Stellar kinematics and metallicities in the ultra-faint dwarf galaxy Reticulum II Article (Published Version) Romer, Kathy and The DES Collaboration, et al (2015) Stellar kinematics and metallicities in the ultra-faint dwarf galaxy Reticulum II. Astrophysical Journal, 808 (95). ISSN 0004-637X This version is available from Sussex Research Online: http://sro.sussex.ac.uk/id/eprint/61755/ This document is made available in accordance with publisher policies and may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the URL above for details on accessing the published version. Copyright and reuse: Sussex Research Online is a digital repository of the research output of the University. Copyright and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable, the material made available in SRO has been checked for eligibility before being made available. Copies of full text items generally can be reproduced, displayed or performed and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. http://sro.sussex.ac.uk The Astrophysical Journal, 808:95 (14pp), 2015 July 20 doi:10.1088/0004-637X/808/1/95 © 2015. The American Astronomical Society. All rights reserved. STELLAR KINEMATICS AND METALLICITIES IN THE ULTRA-FAINT DWARF GALAXY RETICULUM II*†‡ J. D. Simon1, A. Drlica-Wagner2,T.S.Li3, B. Nord2, M. Geha4, K. Bechtol5, E. Balbinot6,7, E. Buckley-Geer2, H. Lin2, J. Marshall3, B. Santiago7,8, L. Strigari3, M. Wang3, R. H. Wechsler9,10,11, B. Yanny2, T. Abbott12, A. H. Bauer13, G. M. Bernstein14, E. Bertin15,16, D. Brooks17, D. L. Burke10,11, D. Capozzi18, A. Carnero Rosell7,19, M. Carrasco Kind20,21,C.B.D’Andrea18, L. N. da Costa7,19, D. L. DePoy3, S. Desai22, H. T. Diehl2, S. Dodelson2,5, C. E. Cunha10, J. Estrada2, A. E. Evrard23, A. Fausti Neto7, E. Fernandez24, D. A. Finley2, B. Flaugher2, J. Frieman2,5, E. Gaztanaga13, D. Gerdes23, D. Gruen25,26, R. A. Gruendl20,21, K. Honscheid27,28, D. James12, S. Kent2, K. Kuehn29, N. Kuropatkin2, O. Lahav17, M. A. G. Maia7,19, M. March17, P. Martini27,30, C. J. Miller23,31, R. Miquel24, R. Ogando7,19, A. K. Romer32, A. Roodman10,11, E. S. Rykoff10,11, M. Sako14, E. Sanchez33, M. Schubnell23, I. Sevilla20,33, R. C. Smith12, M. Soares-Santos2, F. Sobreira2,7, E. Suchyta27,28, M. E. C. Swanson21, G. Tarle23, J. Thaler34, D. Tucker2, V. Vikram35, A. R. Walker12, and W. Wester2 (The DES Collaboration) 1 Carnegie Observatories, 813 Santa Barbara St., Pasadena, CA 91101, USA 2 Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510, USA 3 George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, and Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA 4 Astronomy Department, Yale University, New Haven, CT 06520, USA 5 Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA 6 Department of Physics, University of Surrey, Guildford GU2 7XH, UK 7 Laboratório Interinstitucional de e-Astronomia—LIneA, Rua Gal. José Cristino 77, Rio de Janeiro, RJ-20921-400, Brazil 8 Instituto de Física, UFRGS, Caixa Postal 15051, Porto Alegre, RS-91501-970, Brazil 9 Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA 10 Kavli Institute for Particle Astrophysics & Cosmology, P.O. Box 2450, Stanford University, Stanford, CA 94305, USA 11 SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA 12 Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile 13 Institut de Ciències de l’Espai, IEEC-CSIC, Campus UAB, Facultat de Ciències, Torre C5 par-2, E-08193 Bellaterra, Barcelona, Spain 14 Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA 15 Sorbonne Universités, UPMC Univ Paris 06, UMR 7095, Institut d’Astrophysique de Paris, F-75014, Paris, France 16 Institut d’Astrophysique de Paris, Univ. Pierre et Marie Curie & CNRS UMR7095, F-75014 Paris, France 17 Department of Physics & Astronomy, University College London, Gower Street, London, WC1E 6BT, UK 18 Institute of Cosmology & Gravitation, University of Portsmouth, Portsmouth, PO1 3FX, UK 19 Observatório Nacional, Rua Gal. José Cristino 77, Rio de Janeiro, RJ-20921-400, Brazil 20 Department of Astronomy, University of Illinois, 1002 W. Green Street, Urbana, IL 61801, USA 21 National Center for Supercomputing Applications, 1205 West Clark St., Urbana, IL 61801, USA 22 Department of Physics, Ludwig-Maximilians-Universität, Scheinerstr.1, D-81679 München, Germany 23 Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA 24 Institut de Física d’Altes Energies, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain 25 Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, D-85748 Garching, Germany 26 University Observatory Munich, Scheinerstrasse 1, D-81679 Munich, Germany 27 Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA 28 Department of Physics, The Ohio State University, Columbus, OH 43210, USA 29 Australian Astronomical Observatory, North Ryde, NSW 2113, Australia 30 Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA 31 Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA 32 Department of Physics and Astronomy, Pevensey Building, University of Sussex, Brighton, BN1 9QH, UK 33 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain 34 Department of Astronomy, University of Illinois, 1002 W. Green Street, Urbana, IL 61801, USA 35 Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA Received 2015 April 12; accepted 2015 June 16; published 2015 July 23 ABSTRACT We present Magellan/M2FS, Very Large Telescope/GIRAFFE, and Gemini South/GMOS spectroscopy of the newly discovered Milky Way satellite Reticulum II. Based on the spectra of 25 Ret II member stars selected from Dark Energy Survey imaging, we measure a mean heliocentric velocity of 62.8 0.5 km s-1 and a velocity -1 dispersion of 3.3 0.7 km s . The mass-to-light ratio of Ret II within its half-light radius is 470 210 ML, demonstrating that it is a strongly dark matter-dominated system. Despite its spatial proximity to the Magellanic Clouds, the radial velocity of Ret II differs from that of the LMC and SMC by 199 and 83 km s-1, respectively, suggesting that it is not gravitationally bound to the Magellanic system. The likely member stars of Ret II span * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. † Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., undera cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). ‡ Based on data obtained from the ESO Science Archive Facility under request number 157689. 1 The Astrophysical Journal, 808:95 (14pp), 2015 July 20 Simon et al. 1.3 dex in metallicity, with a dispersion of 0.28 ± 0.09 dex, and we identify several extremely metal-poor stars with [Fe/H] <-3. In combination with its luminosity, size, and ellipticity, these results confirm that Ret II is an ultra-faint dwarf galaxy. With a mean metallicity of [Fe/H]=- 2.65 0.07, Ret II matches Segue 1 as the most metal-poor galaxy known. Although Ret II is the third-closest dwarf galaxy to the Milky Way, the line-of-sight 25- ◦ integral of the dark matter density squared is log10 (J )= 18.8 0.6 GeV cm within 0 .2, indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies. Key words: dark matter – galaxies: dwarf – galaxies: individual (Reticulum II) – galaxies: stellar content – Local Group – stars: abundances 1. INTRODUCTION 2. OBSERVATIONS AND DATA REDUCTION The population of known dwarf galaxies orbiting the Milky 2.1. Target Selection Way has grown rapidly over the last decade, with the discovery Spectroscopic targets were selected from the object catalog of the ultra-faint dwarfs by the Sloan Digital Sky Survey derived from the coadded images of the first internal annual (SDSS) more than doubling the size of our Galaxy’s retinue of release of DES data (Y1A1; Sevilla et al. 2011; Desai satellites (e.g., Willman et al. 2005; Zucker et al. 2006; et al. 2012; Mohr et al. 2012; Balbinot et al. 2015;R.A. Belokurov et al. 2007). These extreme objects are the focus of Gruendl et al., 2015 in preparation). We identified objects as an enormous variety of ongoing work, ranging from their ( stars based on the spread_ model quantity output by SExtractor internal kinematics e.g., Martin et al. 2007; Simon & ( ) Geha 2007; Koposov et al. 2011), metallicities (e.g., Kirby Bertin 2011; Desai et al. 2012 . Our stellar sample consists of et al. 2008; Norris et al. 2010), and chemical abundance well-measured objects with ∣spread_ model _ i∣ < 0.002 and patterns (e.g., Koch et al. 2008; Frebel et al. 2010, 2014; flags_{ g , r , i }< 4.