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The Saga Survey: I. Satellite Galaxy Populations Around Eight Milky Way Analogs

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The Saga Survey: I. Satellite Galaxy Populations Around Eight Milky Way Analogs DRAFT VERSION SEPTEMBER 19, 2017 Preprint typeset using LATEX style AASTeX6 v. 1.0 THE SAGA SURVEY: I. SATELLITE GALAXY POPULATIONS AROUND EIGHT MILKY WAY ANALOGS MARLA GEHA1 ,RISA H. WECHSLER2,3 ,YAO-YUAN MAO4 ,ERIK J. TOLLERUD5 ,BENJAMIN WEINER6 ,REBECCA BERNSTEIN7 , BEN HOYLE8,9 ,SEBASTIAN MARCHI10 ,PHIL J. MARSHALL3 ,RICARDO MUÑOZ10 , AND YU LU7 1Department of Astronomy, Yale University, New Haven, CT 06520, USA 2Kavli Institute for Particle Astrophysics and Cosmology & Department of Physics, Stanford University, Stanford, CA 94305, USA 3SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA 4Department of Physics and Astronomy & Pittsburgh Particle Physics, Astrophysics and Cosmology Center (PITT PACC), University of Pittsburgh, Pittsburgh, PA 15260, USA 5Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA 6Department of Astronomy, University of Arizona, Tucson, AZ, USA 7The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA 8Universitaets-Sternwarte, Fakultaet für Physik, Ludwig-Maximilians Universitaet Muenchen, Scheinerstr. 1, D-81679 Muenchen, Germany 9Max Planck Institute für Extraterrestrial Physics, Giessenbachstr. 1, D-85748 Garching, Germany 10Departamento de Astronomia, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile ABSTRACT We present the survey strategy and early results of the “Satellites Around Galactic Analogs” (SAGA) Survey. The SAGA Survey’s goal is to measure the distribution of satellite galaxies around 100 systems analogous to the Milky Way down to the luminosity of the Leo I dwarf galaxy (Mr < -12:3). We define a Milky Way analog based on K-band luminosity and local environment. Here, we present satellite luminosity functions for 8 Milky Way analog galaxies between 20 to 40 Mpc. These systems have nearly complete spectroscopic coverage of candidate satellites within the projected host virial radius down to ro < 20:75 using low redshift gri color criteria. We have discovered a total of 25 new satellite galaxies: 14 new satellite galaxies meet our formal criteria around our complete host systems, plus 11 additional satellites in either incompletely surveyed hosts or below our formal magnitude limit. Combined with 13 previously known satellites, there are a total of 27 satellites around 8 complete Milky Way analog hosts. We find a wide distribution in the number of satellites per host, from 1 to 9, in the luminosity range for which there are five Milky Way satellites. Standard abundance matching extrapolated from higher luminosities predicts less scatter between hosts and a steeper luminosity function slope than observed. We find that the majority of satellites (26 of 27) are star-forming. These early results indicate that the Milky Way has a different satellite population than typical in our sample, potentially changing the physical interpretation of measurements based only on the Milky Way’s satellite galaxies. Keywords: galaxies: dwarf – galaxies: halos – galaxies: luminosity function, mass function – galaxies: struc- ture – Local Group 1. INTRODUCTION Matter model (ΛCDM). ΛCDM simulations including only The Milky Way is the most well-studied galaxy in the Uni- dark matter, combined with simple galaxy formation pre- verse (e.g., Bland-Hawthorn & Gerhard 2016). From a cos- scriptions, over-predict both the number of satellite galaxies arXiv:1705.06743v3 [astro-ph.GA] 16 Sep 2017 mological and galaxy formation perspective, one of the more observed around the Milky Way and their central mass den- informative components of the Milky Way is its population of sities (the "too-big-to-fail" problem; Boylan-Kolchin et al. dwarf galaxy satellites. While the number of faint satellites 2012). Stated differently, ΛCDM predicts large numbers of dark matter subhalos that either do not exist around the Milky (Mr > -10) is steadily increasing due to discoveries in on- going large-area imaging surveys (e.g., Koposov et al. 2015; Way, do not host bright satellite galaxies, or are not as dense Drlica-Wagner et al. 2015), the number of bright satellites as expected (e.g., Garrison-Kimmel et al. 2014a). It has been suggested that a realistic treatment of baryonic physics and (Mr < -10) has remained unchanged since the discovery of the disrupting Sagittarius dwarf spheroidal galaxy over 20 the stochastic nature of star formation can fix these discrep- years ago (Ibata et al. 1994). The population of bright satel- ancies (e.g., Brooks & Zolotov 2014; Guo et al. 2015; Wet- lite galaxies around the Milky Way is thus largely complete. zel et al. 2016; Brooks et al. 2017). Other authors have sug- The properties of the Milky Way’s brightest satellites do gested that the discrepancy favors alternative models to CDM not agree with predictions of the simplest galaxy forma- (e.g., Lovell et al. 2012; Polisensky & Ricotti 2014). While tion models based on simulations of the Lambda Cold Dark some of these discrepancies may be solved if the Milky Way 2 GEHA ET AL. has a lower mass (e.g., Vera-Ciro et al. 2013; Dierickx & In this paper, we present the survey strategy and early Loeb 2017), similar results hold for M31 in the Local Group results of the Satellites Around Galactic Analogs (SAGA) (Tollerud et al. 2014). Survey1. The goal of the SAGA Survey is to obtain It is possible that the Local Group satellites are not rep- spectroscopically-confirmed complete satellite luminosity resentative of typical galaxies at this mass scale (e.g., Pur- functions within the viral radius of 100 Milky Way analogs cell & Zentner 2012; Jiang & van den Bosch 2016). Sev- in the distance range 20 - 40 Mpc down to Mr = -12:3. eral studies have considered the question of how typical the The paper is organized as follows. In§2, we describe the Milky Way is in terms of its bright satellite population (Liu SAGA Survey strategy including our definition and selection et al. 2011; Guo et al. 2011; Tollerud et al. 2011; James of Milky Way host analogs. In§3, we detail the observing & Ivory 2011; Strigari & Wechsler 2012; Robotham et al. facilities used to obtain redshifts for over 17,000 candidate 2012). Most of these studies use the Sloan Digital Sky Sur- satellite galaxies. In§4 and§5, we describe our efforts to vey (SDSS) whose spectroscopic magnitude limit of r = 17:7 improve targeting efficiency for low redshift galaxies and ex- corresponds to satellites in the nearby Universe that are sim- plore possible biases in our existing redshift survey. Finally, ilar to the Large and Small Magellanic Clouds (Mr = -18:6 in§6 we present results based on satellites discovered around and -17:2, respectively). These studies find that our Galaxy eight Milky Way host galaxies. is unusual, but not yet uncomfortably so, in its bright satellite All distance-dependent parameters in this paper are calcu- -1 -1 population. The Milky Way analogs on average have only 0.3 lated assuming H0 = 70 km s Mpc . Magnitudes and col- satellites brighter than these luminosities, versus two for the ors are extinction corrected (as denoted with a subscript ’o’, Milky Way (Liu et al. 2011). The distribution of these bright e.g., ro) using Schlegel et al. (1998) as reported by SDSS satellites is also remarkably consistent with simulations us- DR12 and K-corrected to redshift zero using the kcorrect ing fairly straightforward assumptions about the galaxy-halo v4_2 software package (Blanton & Roweis 2007). connection (Busha et al. 2011; Rodríguez-Puebla et al. 2013; Kang et al. 2016). It is below these luminosities that the 2. THE SAGA SURVEY DESIGN Milky Way’s satellite properties diverge from simple galaxy The goal of the SAGA survey is to characterize the satellite formation predictions. This suggests a search for such satel- galaxy population around 100 Milky Way analogs within the lites around a large sample of hosts. virial radius down to an absolute magnitude of Mr;o = -12:3. Identifying satellites fainter than the Magellanic Clouds in In the Milky Way, there are five satellites brighter than this a statistical sample of host galaxies is observationally chal- magnitude limit; the dimmest of these, Leo I, has a luminos- 6 lenging. For hosts within 10 Mpc, satellites can be reliably ity of Mr;o = -12:3 and a stellar mass of M∗ = 3 × 10 M distinguished based on size, surface brightness, and, in the (McConnachie 2012). We chose Milky Way analog galax- most nearby cases, by resolved stars (e.g., Crnojevic´ et al. ies from a largely complete list of galaxies in the local uni- 2016; Javanmardi et al. 2016; Danieli et al. 2017). How- verse (§ 2.1), and describe a well-defined set of criteria for ever, this volume contains only a handful of Milky Way- selecting Milky Way analog galaxies (§ 2.2). We first mo- like galaxies, and is thus insufficient to answer the statis- tivate our Galactic analog selection over the distance range tical questions outlined above. Low mass galaxies around 20 to 40 Mpc, and then simulate the properties of Milky Way Milky Way analogs beyond 10 Mpc are difficult to distin- satellites observed at these distances (§ 2.3). guish from the far more numerous background galaxy popu- lation using photometry alone. Previous studies have focused 2.1. The Master List: A Complete Galaxy Catalog in a on spectroscopic follow-up of single hosts (Spencer et al. 40 Mpc Volume 2014), or attempted to constrain satellite populations statisti- We select Milky Way analog galaxies from a catalog of cally (Speller & Taylor 2014). Photometric redshifts do not galaxies that is as complete as possible within the survey vol- perform well at low redshifts (see § 4.1), thus a wide-area ume in order to better understand our selection function. To spectroscopic survey is required to quantify satellite pop- do this requires a catalog of all bright galaxies in the Local ulations.
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