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And young Companions of lack a with otbyn h oa ru,bsdo epiaigwt Subaru/H satellit with galaxy imaging dwarf deep on known D based faintest lies Group, J074238+652501-dw the Local CASH of the discovery beyond host the report We o uioiydaf nteLclGop h ako ihrdetected either of lack The Group. o Local an the in is dwarfs luminosity J074238+652501-dw low MADCASH that suggesting populations, ae itneo 3 of distance lated Kirby ≈ 5,6 . p.Ti e wr has dwarf new This Mpc. 3.2 olnKlhne l 2012 al. et Boylan-Kolchin rcF Bell F. Eric , 11 1. oetLupton Robert , akmte,glxe:daf aais omto,glxe:halos galaxies: formation, galaxies: dwarf, galaxies: matter, dark INTRODUCTION A T E tl ATX .1.0 v. AASTeX6 style X 2 lpne l 1999 al. et Klypin ai .Sand J. David , ∼ . 39 7 -0stlie tlata asv sMDAHJ074238+652501-dw MADCASH as massive as least at satellites 4-10 enP Brodie P. Jean , ± ASSIA AAYNC20 T32MPC 3.2 AT 2403 NGC GALAXY SPIRAL MASS 0 . 1Mc h oo-antd iga eel oeiec fyugs young of evidence no reveals diagram color-magnitude The Mpc. 41 4 niaH .Peter G. H. Annika , ,btsignificant but ), 3 ∼ M alPrice Paul , 5kci rjcinfo G 43 wr prlglx at galaxy spiral dwarf a 2403, NGC from projection in kpc 35 g n “too and , = 8 eiaCrnojevi Denija , − at rz A904 USA 95064, CA Cruz, Santa y rneo,N 84,USA 08544, NJ Princeton, ty, ead C291,Psdn,C 12,USA 91125, CA Pasadena, 249-17, MC levard, 7 . lioe D228 USA 21218, MD altimore, 4 .Bx100 tto ocs igtn NKL74 Canada 7B4, K7L ON Kingston, Forces, Station 17000, Box O. est,Hwhr I 12 Australia 3122, VIC Hawthorn versity, vne aefr,P 94,UA jeff[email protected] USA; 19041, PA Haverford, Avenue, ABSTRACT nvriyAe n ro,M 80,USA 48109, MI Arbor, Ann Ave, University h ue’ nvriy igtn nai,Cnd,KL3N6 K7L Canada, Ontario, Kingston, University, Queen’s c,T 90-01 USA 79409-1051, TX ock, iy n ahntnSur,SnJse A912 USA 95192, Jos´e, CA San Square, Washington One sity, ± prtdb h ainlAtooia bevtr fJapan of Observatory Astronomical National the by operated iy atLnig I484 USA 48824, MI Lansing, East sity, 4 n Z871 USA; 85721, AZ on, 0 h hoSaeUiest,Clmu,O 31,USA 43210, OH Columbus, University, State Ohio The ehWillman Beth , . n aflgtrdu f168 of radius half-light a and 4 12 GALEX ao .Romanowsky J. Aaron , an wrsmyb soitdwt h ag Magel- Large the (LMC; with Cloud associated lanic be may dwarfs faint 2015 (e.g., forward going 2014 discovered systems more observa- be Many between should expectations. gap theoretical the and closed tions partially has Group therein) Local the recently in most discoveries (LG; dwarf faint of boom cent (e.g., physics onic c ´ 3 .Itiunl,svrlo h el discovered ; newly the of several Intriguingly, ). U/U u nacmn sconsistent is enhancement flux NUV/FUV ucnA Forbes A. Duncan , ehae l 2016 al. et Jethwa 2 nnhnKarunakaran Ananthan , H d ea-ordafsmlrto similar dwarf metal-poor ld, I a ( gas h tla usrcueand substructure stellar the f ezle l 2016 al. et Wetzel prSpieCm MAD- Suprime-Cam. yper orab ta.2016 al. et Torrealba fa M stellar-mass LMC an of e eho ta.2015 al. et Bechtol la ouain.Models populations. ellar aelncAao Dwarf Analog Magellanic ,13 8, M 1 oeta h n such one the than more ; HI n a Strader Jay and , 9 ± ae ta.2016 al. et Sales MAINT H LOW THE TO OMPANION oahnHargis Jonathan , /L 0p,a h calcu- the at pc, 70 V < 0 . 69 .Lkws,tere- the Likewise, ). ruda around 5 M Kristine , ; n references and , ⊙ m ooo tal. et Koposov tellar mn oth- among , /L agse al. et Hargis . 14 ⊙ 10 , Evan , 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. 2011)2. The Processing Version 2 (Schlafly et al. 2012; Tonry et al. GHOSTS fields were observed with the F606W and 2012; Magnier et al. 2013). The images are transformed F814W filters. Artificial star tests showed that the ACS to a common reference frame and coadded with con- data are > 90% complete to the magnitude limit of our servative clipping to remove artifacts that appear on HSC photometry (Radburn-Smith et al. 2011). Using a a single visit. Coadded images are used for all the matching radius of 1′′, we recover half of the HST/ACS photometric and astrometric measurements reported stellar sources (i.e., we are 50% complete) at a magni- below. We create a merged source list from deblended tude of F814W≈ 26.0, which corresponds to i ≈ 26.4 in catalogs in each band, and apply the same centroid and our HSC data. aperture or model (generally derived from the i-band 4. A NEW DWARF GALAXY COMPANION image) to measure each object’s flux in both bands. Point sources are separated from extended sources We visually identified a candidate dwarf galaxy ∼35 by removing objects for which the model and PSF kpc to the east of NGC 2403 in projection (Figure 2), fluxes differ by more than three times the flux error which we dub MADCASH J074238+652501-dw. At for that object. All stellar magnitudes presented in this radius, MADCASH J074238+652501-dw is just be- this work are derived from PSF photometry. We then yond the field of view of previous work on the halo match our catalogs to SDSS DR9 (Ahn et al. 2012), of NGC 2403 (Barker et al. 2012). This dwarf shows and transform to gSDSS and iSDSS magnitudes with an no sign of a nuclear star cluster or disturbed morphol- offset and color term. The transformed magnitudes ogy. NGC 2403 has one other known dwarf compan- have ∼ 0.03 mag scatter about the SDSS values. All ion, the bright dSph/dE galaxy DDO 44 (MB = −12.1; magnitudes presented henceforth are on the SDSS photometric system, corrected for extinction using 2 the Schlegel et al. (1998) maps with coefficients from Data products available at https://archive.stsci.edu/pub/hlsp/ghosts/index.html 4

Figure 3. Left panel: Color magnitude diagram of point sources within 20′′ of MADCASH J074238+652501-dw. Typical photometric errors as a function of magnitude are shown at the left side of this panel. The other three panels show CMDs of nearby “blank-sky” regions of the same size. In each panel, we show Padova isochrones of old (10-Gyr) populations with metallicities of [M/H]=−2.2, −1.5, −1.0, and −0.5, at D = 3.39 Mpc (the distance to MADCASH J074238+652501-dw; see Section 4.1). There is an overdensity of resolved sources at the position of the dwarf, consistent with an old, metal-poor RGB, extending ∼2 mags below the RGB tip.

◦ Karachentsev et al. 2013) roughly 1.3 (∼ 73 kpc in pro- (g − i)0 > 1.0. We linearly interpolate 10 and 14 Gyr jection) to the north. isochrones shifted to the distance modulus listed in The left panel of Fig. 3 shows a color–magnitude dia- Table 1, and assign each star the metallicity of the gram of point sources within 20′′ of the center of MAD- interpolated isochrone that passes through its color CASH J074238+652501-dw. For comparison, other and magnitude. The mean metallicity of the 13 stars panels show CMDs in randomly selected nearby back- is [Fe/H] = −1.6(−1.7) for the 10 (14) Gyr isochrones, ground fields (also with 20′′ radius). Each panel con- with standard deviation of 0.4 dex. tains PARSEC isochrones (Bressan et al. 2012) for old (10 Gyr) populations at our derived distance to MAD- 4.1. Distance CASH J074238+652501-dw (3.39 Mpc; see Sec. 4.1) We estimate the distance to MAD- and metallicities ([M/H]) of -2.2, -1.5, -1.0, and -0.5 CASH J074238+652501-dw using the TRGB method (assuming a solar metallicity of Z⊙ = 0.0152). The (Lee et al. 1993). The TRGB absolute magnitude is most metal-poor isochrones follow the red giant branch estimated by averaging the magnitudes of the brightest (RGB) of MADCASH J074238+652501-dw closely, with metal-poor ([M/H] = −2.2, −1.5, and − 1.0), old (10 little evidence of younger populations blueward of the Gyr) stars in the PARSEC isochrones; we adopt a TRGB RGB or more metal-rich RGB stars following the red- TRGB magnitude of Mi = −3.47 ± 0.05. We dest of the isochrones. Two conclusions can be drawn locate the TRGB of MADCASH J074238+652501-dw from Figure 3 – first, that there is an obvious stel- using stars within 20′′ of the dwarf center, keeping lar excess relative to neighboring regions, and sec- only stars with 0.8 < (g − i)0 < 2.1 (the color range ondly that the excess stars predominantly cluster around of the metal-poor RGB; see Fig. 3). We bin these in the old, metal-poor ([M/H] < −1.0) locus of the magnitude to create a luminosity function, then use isochrones we have overlaid. Assuming that the stars in a zero-sum Sobel edge-detection filter to locate the MADCASH J074238+652501-dw mostly cluster around transition in stellar density corresponding to the RGB [Fe/H] = -2, then we conclude that the galaxy does not tip. We repeat this for bins of different widths (from host populations significantly younger than 10 Gyr. 0.15 to 0.2 mag in steps of 0.01) and then adopt the We estimate the mean metallicity of MAD- mean value; the standard deviation is taken as the 3 CASH J074238+652501-dw by comparing the CMD uncertainty. We measure i0,TRGB = 24.18 ± 0.20 for positions of stars with PARSEC isochrones. In order to eliminate contamination by non-members, we select 3 the 13 stars in the left panel of Fig. 3 with i0 < 25 and We attempted to apply the maximum likelihood method of Makarov et al. (2006), but it failed to converge due to the small 5

MADCASH J074238+652501-dw (in agreement with the distance to NGC 2403 from previous work; e.g., Bellazzini 2008), corresponding to m−M = 27.65±0.26 (D = 3.39 ± 0.41 Mpc) for the new dwarf galaxy. For comparison, we also perform the TRGB analysis on stars near the main body of NGC 2403, and find i0,TRGB = 23.92 ± 0.11 for NGC 2403, or a distance modulus of m − M = 27.39 ± 0.16 (D = 3.01 ± 0.23 Mpc). This agrees with typical measurements of the distance to NGC 2403 within the uncertainties (e.g., Radburn-Smith et al. 2011: m − M = 27.51 ± 0.07) .

4.2. Structural parameters To derive structural parameters of MAD- CASH J074238+652501-dw, we select RGB candidates centered on the three most metal-poor isochrones in Figure 4. Absolute V -band magnitude (MV ) and half-light radius (rh) of MADCASH J074238+652501-dw (large red Figure 3, with magnitudes 23.7 < i0 < 26.2, within ′ ′ star) placed in context with other known systems. Black a 5 × 5 box centered on the dwarf. This catalog points and triangles are data for MW and M31 dwarf galax- was passed to a maximum likelihood estimator of ies (e.g., McConnachie 2012), and blue squares are members the dwarf structural parameters using the Sand et al. of the NGC 3109 association, including Antlia and Antlia B (Sand et al. 2015). The small, red, downward-pointing trian- (2012) implementation of the Martin et al. (2008) gle represents DDO 44, another known satellite of NGC 2403 method. The resulting structural parameters for (Karachentsev et al. 1999). MADCASH J074238+652501-dw are given in Table 1, with uncertainties determined via 1000 bootstrap MADCASH J074238+652501-dw upon the observed re- resamplings of the data. lationship for LG dwarfs (Figure 4). This suggests The central position of MADCASH J074238+652501- that even though MADCASH J074238+652501-dw has dw is well constrained, but the additional parameters evolved in a different environment than MW/M31 are poorly measured due to the small number of re- dwarfs, the physical processes that determine its proper- solved stars in the dwarf. We derive a half-light radius ties are similar to those in more massive hosts’ halos. If ′′ of rh = 10.2 ± 3.0 , corresponding to 168 pc at a dis- MADCASH J074238+652501-dw also follows other scal- tance of 3.39 Mpc. For the ellipticity, we derive only an ing relations for LG dwarf galaxies McConnachie (2012, upper limit of ǫ < 0.42 (within 68% confidence limits). e.g.,), then this dwarf with MV = −7.7 should have a Because the ellipticity is poorly constrained, we cannot dynamical mass-to-light ratio of ∼ 100, dynamical mass 6 reliably measure the dwarf’s position angle; the value of of Mdyn(< rh) ∼ 5 × 10 M⊙, and a mean metallicity θ = 29◦ corresponds to the likelihood maximum, but is of [Fe/H]∼−2.0. This metallicity is consistent with our unconstrained. inference of an overall metal-poor population in MAD- CASH J074238+652501-dw. 4.3. Luminosity and stellar populations The sparse CMD precludes a precise quantification of We estimate the luminosity of MAD- the SFH beyond our previous statement that the galaxy CASH J074238+652501-dw by measuring the inte- does not host stars significantly younger than 10 Gyr. ′′ grated flux in a circular aperture of radius rh = 10.2 and central position as measured in Section 4.2 (see, 4.4. GALEX UV e.g., Sand et al. 2014, 2015). After subtracting the We discern no FUV or NUV flux enhancement in average background from 100 equal-area apertures in available GALEX imaging at the position of MAD- random positions throughout the same CCD frame and CASH J074238+652501-dw. The nearest source in the multiplying the flux by two to account for our half-light MAST/GALEX GR6 catalog, at ∼ 0.86′ from the center radius aperture, we find integrated luminosities of of the dwarf, is faint in the FUV, with no NUV detec- Mg = −7.4 ± 0.4 and Mi = −8.1 ± 0.6. This translates tion. This source has no obvious counterpart in our HSC to MV = −7.7 ± 0.7 using the filter transformations of images. Jordi et al. (2006). Using tiles from the GALEX All-Sky Imag- The measured luminosity and half-light radius place ing Survey, we measure the FUV flux of MAD- CASH J074238+652501-dw, using the same aperture and background-subtraction technique used in estimat- number of resolved stars near the TRGB of the dwarf. ing the optical luminosity in Section 4.3. This flux 6 was converted into a luminosity using our adopted FWHM = 9.1′(8.97 kpc) GBT beam at this frequency. TRGB distance modulus of 27.65, adopting an ex- This non-detection combined with the measured dis- tinction coefficient AFUV = 7.9 E(B − V ) (Lee et al. tance and luminosity suggest that a putative HI coun- 2009). We convert this to a star formation rate (SFR) terpart has a 5σ, 15 km s−1 HI mass upper limit of 4 using the relation between SFR and UV continuum MHI,lim = 7.1 × 10 M⊙ and MHI/LV = 0.69 M⊙/L⊙. −1 from McQuinn et al. (2015): SFR(M⊙ yr )=2.04 × The satellite is therefore gas-poor, similar to what is −28 −1 −1 10 Lν (erg s Hz ), and find an upper limit on found for other dwarf spheroidals in the Local Volume the SFR in MADCASH J074238+652501-dw of 4.4 × (Grcevich & Putman 2009; Spekkens et al. 2014). −6 −1 10 M⊙ yr . The same relation using the GALEX FUV aperture magnitude for MW dwarf Leo T from 5. CONCLUSIONS −6 −1 Lee et al. (2011) yields SFRLeoT ≈ 5.6 × 10 M⊙ yr , We report the discovery of a faint (M = −7.4 ± 0.4) suggesting that MADCASH J074238+652501-dw is at g dwarf galaxy companion of the LMC analog NGC 2403 most forming stars at a similar rate as Leo T. (∼ 2× LMC stellar mass) in imaging data from the MADCASH survey using Hyper Suprime-Cam on the Table 1. Properties of MAD- Subaru telescope. From resolved stars reaching ∼ CASH J074238+652501-dw 2 magnitudes below the RGB tip, we show that the new dwarf, MADCASH J074238+652501-dw, has predomi- Parameter Value nantly old, metal-poor stellar populations (∼ 10 Gyr, RA (hh:mm:ss) 07 : 42 : 38.887 ± 2.6′′ [M/H] ∼ −2) similar to those in the LG ultra-faint ′′ galaxies. Using non-detections in HI and UV observa- Decl (dd:mm:ss) +65 : 25 : 01.89 ± 3.2 tions, we place upper limits on the available gas reservoir m − M (mag) 27.65 ± 0.26 and star formation rate, confirming this as a gas-poor ± D (Mpc) 3.39 0.41 system with old stellar populations. Our derived dis- Mg (mag) −7.4 ± 0.4 tance modulus of m − M = 27.65 ± 0.26 places MAD- MV (mag) −7.7 ± 0.7 CASH J074238+652501-dw near NGC 2403, bolstering rh (arcsec) 10.2 ± 3.0 the case that it is a faint satellite of this Local Volume rh (pc) 168 ± 70 LMC analog. ǫ < 0.42 (68% CL) Should we have expected to find only one new θ (deg.) 29◦(unconstrained) satellite? NGC 2403 has one previously known 5 −2 satellite, the dwarf galaxy DDO 44 (MV ∼ µ0 (mag arcsec ) 25.9 ± 0.7 5 −12.5, [Fe/H] ∼ −1.7; Karachentsev et al. 1999), star ∼ × M (M⊙) 1 10 which is outside our current footprint. The stel- MHI/LV (M⊙/L⊙) < 0.69 lar masses of this satellite and the newly discovered 4 7 MHI(M⊙) < 7.1 × 10 dwarf are Mstar,DDO 44 ∼ 6 × 10 M⊙ (estimated −6 SFR (M⊙/yr) ≤ 4.4 × 10 from the K-band magnitude from Karachentsev et al. 2013, assuming M/L = 1) and Mstar ∼ 1 × 5 10 M⊙ for MADCASH J074238+652501-dw. Based on abundance-matching relations (Moster et al. 2013; Garrison-Kimmel et al. 2014), applied to subhalo 4.5. GBT H I observations mass functions from dark-matter-only simulations Using the Robert C. Byrd Green Bank Telescope4, (e.g., Garrison-Kimmel et al. 2014), we expect 3- we obtained position-switched HI observations of MAD- 11 dwarf galaxies at least as massive as MAD- CASH J074238+652501-dw through Director’s Discre- CASH J074238+652501-dw in NGC 2403’s virial vol- tionary Time (AGBT-16A-462; PI:Spekkens) on 2016 ume. If satellites follow the subhalo distribution in May 9 and 11. The GBT spectrum in the velocity dark-matter-only simulations (Han et al. 2016), which −1 ranges −1000 ≤ VLSRK ≤ −50 km s and 50 ≤ is nearly isothermal and consistent with the distribution −1 VLSRK ≤ 1000 km s has rms noise of σ = 0.35 mJy of satellites at high redshift (Nierenberg et al. 2011), we at a spectral resolution of 15 km s−1. We do not find any HI emission in these velocity ranges within the 5 Karachentsev et al. (2002) suggested NGC 2366, Holmberg II, UGC 4483, and KDG052 may also be companions of NGC 2403. Of these, only NGC 2366 (MB = −16.1) has NGC 2403 designated 4 The National Radio Astronomy Observatory is a facility of the as its main tidal disturber by Karachentsev et al. 2014; all others National Science Foundation operated under cooperative agree- are predominantly influenced by M81. For this work, we assume ment by Associated Universities, Inc. that these galaxies are not satellites of NGC 2403. 7 should have found 2-3 satellites in our footprint with NASA contract NAS5-26555. Support for MAST for 5 Mstar & 1×10 M⊙. This suggests that we should find a non-HST data is provided by the NASA Office of Space factor of 2-3 more in a complete survey of the remaining Science via grant NNX09AF08G and by other grants ∼ 55% of NGC 2403’s virial volume. While this simple and contracts. estimate suggests that one dwarf galaxy is fewer than The Pan-STARRS1 Surveys have been made possible we expect to find in our current data, we must observe through contributions of the Institute for Astronomy, the entire virial volume of NGC 2403 to fully assess the the University of Hawaii, the Pan-STARRS Project Of- significance of its satellite abundance. Placing definitive fice, the Max-Planck Society and its participating in- constraints on cosmological models will require mapping stitutes, the Max Planck Institute for Astronomy, Hei- the virial halos of the ensemble of hosts, which sample delberg and the Max Planck Institute for Extraterres- a variety of environments, in our MADCASH program. trial Physics, Garching, The Johns Hopkins Univer- sity, Durham University, the University of Edinburgh, We thank Fumiaki Nakata and Rita Morris for as- Queen’s University Belfast, the Harvard-Smithsonian sistance at the Subaru Telescope, Mike Beasley for at- Center for Astrophysics, the Las Cumbres Observatory tempting to obtain a spectrum of the new dwarf, the Global Telescope Network Incorporated, the National referee for helpful comments, and Michael Wood-Vasey Central University of Taiwan, the Space Telescope Sci- for conversations that helped improve our photome- ence Institute, the National Aeronautics and Space Ad- try. JLC and BW acknowledge support by NSF Fac- ministration under Grant No. NNX08AR22G issued ulty Early Career Development (CAREER) award AST- through the Planetary Science Division of the NASA 1151462. DJS acknowledges support from NSF grant Science Mission Directorate, the National Science Foun- AST-1412504. The work of DJS was performed at the dation under Grant AST-1238877, the University of Aspen Center for Physics, which is supported by NSF Maryland, Eotvos Lorand University (ELTE), and the grant PHY-1066293. JPB and AR are supported by Los Alamos National Laboratory. NSF grant AST-1211995. Some data presented here were obtained from the Mikulski Archive for Space Tele- Facilities: Subaru (Hyper Suprime-Cam), GALEX, scopes (MAST). STScI is operated by the Association GBT, HST (ACS) of Universities for Research in Astronomy, Inc., under

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