dwarf spheroidal galaxies and is consistent with the presence of a massive dark halo in And II.
• Gallart, C., Freedman, W. L., Mateo, M., Chiosi, C., Thompson, I. B., Aparicio, A., Bertelli, G., Hodge, P. W., Lee, M. G., Olszewski, E. W., Saha, A., Stetson, P. B., & Suntzeff, N. B. (1999). Hubble Space Telescope observations of the local group dwarf galaxy Leo I. Astrophysical Journal Letters, 514(2 PART 1), 665-674.
  More info

  Abstract: We present deep HST F555W (V) and F814W (I) observations of a central field in the Local Group dwarf spheroidal (dSph) galaxy Leo I. The resulting color-magnitude diagram (CMD) reaches I ≃ 26 and reveals the oldest ≃ 10-15 Gyr old turnoffs. Nevertheless, a horizontal branch is not obvious in the CMD. Given the low metallicity of the galaxy, this likely indicates that the first substantial star formation in the galaxy may have been somehow delayed in Leo I in comparison with the other dSph satellites of the Milky Way. The subgiant region is well and uniformly populated from the oldest turnoffs up to the 1 Gyr old turnoff, indicating that star formation has proceeded in a continuous way, with possible variations in intensity but no big gaps between successive bursts, over the galaxy's lifetime. The structure of the red clump of core He-burning stars is consistent with the large amount of intermediate-age population inferred from the main sequence and the subgiant region. In spite of the lack of gas in Leo I, the CMD clearly shows star formation continuing until 1 Gyr ago and possibly until a few hundred Myr ago in the central part of the galaxy.
• Olszewski, E. W. (1999). Star clusters in local group galaxies. Astrophysics and Space Science, 267(1-4), 201-213.
• Mateo, M., Olszewski, E. W., & Morrison, H. L. (1998). Tracing the outer structure of the Sagittarius dwarf galaxy: Detections at angular distances between 10° and 34°. Astrophysical Journal Letters, 508(1 PART II), L55-L59.
  More info

  Abstract: We have obtained deep photometric data in 24 fields along the southeast extension of the major axis of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy and in four fields along the northwest extension. Using star counts at the expected position of the Sgr upper main sequence within the resulting color-magnitude diagrams, we unambiguously detect Sgr stars in the southeast over the range 10°-34° from the galaxy's center. If Sgr is symmetric, this implies a true major-axis diameter of at least 68°, or nearly 30 kpc if all portions of Sgr are equally distant from the Sun. Star counts parallel to the galaxy's minor axis reveal that Sgr remains quite broad far from its center. This suggests that the outer portions of Sgr resemble a stream rather than an extension of the ellipsoidal inner regions of the galaxy. The inferred V-band surface brightness (SB) profile ranges from 27.3 to 30.5 mag arcsec-2 over this radial range and exhibits a change in slope ∼20° from the center of Sgr. The scale length of the outer SB profile is 17°.2, compared with 4°.7 in the central region of Sgr. We speculate that this break in the SB profile represents a transition from the main body of Sgr to a more extended "Sgr stream." By integrating the SB profile, we estimate that the absolute visual magnitude of Sgr lies in the range -13.4 to - 14.6, depending on the assumed structure of Sgr; an upper limit to the luminosity of Sgr is therefore L∼ 5.8 × 107 L⊙. This result lowers the M/LV ratio inferred for Sgr down to ∼10, which is consistent with values observed in the most luminous dSph companions of the Milky Way. © 1998. The American Astronomical Society. All rights reserved.
• Mateo, M., Olszewski, E. W., Vogt, S. S., & Keane, M. J. (1998). The internal kinematics of the Leo I dwarf spheroidal galaxy: Dark matter at the fringe of the milky way. Astronomical Journal, 116(5), 2315-2327.
  More info

  Abstract: We present radial velocities of 33 red giants in the Leo I dwarf spheroidal galaxy (dSph) obtained from spectra taken with the HIRES echelle spectrograph on the Keck Telescope. These data have a mean precision of 2.2 km s-1 and lead to estimates of the central velocity dispersion and systemic velocity of Leo I of 8.8 + 1.3 km s-1 and 287.0 ± 1.9 km s-1, respectively. The systemic velocity confirms past results that Leo I has an unusually large galactocentric velocity, implying the presence of a massive dark halo in the Milky Way or an extended dark component pervading the Local Group. The V-band M/L ratio of Leo I is in the range 3.5-5.6. We have produced a set of models that accounts for the effects of stellar evolution on the global mass-to-light ratio of a composite population. Because Leo I contains a dominant intermediate-age population, we find that the V-band mass-to-light ratio of Leo I would be in the range 6-13 if it were composed exclusively of old stars such as those found in globular clusters. This suggests that Leo I probably does contain a significant dark halo. The mass of this halo is approximately 2 × 107 M⊙, similar to the dark halo masses inferred for all other galactic dSph galaxies. Because Leo I is isolated and has passed the Milky Way at most once in the past, external tides could not plausibly have inflated its central dispersion to the observed value. We also considered whether modified Newtonian dynamics (MOND) could account for the internal kinematics of Leo I and conclude that this alternative gravitational model can account for the Leo I kinematics adequately without requiring a dark halo.
• Olsen, K. A., Hodge, P. W., Mateo, M., Olszewski, E. W., Schommer, R. A., Suntzeff, N. B., & Walker, A. R. (1998). HST colour-magnitude diagrams of six old globular clusters in the LMC. Monthly Notices of the Royal Astronomical Society, 300(3), 665-685.
  More info

  Abstract: We report on HST observations of six candidate old globular clusters in the Large Magellanic Cloud (LMC): NGC 1754, 1835, 1898, 1916, 2005 and 2019. Deep exposures with the F555W and F814W filters provide us with colour-magnitude diagrams that reach to an apparent magnitude in V of ∼25, well below the main-sequence turn-off. These particular clusters are associated with significantly high LMC field star densities and care was taken to subtract the field stars from the cluster colour-magnitude diagrams accurately. In two cases there is significant variable reddening across at least part of the image, but only for NGC 1916 does the differential reddening preclude accurate measurements of the CMD characteristics. The morphologies of the colour-magnitude diagrams match well those of Galactic globular clusters of similar metallicity. All six have well-developed horizontal branches, while four clearly have stars on both sides of the RR Lyrae gap. The abundances obtained from measurements of the height of the red giant branch above the level of the horizontal branch are 0.3 dex higher, on average, than previously measured spectroscopic abundances. Detailed comparisons with Galactic globular cluster fiducials show that all six clusters are old objects, very similar in age to classical Galactic globulars such as M5, with little age spread among the clusters. This result is consistent with ages derived by measuring the magnitude difference between the horizontal branch and main-sequence turn-off. We also find a similar chronology by comparing the horizontal branch morphologies and abundances with the horizontal branch evolutionary tracks of Lee, Demarque & Zinn. Our results imply that the LMC formed at the same time as the Milky Way Galaxy.
• Minniti, D., Liebert, J., Olszewski, E. W., & D., S. (1996). Background giants in the field of the globular cluster M22: Kinematics of the galactic bulge. Astronomical Journal, 112(2), 590-600.
  More info

  Abstract: We report radial velocities, metallicities, and near-IR photometry for about 100 giants in the field of the globular cluster M22, at l,b=(9.9°,-7.6°). Most of them are bulge giants, non-members of the cluster from proper motions. Dividing the sample according to metallicity, we find that the giants more metal-poor than [Fe/H]≈-1 have halo-like kinematics, while the more metal-rich giants have higher rotation and smaller velocity dispersion, which we identify with the bulge. Such dependences of kinematics on metal abundance (which may be due to a mix of populations) are predicted by dissipational collapse models of formation for the inner Galaxy. The 11 M giants in the field follow bulge rather than halo kinematics. © 1996 American Astronomical Society.
• Olszewski, E. W., Pryor, C., & Armandroff, T. E. (1996). The mass-to-light ratios of the Draco and Ursa minor dwarf spheroidal galaxies. II. The binary population and its effect on the measured velocity dispersions of dwarf spheroidals. Astronomical Journal, 111(2), 750-767.
  More info

  Abstract: We use a large set of radial velocities in the Ursa Minor and Draco dwarf spheroidal galaxies to search for binary stars and to infer the binary frequency. Of the 118 stars in our sample with multiple observations, six are velocity variables with χ2 probabilities below 0.001. We use Monte Carlo simulations that mimic our observations to determine the efficiency with which our observations find binary stars. Our best, though significantly uncertain, estimate of the binary frequency for stars near the turnoff in Draco and UMi is 0.2-0.3 per decade of period in the vicinity of periods of one year, which is 3-5X that found for the solar neighborhood. This frequency is high enough that binary stars might significantly affect the measured velocity dispersions of some dwarf spheroidal galaxies according to some previous numerical experiments. However, in the course of performing our own experiments, we discovered that this previous work had inadvertently overestimated binary orbital velocities. Our first set of simulations of the effects of binaries is based on the observed scatter in the individual velocity measurements for the multiply-observed Draco and Ursa Minor stars. This scatter is small compared to measured velocity dispersions and, so, the effect of binaries on the dispersions is slight. This result is supported by our second set of experiments, which are based on a model binary population normalized by the observed binary frequency in Draco and Ursa Minor. We conclude that binary stars have had no significant effect on the measured velocity dispersion and inferred mass-to-light ratio of any dwarf spheroidal galaxy. © 1996 American Astronomical Society.
• Olszewski, E. W., Suntzeff, N. B., & Mateo, M. (1996). Old and intermediate-age stellar populations in the Magellanic Clouds. Annual Review of Astronomy and Astrophysics, 34(1), 511-550.
  More info

  Abstract: The Magellanic Clouds have galactocentric distances of 50 and 63 kiloparsecs, making it possible to probe the older populations of clusters and stars in some detail. Although it is clear that both galaxies contain an old population, it is not yet certain whether this population is coeval with the date of formation of the oldest globulars in the Milky Way. The kinematics of this old population in the Large Magellanic Cloud (LMC) are surprising; no component of this old population is currently measured to be part of a hot halo supported by velocity dispersion. Spectroscopy of field stars is beginning to show the existence of a small population of stars with abundances [Fe/H] less than -1.4. These stars will help to unravel the star-formation history when the next generation of telescopes are commissioned. Asymptotic giant branch stars, long-period variables, planetary nebulae, and horizontal-branch clump stars can be used to trace the extent and kinematics of the intermediate-age population. Deep color-magnitude diagrams can be used to derive the relative proportions of stars older than 1 Gyr. The age distribution of populous clusters and the age-metallicity relation are used to compare the evolution of the two Magellanic Clouds to each other. The issue of where the LMC's metals originated is explored, as is the question of what triggers star formation in the Clouds.
• Armandroff, T. E., Olszewski, E. W., & Pryor, C. (1995). The mass-to-light ratios of the draco and ursa minor dwarf spheroidal galaxies. I. Radial velocities from multifiber spectroscopy. Astronomical Journal, 110(5), 2131-2165.
  More info

  Abstract: We have measured 206 radial velocities for 94 probable members in the Ursa Minor dwarf spheroidal galaxy and 167 velocities for 91 probable members in the Draco dwarf spheroidal galaxy. These velocities were obtained using the KPNO 4 m telescope with the Hydra multifiber positioner and bench spectrograph in 1992, 1993, and 1994. Both simulations and repeated measurements show that the median uncertainty in these velocities is 4.0 km/s. Our velocities are compared with the other major datasets for Draco and Ursa Minor. Aside from a small zero-point difference, there is excellent agreement between the Hydra velocities and the Olszewski et al. MMT echelle velocities. Comparison with the Hargreaves et al. Ursa Minor velocities reveals inconsistencies which we ascribe to underestimated errors in the Hargreaves et al. data. After correcting for zero-point differences, we combine the three major sets of velocities. Our spectra recovered all of the known Carbon (C) stars in Draco and Ursa Minor and revealed one new C star in Draco and two in Ursa Minor. In addition, one star whose velocity is consistent with membership in Ursa Minor has a deep and wide Hβ absorption line in its spectrum. An initial discussion of the kinematics of the two galaxies is presented. The systemic velocities of Draco and Ursa Minor are -293.3±1.0 and -247.4±1.0 km s-1, respectively. Ursa Minor shows apparent rotation about a position angle of 75° with an amplitude of about 3 km s-1; the morphological major axis is 530°±5°. No such rotation is found in Draco. We find a velocity dispersion of 10.4±0.9 km s-1 for Ursa Minor including all stars and 8.8±0.8 km s-1 excluding the star with the most extreme velocity. In Draco we find 10.7±0.9 km s-1 including all stars and 8.5±0.7 km s-1 excluding the three stars with the most extreme velocities. These dispersions are consistent with the dispersions measured in the other two studies and confirm that Draco and Ursa Minor have the largest mass-to-light ratios of any of the dwarf spheroidals. © 1995 American Astronomical Society.
• Fullton, L. K., Carney, B. W., Olszewski, E. W., Zinn, R., Demarque, P., Janes, K. A., Costa, G. D., & Seitzer, P. (1995). A VI_C color-magnitude diagram of the globular cluster NGC 6352 from Hubble Space Telescope planetary camera observations. Astronomical Journal, 110(2), 652-681.
  More info

  Abstract: The globular cluster NGC 6352, which on the basis of its Galactic position, radial velocity, and [Fe/H] is a member of the (thick) disk system of globular clusters, has been observed to below the main-sequence turnoff (MSTO) using the Hubble Space Telescope. These observations, which were obtained before the repair mission, were analyzed using the flux-conserving iterative/recursive deconvolution algorithm developed at the University of North Carolina. This algorithm can produce more precise photometry than standard PSF-fitting methods with the aberrated images. The V, (V-IC) color-magnitude diagram constructed from these observations places the cluster turnoff at V=18.80±0.10. Observations with the CTIO 0.9 m telescope have been used to photometer the brighter stars in the cluster, providing a calibration of the HST data and an estimate of the cluster's reddening, E(B-V)=0.21±0.03. From spectroscopic observations of the strengths of the Ca II triplet lines in red giants in the cluster and from previous measurements in the literature, we find that NGC 6352 is only slightly more metal rich (Δ[Fe/H]=0.08±0.05) than the prototypical disk globular cluster, 47 Tuc. From the difference in V magnitude between the horizontal branch and the MSTO, we find that NGC 6352 is essentially the same age as 47 Tuc (formally, older by 0.7±2.2 Gyr). Comparisons with the latest Yale isochrones support this result and yield 14.5±2 Gyr for the age of 47 Tuc from the photometry of Hesser et al. [PASP, 99, 739 (1987)], which is consistent with other determinations. The old age obtained for NGC 6352 provides additional evidence that the disk system of globular clusters is very old and is in fact older or comparable in age to several globular clusters populating the Galactic halo. © 1995 American Astronomical Society.
• Nemec, J. M., Mateo, M., Burke, M., & Olszewski, E. W. (1995). SX phoenecis stars in the extremely metal-poor globular cluster NGC 5053. Astronomical Journal, 110(3), 1186-1228.
  More info

  Abstract: The results of a major search for photometrically variable blue straggler stars (BSs) in the extremely metal-poor globular cluster NGC 5053 are presented. The survey is based on photometry of over 200 CCD frames (BVI passbands) taken on 18 nights between 1985 and 1994. Five of the 16 BSs monitored for variability are identified as SX Phe stars and their photometric characteristics derived. These five stars are among the shortest-period (49
• Olszewski, E. W., Aaronson, M., & Hill, J. M. (1995). Nine seasons of velocity measurements in the draco and ursa minor dwarf spheroidal galaxies with the MMT echelle. Astronomical Journal, 110(5), 2120-2130.
  More info

  Abstract: We have used the Multiple Mirror Telescope echelle spectrograph to measure 112 velocities of 42 stars in the Draco and Ursa Minor dwarf spheroidal galaxies and three velocities of three foreground stars between 1982 April and 1990 September. We used 11 A resolution spectra obtained with the MX multifiber spectrograph at the Steward 90″ to find additional giant candidates; 5 UMi and 13 Draco stars were then observed at the MMT and added to the original sample of velocity members. In addition, the MX spectra were used to eliminate 74 stars in the direction of UMI and 59 stars in Draco as likely foreground dwarfs. We detected 7 velocity variables, defined as those stars whose probability of exceeding the measured χ2 by chance is less than 1.5%. Three of these stars are Carbon (C) stars (UMi K and VA 335 and Draco C1); two have emission lines (Draco C1 and UMi M). We show that the C star Draco C4, with a proper motion membership probability of 7%, has a velocity consistent with membership. It is not surprising that these C (most likely CH) stars are binaries because McClure has shown that most Galactic CH stars are in binary systems. Of the remaining 35 stars, only 4 are velocity variables, with measured velocity extrema of 29.1 km s-1 (UMi M), 7.2 km s-1 (Draco XI-2), 9.0 km s-1 (Draco 24), and 8.3 km s-1 (Draco 473). The velocity dispersions are 10.1±1.7 km s-1 for UMi, and 9.9±1.4 km s-1 for Draco. These dispersions change to 10.5±2.0 for UMi, and 8.2±1.3 for Draco if we eliminate the velocity variables. Our dispersion for UMi differs from that of Hargreaves et al. [MNRAS, 271, 693 (1994b)] by 1.3σ of the combined errors. These velocities are combined with the one-component King models of Pryor & Kormendy [AJ, 100, 127 (1990)] to give script M sign/L= 73 for UMi, and 77 for Draco. © 1995 American Astronomical Society.
• Vogt, S. S., Mateo, M., Olszewski, E. W., & Keane, M. J. (1995). Internal kinematics of the Leo II dwarf spheroidal galaxy. Astronomical Journal, 109(1), 151-163.
  More info

  Abstract: We have obtained radial velocities of 31 red giants in the Leo II dwarf spheroidal (dSph) galaxy using the HIRES echelle spectrograph of the Keck 10 m telescope. These stars were selected using CCD photometry obtained with the Palomar 1.5 m telescope. The radial velocity errors were determined from repeat measurements of Leo II giants and faint radial velocity standards; the average error for the Leo II giants is 2.4 km s-1. The systemic velocity of Leo II is 76.0±1.3 km s-1, and the velocity dispersion is 6.7±1.1 km s-1. Adopting the standard assumptions (mass follows light, isotropy, and equilibrium) we derive a "global" V-band mass-to-light ratio (Mtot/Ltot) = 11.1±3.8, and a central value (ρ0/I0) = 11.6±3.2 (both in solar units). These values are larger than the M/L ratios measured for "local" stellar populations (star clusters or the solar neighborhood). Monte Carlo simulations show that a large population of binaries probably has not inflated a small intrinsic velocity dispersion to the observed value. A model-independent lower limit of the central mass density of Leo II yields ρ0,min=0.074M⊙ pc-3, larger than the central luminous mass density in Leo II for (M/L)1um=2.0. This demands an extensive dark halo in the outer regions of the galaxy to account for the observed dispersion. We conclude that Leo II contains a significant dark matter component. The total mass of the dark halo is 0.9 × 107 M⊙, consistent with the masses inferred in all well-studied dSph galaxies. Because it is remote from the Galaxy, tides cannot plausibly affect the velocity dispersion of Leo II. However, we cannot use these results to rule out the possibility that tides may influence the kinematics of other dSph galaxies found closer to the Milky Way.
• Mateo, M., Olszewski, E. W., Pryor, C., Welch, D. L., & Fischer, P. (1993). The Carina dwarf spheroidal galaxy: How dark is it?. Astronomical Journal, 105(2), 510-526.
  More info

  Abstract: We have obtained precise (〈σ〉=3.1 km s-1) radial velocities of 23 photometrically-selected normal giant stars located near the center of the Carina dwarf spheroidal (dSph) galaxy. Of this sample, 17 stars are Carina members based on their heliocentric radial velocities in excess of 200 km s-1. From this subsample, we derive a mean systemic velocity for Carina of 223.1±1.8 km s-1, and a central velocity dispersion of 6.8 km s-1. Using the luminosity function of the brightest stars in Carina, we have estimated the central surface brightness of the galaxy to be Σ0,V=25.2±0.3. Assuming the mass distribution follows that of the visible material in Carina, we derived the V band mass-to-light ratio to be (script M sign/L)V=39±23, based on published core parameters of the galaxy and using estimates of the integrated mass and luminosity. We have also fit the kinematic and structural data for Carina to a variety of one- and two-component Michie-King models. Those models in which the velocity dispersion of the visible matter is extremely anisotropic are ruled out by the surface brightness profile; acceptable fits correspond to models with global V-band script M sign/L ratios in excess of 35. These fits do not allow us to place any constraints on the distribution of dark matter in Carina except that the central mass density must be ≳0.02 script M sign⊙pc-3. We consider and reject the possibility that atmospheric or binary motions among the program stars may have artificially inflated a small intrinsic velocity dispersion to the observed value. The inferred dark matter content of Carina is consistent with the overall trend of increasing script M sign/L ratio with decreasing total luminosity exhibited by the best-studied dSph galaxies. We conclude that an assumed highly radially anisotropic velocity dispersion tensor in low-luminosity dSph galaxies is unlikely to be able to reduce the script M sign/L ratios within the visible volumes of these systems significantly. Similarly, we conclude that tidal forces probably do not play a significant role in generating the large range of script M sign/L ratios inferred for dSph systems. The current data for dSph galaxies are consistent with the conjecture that these systems all have similar dark matter halos with total masses in the range 1-5×107 script M sign⊙.
• Suntzeff, N. B., Mateo, M., Terndrup, D. M., Olszewski, E. W., Geisler, D., & Weller, W. (1993). Spectroscopy of giants in the Sextans dwarf spheroidal galaxy. Astrophysical Journal Letters, 418(1), 208-228.
  More info

  Abstract: We have obtained spectra in the region of the Ca II infrared triplet at 8500 Å for 80 stars in the direction of the Sextans dwarf spheroidal galaxy. We have found 43 radial velocity members of the Sextans galaxy and 36 field stars The mean velocity of the Sextans galaxy is 227.9 ± 1.8 km s-1, in agreement with the previously determined value of 230 ± 6 km s-1 from Da Costa et al. (1991). We find that the intrinsic velocity dispersion of Sextans is 6.2 ± 0.9 km s-1 based on 33 stars. The two standard methods for estimating the total mass-to-light ratio yield (M/L)tot = 54+44-24 and 30+20-13, implying significant amounts of dark matter are present, although we cannot rule out binary contamination as the cause of the large observed velocity dispersion with only a single epoch of observations. The mean metallicity of Sextans is 〈[Fe/H]〉 = -2.05 ± 0.04, with an intrinsic star-to-star metallicity dispersion of 0.19 ± 0.02 dex in [Fe/H]. The abundance measurements based on the Ca II line strengths are found to be correlated with the color of the star, supporting the existence of a real metallicity dispersion. The mean metallicity is in excellent agreement with the general trend of metallicity-absolute galaxian magnitude for the dwarf spheroidal galaxies that are satellites to the Milky Way galaxy. The horizontal branch of Sextans is quite red for the derived mean metallicity implying that Sextans, like Draco, is a "second parameter" object that may be a few Gyr younger than the typical Galactic globular cluster.
• Welch, D. L., Mateo, M., Olszewski, E. W., Fischer, P., & Takamiya, M. (1993). The variable stars of the young LMC cluster NGC 2164. Astronomical Journal, 105(1), 146-154.
  More info

  Abstract: The young, populous Large Magellanic Cloud (LMC) cluster NGC 2164 has been searched for variable stars. From (up to) 64 epochs of new BV CCD photometry obtained between December 1989 and December 1991 we have discovered one new member (overtone) classical Cepheid variable with a period of 3.772 days. We also present photometry for a previously unknown field overtone Cepheid variable with a period of 3.4626 days and the longer-period HV 12078 which has a period of 10.6878 days. It is suggested that HV 12078 may be a member of the nearby, young cluster NGC 2156 which is also contained within our field on most frames. We have investigated the lightcurve shape of the NGC 2164 member and find that its Fourier phase φ21 = 3.1 which is in agreement with the sequence of longer-period s-Cepheids suggested by Antonello et al. [A&A, 236, 138 (1990)]. The clear separation of fundamental and overtone pulsators in the period-luminosity-color relation of known LMC cluster Cepheids leaves no doubt that the NGC 2164 member is a true overtone. Our best images of NGC 2164 (with the smallest FWHM) reveal the superluminous giant candidates of Flower & Hodge (1975) to have double or multiple components, suggesting that (in this cluster, at least) these objects can be accounted for by near-superpositions of ordinary stars. We discuss a possible origin for their appearance in earlier color-magnitude diagrams. We speculate that the increased dispersion observed in the Small Magellanic Cloud infrared period-luminosity (P-L) and period-luminosity-color (P-L-C) relations, relative to those of the LMC, may be due to a different long-period cutoff in the period distribution of overtone pulsators.
• Minniti, D., D., S., Olszewski, E. W., & Hill, J. M. (1992). Rotation of the galactic bulge. Astrophysical Journal Letters, 393(2 PART II), L47-L50.
  More info

  Abstract: We present results from spectroscopic observations of K giants in two fields towards the Galactic bulge. The mean Galactocentric velocity and velocity dispersion for a field at l, b = (8, 7) are 〈V〉 = 45 ± 10 km s-1 and σ = 85 ± 7 km s-1. For a field at l, b = (12, 3) we find 〈V〉 = 77 ± 9 km s-1 and σ = 68 ± 6 km s-1. The bulge of the Galaxy is found to be rotating, and its velocity dispersion decreases with increasing Galactocentric distance.
• Schommer, R. A., Olszewski, E. W., Suntzeff, N. B., & Harris, H. C. (1992). Spectroscopy of giants in LMC clusters. II. Kinematics of the cluster sample. Astronomical Journal, 103(2), 447-459.
  More info

  Abstract: We analyze velocities for 83 star clusters in the LMC, based on individual stellar velocities measured at the Calcium triplet. One-half of the clusters are objects in the outer parts of the LMC (radius≥5°), which had no previous velocity determinations. Published velocities for intermediate and old clusters are shown to have had systematic errors. We compare our new velocities with various rotation curve analyses of the LMC, and test aspects of the twisted disk model proposed by Freeman et al. [AJ, 272, 488 (1983) ]. Our outer cluster sample and the intermediate age inner clusters form a disk which agrees with the parameters of the optical isophotes and inner H I rotation curve. The oldest clusters still present an enigma; they do not have the kinematics of an isothermal, or slowly rotating, pressure-supported halo. These oldest objects rotate with an amplitude comparable to that of the younger disk and have a small velocity dispersion, although the parameters are poorly determined in this small sample. When the transverse motion of the LMC is taken into account, a single rotating disk solution fits the old and intermediate-aged clusters and other tracers (i.e., there is no need for an additional "tilted disk" system). In the inner 2° the old clusters still exhibit peculiar velocities, as do the CH stars and the old long-period variables. The rotation curve of the LMC does not show signs of a Keplerian falloff out to at least 5-6 disk scale lengths, implying the existence of dark matter associated with the LMC. The mass of the LMC is larger than 1.5×1010 script M sign ⊙. The integrated mass-to-light ratio of the LMC is ∼10.
• Suntzeff, N. B., Schommer, R. A., Olszewski, E. W., & Walker, A. R. (1992). Spectroscopy of giants in LMC clusters. III. Velocities and abundances for NGC 1841 and reticulum and the properties of the metal-poor clusters. Astronomical Journal, 104(5), 1743-1764.
  More info

  Abstract: Velocities and metallicities for giant star members of the distant globular clusters Reticulum (C 0435 -590) and NGC 1841 (C 0444-840) based on spectroscopy with the CTIO Argus multi-fiber system show that these two clusters are likely members of the Large Magellanic Cloud (LMC). The velocity, [Fe/H] metallicity, and absolute visual magnitude for Reticulum are 241.5±1.5 km s-1, -1.71, and -6.0±0.2, and for NGC 1841, 212.5±1.8 km s-1, -2.11, and -7.9±0.2. The error in the metallicities is ∼0.1 dex and is dominated by the uncertainties in the metallicities of the Galactic globular clusters used in the calibrations. From the velocities of 12 member giants we derive an intrinsic dispersion of 2.1-2.1+1.3 km s-1, a mass of 1.4-1.4+2.4×105 script M sign⊙, and a script M sign/LV=7.3-7.3+12.0 for Reticulum. For NGC 1841 (13 members), the dispersion is 3.6-2.1+1.4 km s-1, the mass is 2.8-2.2+2.5×105 script M sign⊙, and the script M sign/LV value is 2.4-1.9+2.2. NGC 1841 is apparently in front of the LMC by ∼5 kpc, based on the mean magnitude of the RR Lyraes. We review the properties of the LMC Population II clusters and field stars. The LMC has 13 known population II clusters. The mean metallicity of the LMC clusters is 0.3 dex lower than the ensemble average for the Galactic globular clusters, continuing the trend seen in other galaxies that the less luminous galaxies have less metal-rich globular cluster systems. However, the mean cluster metallicity, the absolute magnitude distribution of the clusters, and the relative number of RR Lyraes per unit cluster luminosity are very similar to the Galactic globular cluster population outside of the solar circle. The ratio of luminous mass in the clusters to that in the field population II is 0.02 for both the LMC and the Galaxy. We also find that the total number of Population II clusters, the total luminosity in clusters, and the total luminosity in the Population II field stars all scale as the ratio of the total luminosities of the LMC and the Galaxy. We argue that these characteristics are consistent with the Searle & Zinn (1978) and Zinn (1980) scenario that the Galactic halo formed out of subunits as large as the LMC, and not the tidal disruption of globular clusters. The LMC Population II clusters have a total absolute magnitude MV=-10.8 and luminous mass (for script M sign/L=2.0) of 3.6×106 script M sign⊙. The absolute magnitude for the field Population II, based on the number of RR Lyraes out to NGC 1841 as estimated by Kinman, et al. (1992), is -15.2, corresponding to a mass of 2.1×108 script M sign⊙. The averaged cluster population contains 13.6 RR Lyraes for MV= -7.5 in luminosity.
• Mateo, M., Olszewski, E., Welch, D. L., Fischer, P., & Kunkel, W. (1991). A kinematic study of the fornax dwarf spheroidal galaxy. Astronomical Journal, 102(3), 914-926.
  More info

  Abstract: We have obtained precise radial velocities of 44 stars and four globular clusters located in two fields of the Fornax dwarf spheroidal (dSph) galaxy. The fields are located in the center of Fornax, and about two core radii SW of the galaxy center along the major axis. Our results are based on photon-counting Echelle spectroscopy with a resolution of approximately 14 km s-1. We also present BV CCD photometry of the giant branch of Fornax in both fields. Using a variety of kinematic and photometric criteria, we have identified 10-12 probable nonmembers in our sample of spectroscopically observed stars. Based on the most probable members, the mean heliocentric systemic velocity of Fornax is 53.0 ± 1.8 km s-1, with no evidence for any significant rotation about the minor axis. The intrinsic velocity dispersion of the stars in the central field of Fornax is 9.9 ± 1.7 km s-1 (12.1 ± 1.9 km s-1), while for the outer field the velocity dispersion is 12.0 ± 2.8 km s-1 (13.6 ± 3.0 km s-1). The first values refer to dispersions estimated using only the most probable members; the second values show the effect of including a marginal nonmember in each sample. The true central velocity dispersion is ≤1.6 km s-1 larger than the observed central dispersions for a number of reasonable models. The central mass-to-light ratio (script M sign/L) of Fornax is 12.3 ± 4.5, or 11.4 ± 4.2 based on global galaxy parameters or core fitting, respectively; the full range of acceptable script M sign̈/L ratios is between 5.3 and 26. The uncertainties in the published structural parameters of Fornax are the dominant source of this large range. The observed velocity dispersion "profile" suggests that any dark matter (DM) halo in Fornax is more extended than the visible material. A model-independent lower limit to the central mass density of Fornax also implies that the galaxy contains a significant DM component. If the velocity dispersion in the inner parts of Fornax is assumed to be isotropic, then the central mass density of DM is 0.07 ± 0.03 script M sign⊙ pc-3. This is comparable to the central DM densities observed in gas-rich dwarf irregular galaxies, but is lower than in other dSph systems. Whether this latter effect is related to galaxy luminosity or Galactic tidal effects is unclear.
• Olszewski, E. W., Schommer, R. A., Suntzeff, N. B., & Harris, H. C. (1991). Spectroscopy of giants in LMC clusters. I. Velocities, abundances, and the age-metallicity relation. Astronomical Journal, 101(2), 515-537.
  More info

  Abstract: We report velocities and equivalent widths derived from spectra taken at the calcium triplet (λ∼8500 Å) for more than 150 stars in ∼80 different LMC star clusters. Velocities of the individual program stars are accurate to 5 km s-1. The sum of the pseudo-equivalent widths of the three calcium lines are compared with those of Galactic calibrators; these measures provide a sensitive abundance indicator for - 2.3
• Aaronson, M., Blanco, V. M., Cook, K. H., Olszewski, E. W., & Schechter, P. L. (1990). Northern milky way carbon stars: New candidates, JHK photometry, and radial velocities. Astrophysical Journal, Supplement Series, 73(4), 841-862.
  More info

  Abstract: Data are presented for low-latitude northern Milky Way carbon stars. Coordinates and cross identifications are given for carbon stars in nine fields thought to be unusually transparent. Of these, 142 are confirmed new discoveries. Five hundred thirty-eight JHK photometric observations are reported for 480 stars. Six hundred twenty velocity measurements are presented for 424 stars. Improved coordinates are given for many previously discovered stars. Averaged JHK photometry and velocities are summarized for a sample of 400 stars.
• Mateo, M., Harris, H. C., Nemec, J., & Olszewski, E. W. (1990). Blue stragglers as remnants of stellar mergers: The discovery of short-period eclipsing binaries in the globular cluster NGC 5466. Astronomical Journal, 100(2), 469-484.
  More info

  Abstract: We have obtained numerous V and B CCD images of the central region of the metal-poor globular cluster NGC 5466 to search for short-period variables among the known blue stragglers in the cluster. Of the nine variable blue stragglers discovered in this survey, three are eclipsing binaries with periods between 0.298 and 0.511 days. The remaining six variable blue stragglers are pulsating SX Phe stars, and their properties will be described in detail elsewhere. Of the three eclipsing systems, two are contact binaries (W UMa stars), while the third is a very short-period semidetached (Algol) or fully detached binary. The photometric properties of all three stars are consistent with cluster membership. We compare the number of close binaries and the total number of blue stragglers in NGC 5466 with theoretical estimates of the timescales required for contact systems to merge into single stars and the lifetimes of blue stragglers. This comparison is consistent with the claim that all of the noneclipsing blue stragglers in NGC 5466 formed as the result of mergers of the components in close binary systems, although the uncertainties in our analysis are sufficiently large that we cannot rule out the possibility that other mechanisms have produced some fraction of the blue stragglers in the cluster. The observed number of close binaries among well-studied blue stragglers in intermediate-age and old open clusters and globular clusters is not inconsistent with the merger hypothesis.
• Mateo, M., Olszewski, E. W., & Madore, B. F. (1990). Cepheids in Magellanic Cloud star clusters: Fundamental and overtone pulsators in NGC 2157. Astrophysical Journal, 353(1 PART 2), L11-L15.
  More info

  Abstract: As part of a systematic CCD survey to search for and study Cepheids in young Magellanic Cloud star clusters, we report the discovery of three Cepheids in the Large Magellanic Cloud star cluster NGC 2157. Two of these variables have periods of approximately 3 days, while the third has a period of ∼7.7 days. The photometric properties of the two short-period Cepheids are consistent with their being overtone pulsators, while the long-period Cepheid is more likely pulsating in the fundamental mode. We reach the same conclusion by calculating the pulsational masses for the new Cepheids using a consistent set of theoretical relations. The evolutionary/pulsational mass discrepancy known for Galactic Cepheids reappears for these extragalactic Cepheids with our assumption that the short-period NGC 2157 variables are overtone pulsators. Mass loss can account for neither the observed period range nor the mass discrepancy unless extremely large and unprecedented mass-loss rates are assumed. Stellar models incorporating modified treatments of convection may provide a solution for this new version of the Cepheid mass discrepancy. Our observations of the NGC 2157 Cepheids along with recent results for other Magellanic Cloud cluster Cepheids suggest that the fraction of short-period (≲ 7 days) Cepheids that are first overtone pulsators may be much larger than is generally believed.
• Aaronson, M., & Olszewski, E. W. (1984). OPTICAL COUNTERPARTS OF UNIDENTIFIED IRAS POINT SOURCES: INFRARED LUMINOUS GALAXIES.. Nature, 309(5967), 414-417.
  More info

  Abstract: Deep charge-coupled device imaging has been used to identify a number of IRAS (Infrared Astronomical Satellite) blank field sources with distant galaxies, and indicates that most high-latitude unidentified IRAS point sources have an extragalactic origin. The present sample of galaxies seems to nclude some with the largest known ratio of IR-to-optical luminosity, with values of L(IR)/L(B) ranging upwards of 200. A redshift obtained for one source, 0422 plus 009, indicates that this object has approx. 35 times the absolute IR luminosity of the starburst galaxy M82.

Proceedings Publications

• Mutlu-Pakdil, B. .., Sand, D., Walker, M., Caldwell, N., Carlin, J., Collins, M., Crnojevic, D., Mateo, M., Olszewski, E., Seth, A., Strader, J., Willman, B., & Zaritsky, D. (2020, jan). Signatures of Tidal Disruption in Ultra-Faint Dwarf Galaxies: A Combined HST, Gaia, and MMT/Hectochelle Study of Leo V. In American Astronomical Society Meeting Abstracts \#235, 235.
• Narayan, G., Axelrod, T., Calamida, A., Saha, A., Matheson, T., Olszewski, E., Holberg, J., Holberg, J., Bohlin, R., Stubbs, C., Rest, A., Deustua, S., Sabbi, E., MacKenty, J., Points, S., & Hubeny, I. (2018, jan). Sub-percent Photometry: Faint DA White Dwarf Spectrophotometric Standards for Astrophysical Observatories. In American Astronomical Society Meeting Abstracts #231, 231.
• Johnson, C., Caldwell, N., Rich, R., Mateo, M., Bailey, J., Clarkson, W., Olszewski, E., & Walker, M. (2017, jan). The Multiple Generations and Populations of the Massive Globular Cluster NGC 6273 (M 19). In American Astronomical Society Meeting Abstracts \#229, 229.
• Nidever}, D., {Olsen}, K., {Gruendl}, R., {Besla}, G., {Saha}, A., {Olszewski}, E., {Munoz}, R., {Gallart}, C., {Monelli}, M., {Walker}, A., {Blum}, R., {Kaleida}, C., {Vivas}, K., {Majewski}, S., {Zaritsky}, D., Marel}, R., {Bell}, E., {Conn}, B., {Stringfellow}, G., , {Jin}, S., et al. (2015, jan). First Results from the Survey of the MAgellanic Stellar History (SMASH). In American Astronomical Society Meeting Abstracts, 225.
• {James}, B., {Koposov}, S., {Stark}, D., {Belokurov}, V., {Pettini}, M., , E. (2015, jan). Uncovering Blue Diffuse Dwarf Galaxies. In American Astronomical Society Meeting Abstracts, 225.
• {Narayan}, G., {Saha}, A., {Matheson}, T., {Holberg}, J., {Olszewski}, E., {Stubbs}, C., {Deustua}, S., {Bohlin}, R., {Gilliland}, R., {Rest}, A., {Sabbi}, E., {MacKenty}, J., , T. (2015, jan). Establishing a Network of Next Generation SED Standards with DA White Dwarfs. In American Astronomical Society Meeting Abstracts, 225.
• {Saha}, A., , E. (2015, may). Old Stellar Populations as Structural Tracer of the Magellanic Cloud Complex. In Fifty Years of Wide Field Studies in the Southern Hemisphere: Resolved Stellar Populations of the Galactic Bulge and Magellanic Clouds, 491.
• {Spencer}, M., {Mateo}, M., {Walker}, M., , E. (2015, jan). The Fraction of Binaries in the Distant Dwarf Spheroidal Leo II. In American Astronomical Society Meeting Abstracts, 225.
• Olsen, K. A., Nidever, D. L., Gruendl, R. A., Blum, R. D., Walker, A. R., Saha, A., Olszewski, E. W., Munoz, R., Kunder, A. M., Kaleida, C. C., Conn, B., Besla, G., Majewski, S. R., Gallart, C., Monelli, M., Stringfellow, G. S., Zaritsky, D. F., Chu, Y., Van Der Marel, R. P., , Martin, N., et al. (2014, jan). SMASH: The Survey of the MAgellanic Stellar History. In American Astronomical Society Meeting Abstracts #223, 223, #254.44.
• Saha, A., Narayan, G., Matheson, T., Holberg, J. B., Stubbs, C., Deustua, S. E., Bohlin, R., Olszewski, E. W., Gilliland, R. L., Axelrod, T. S., & Rest, A. (2014, jun). Establishing a Network of Next Generation SED Standards with DA White Dwarfs. In American Astronomical Society Meeting Abstracts #224, 224, #319.04.