Draft version August 10, 2016 Preprint typeset using LATEX style AASTeX6 v. 1.0 FIRST RESULTS FROM THE MADCASH SURVEY: A FAINT DWARF GALAXY COMPANION TO THE LOW MASS SPIRAL GALAXY NGC 2403 AT 3.2 MPC 1 Jeffrey L. Carlin2, David J. Sand3, Paul Price4, Beth Willman2, Ananthan Karunakaran5, Kristine Spekkens5,6, Eric F. Bell7, Jean P. Brodie8, Denija Crnojevic´3, Duncan A. Forbes9, Jonathan Hargis10, Evan Kirby11, Robert Lupton4, Annika H. G. Peter12, Aaron J. Romanowsky8, 13, and Jay Strader14 1Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. 2LSST and Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721, USA; Haverford College, Department of Astronomy, 370 Lancaster Avenue, Haverford, PA 19041, USA; jeff[email protected] 3Texas Tech University, Physics Department, Box 41051, Lubbock, TX 79409-1051, USA 4Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA 5Department of Physics, Engineering Physics and Astronomy, Queen’s University, Kingston, Ontario, Canada, K7L 3N6 6Department of Physics, Royal Military College of Canada, P.O. Box 17000, Station Forces, Kingston, ON K7L 7B4, Canada 7Department of Astronomy, University of Michigan, 1085 South University Ave, Ann Arbor, MI 48109, USA 8University of California Observatories, 1156 High Street, Santa Cruz, CA 95064, USA 9Centre for Astrophysics and Supercomputing, Swinburne University, Hawthorn VIC 3122, Australia 10Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 11California Institute of Technology, 1200 E. California Boulevard, MC 249-17, Pasadena, CA 91125, USA 12CCAPP, Department of Physics, and Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA 13Department of Physics and Astronomy, San Jos´eState University, One Washington Square, San Jos´e, CA 95192, USA 14Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA ABSTRACT We report the discovery of the faintest known dwarf galaxy satellite of an LMC stellar-mass host beyond the Local Group, based on deep imaging with Subaru/Hyper Suprime-Cam. MAD- CASH J074238+652501-dw lies ∼35 kpc in projection from NGC 2403, a dwarf spiral galaxy at D≈3.2 Mpc. This new dwarf has Mg = −7.4 ± 0.4 and a half-light radius of 168 ± 70 pc, at the calcu- lated distance of 3.39 ± 0.41 Mpc. The color-magnitude diagram reveals no evidence of young stellar populations, suggesting that MADCASH J074238+652501-dw is an old, metal-poor dwarf similar to low luminosity dwarfs in the Local Group. The lack of either detected HI gas (MHI/LV < 0.69 M⊙/L⊙, based on Green Bank Telescope observations) or GALEX NUV/FUV flux enhancement is consistent with a lack of young stars. This is the first result from the MADCASH (Magellanic Analog Dwarf Companions And Stellar Halos) survey, which is conducting a census of the stellar substructure and faint satellites in the halos of Local Volume LMC analogs via resolved stellar populations. Models predict a total of ∼4-10 satellites at least as massive as MADCASH J074238+652501-dw around a arXiv:1608.02591v1 [astro-ph.GA] 8 Aug 2016 host with the mass of NGC 2403, with 2-3 within our field of view, slightly more than the one such satellite observed in our footprint. Keywords: dark matter, galaxies: dwarf, galaxies: formation, galaxies: halos 1. INTRODUCTION onic physics (e.g., Wetzel et al. 2016). Likewise, the re- The faint end of the galaxy luminosity function is an cent boom of faint dwarf discoveries in the Local Group important probe of the astrophysics associated with the (LG; most recently Torrealba et al. 2016, and references Λ+Cold Dark Matter (ΛCDM) model of galaxy forma- therein) has partially closed the gap between observa- tion. Quantitative verification of this model has met tions and theoretical expectations. Many more systems with challenges on sub-galactic scales (e.g., the “miss- should be discovered going forward (e.g., Hargis et al. ing satellites problem”, Klypin et al. 1999, and “too 2014). Intriguingly, several of the newly discovered big to fail”, Boylan-Kolchin et al. 2012), but significant faint dwarfs may be associated with the Large Magel- progress has been made with the latest generation of nu- lanic Cloud (LMC; Bechtol et al. 2015; Koposov et al. merical simulations, which include a wide range of bary- 2015; Jethwa et al. 2016; Sales et al. 2016, among oth- 2 ers), which is expected to have its own satellite sys- tem in the ΛCDM model (e.g., D’Onghia & Lake 2008; Sales et al. 2011). To comprehensively compare observations with ex- pectations for galaxy formation in a ΛCDM universe, we must also look beyond the LG to measure the abundance and properties of dwarfs around primary galaxies of different masses, morphologies, and en- vironments. This work has already begun for sev- eral systems with masses similar to, or greater than, the Milky Way (MW; e.g., M81: Chiboucas et al. 2013; Cen A: Crnojevi´cet al. 2014, 2016b; NGC 253: Sand et al. 2014; Romanowsky et al. 2016; Toloba et al. 2016). However, little attention has been paid to less- massive hosts (but see Sand et al. 2015, in NGC 3109), which may shed light on the putative dwarf galaxies of the LMC. Figure 1. DSS image of NGC 2403 with the HSC field of view overlaid as green and red circles. The large black circle This paper presents the discovery of MAD- represents an assumed virial radius of 110 kpc. Red cir- CASH J074238+652501-dw, a low-luminosity satellite of cles are fields already observed, encompassing ∼ 45% of the the LMC stellar-mass analog NGC 2403 (D ≈ 3.2 Mpc, virial volume of NGC 2403’s halo. Green fields are the four 9 additional HSC fields planned to complete our mapping of M ∼ 7 × 10 M⊙, or ∼ 2× LMC stellar mass), the star NGC 2403. first result of a program to search for faint dwarfs and map the stellar halos of LMC analogs in the 1 nearby Universe. Prior to our discovery, NGC 2403 model of Guo et al. (2013) . At the time of this writing, had one known satellite (DDO 44, MB ∼ −12.1; the MADCASH team has acquired significant data (and Karachentsev et al. 2013). In Section 2 we briefly upcoming observing time) on NGC 2403, NGC 247 and summarize our survey plans and strategy to map NGC 4214 (through NOAO Gemini-Subaru exchange the halos of nearby LMC-sized systems. Section 3 time: PI Willman, 2016A-0920; and Keck-Subaru: PI discusses the observations and data reduction, and Brodie, 2015B U085HSC, 2016B U138HSC). Section 4 details the properties of the newly discovered The large aperture of the Subaru telescope and the ◦ dwarf galaxy. We conclude in Section 5 by placing 1.5 diameter field of view of the prime focus imager MADCASH J074238+652501-dw in context both with Hyper Suprime-Cam (HSC; Miyazaki et al. 2012) make respect to expectations from ΛCDM and with known this project possible; the HSC field corresponds to ∼80 systems in the Local Volume. kpc at the distance to NGC 2403 (D∼3.2 Mpc). HSC can map the entire virial volume of an LMC-sized halo (see Figure 1) in only seven pointings. We image our fields in g and i band to a depth that is ∼2 magnitudes 2. SURVEY DESCRIPTION below the tip of the red giant branch (TRGB). This is We designed the MADCASH (Magellanic Analog deep enough to identify and characterize dwarf galaxies Dwarf Companions And Stellar Halos) survey to use re- as faint as MV ∼−7. solved stars to map the virial volumes of Local Volume 9 3. OBSERVATIONS AND DATA REDUCTION galaxies (d .4 Mpc) with stellar masses of 1-7×10 M⊙ (roughly 1/3 to 3 times that of the LMC, assuming We observed three NGC 2403 fields with Subaru/HSC (M/L)K = 1). on 2016 Feb 9-10 with exposure times of 10 × 300 s in The Karachentsev et al. (2013) catalog includes four g and 10 × 120 s in i for each field. The seeing was such galaxies – NGC 2403, NGC 247, NGC 4214 and ∼0.5−0.7′′. The fields, shown in Figure 1, extend nearly NGC 404 – that are accessible from the Subaru tele- to the virial radius both to the east and west of the main scope on Mauna Kea and have E(B − V ) < 0.15. Two body of NGC 2403 and sample ∼ 45% of NGC 2403’s additional systems (NGC 55 and NGC 300; with D < virial volume. 3 Mpc) are in the southern sky and could be observed The images were processed using the HSC with the Dark Energy Camera (DECam). The virial pipeline (hscPipe 4.0.1; http://hsca.ipmu.jp; radii of galaxies in this stellar mass range are ∼100-130 kpc, inferred from semi-analytic galaxy catalogs gener- 1 ated from the Millennium-WMAP7 structure formation Searchable at http://gavo.mpa-garching.mpg.de/MyMillennium/. 3 Figure 2. Inset: HSC image (in g-band, ∼ 2.8′ × 1.8′ in size) of the candidate dwarf galaxy MADCASH J074238+652501-dw. The center of NGC 2403 is ∼ 38′ away, or ∼ 35 kpc in projection. Background image: Color composite from SDSS-III (acquired via http://wikisky.org/) of NGC 2403. http://hsc.mtk.nao.ac.jp/pipedoc_e/index.html), Schlafly & Finkbeiner (2011). The average color excess which is based on an earlier version of the LSST pipeline for stars in this region of the sky is E(B − V ) ∼ 0.048. (Axelrod et al. 2010). Images are bias-subtracted, flat- To estimate the photometric completeness of our fielded with dome flats, corrected for the brighter-fatter catalogs, we match our Subaru/HSC data to three effect (Coulton et al., in prep.), and astrometrically HST/ACS fields in the halo of NGC 2403 from the and photometrically calibrated against Pan-STARRS 1 GHOSTS program (Radburn-Smith et al.