Accepted to ApJL, 9 June 2017. Preprint typeset using LATEX style AASTeX6 v. 1.0 D1005+68: A NEW FAINT DWARF GALAXY IN THE M81 GROUP Adam Smercina1, Eric F. Bell1, Colin T. Slater2, Paul A. Price3, Jeremy Bailin4, Antonela Monachesi5,6,7 (Accepted to ApJL, 9 June 2017) 1Department of Astronomy, University of Michigan, 1085 S. University Avenue, Ann Arbor, MI 48109-1107, USA; [email protected] 2Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195-1580, USA 3Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA 4Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35487-0324, USA 5MPA, Garching, Germany 6Departamento de F´ısica y Astronom´ıa, Universidad de La Serena, Av. Juan Cisternas 1200 N, La Serena, Chile 7Instituto de Investigaci´onMultidisciplinar en Ciencia y Tecnolog´ıa,Universidad de La Serena, Ra´ulBitr´an1305, La Serena, Chile ABSTRACT We present the discovery of d1005+68, a new faint dwarf galaxy in the M81 Group, using observations taken with the Subaru Hyper Suprime-Cam. d1005+68's color-magnitude diagram is consistent with a +0:39 distance of 3:98−0:43 Mpc, establishing group membership. We derive an absolute V -band magnitude, +0:38 +39 from stellar isochrone fitting, of MV = −7:94−0:50, with a half-light radius of rh = 188−41 pc. These place d1005+68 within the radius{luminosity locus of Local Group and M81 satellites and among the faintest confirmed satellites outside the Local Group. Assuming an age of 12 Gyr, d1005+68's red giant branch is best fit by an isochrone of [Fe/H] = −1:90 ± 0:24. It has a projected separation from nearby M81 satellite BK5N of only 5 kpc. As this is well within BK5N's virial radius, we speculate that d1005+68 may be a satellite of BK5N. If confirmed, this would make d1005+68 one of the first detected satellites-of-a-satellite. 1. INTRODUCTION to MSP and TBTF). Furthermore, mounting evidence The past decade has seen an awakening in the suggests that both the Milky Way's and M31's satel- field of dwarf galaxy discovery. Large photometric sur- lites form potentially planar structures (Pawlowski et al. veys such as the Sloan Digital Sky Survey (SDSS), the 2013). Though ΛCDM predicts anisotropic accretion Panoramic Survey Telescope Rapid Response System due to infall along cosmic filaments (e.g., Li & Helmi (Pan-STARRS), and the Dark Energy Survey (DES) 2008), potentially resulting in planar satellite distribu- have permitted the discovery of >30 faint and ultrafaint tions (Sawala et al. 2016), the thinness of the Local dwarf galaxy (UFD) candidates in the Local Group (e.g., Group planes remains difficult to replicate. Belokurov et al. 2006; Martin et al. 2013; Drlica-Wagner ΛCDM predicts that all galaxy halos host sub- et al. 2016; Homma et al. 2016). These discoveries have halos, the most massive of which will host luminous informed the nearly two-decade-old \missing satellites satellites. Consequently, many of the satellites around problem" (hereafter MSP; Klypin et al. 1999). This ap- Milky Way{mass galaxies also likely possess, or pos- parent tension between the low-end halo mass function sessed before infall, their own orbiting subhalos. These \satellites-of-satellites" are difficult to detect, owing to arXiv:1706.07039v2 [astro-ph.GA] 22 Jun 2017 slope, predicted by ΛCDM, and the considerably flatter slope of the Milky Way dwarf galaxy luminosity func- their intrinsic faintness. Recent work suggests that sev- tion is a sensitive probe of dark matter properties and eral of the Milky Way satellites nearest to the Magel- galaxy formation in the lowest-mass dark matter halos lanic Clouds may be satellites of the Clouds themselves (e.g., Macci`oet al. 2010; Brooks et al. 2013). Yet, with (Drlica-Wagner et al. 2016), with possibly > 30% of improved understanding, new puzzles have emerged. An Milky Way satellites originating around the Large Mag- apparent dearth of luminous high-velocity subhalos { ellanic Cloud (LMC; Jethwa et al. 2016). the \too big to fail" problem (hereafter TBTF; Boylan- It is clear that our understanding of dwarf galaxy Kolchin et al. 2011) | is an extension of MSP that is populations in the ΛCDM paradigm is currently limited. not alleviated by the discovery of UFDs (see Simon & A key hurdle is that our understanding of dwarf galaxy Geha 2007, Macci`oet al. 2010, Font et al. 2011, and luminosity functions, spatial distributions, and prop- Brooks et al. 2013 for discussion of possible solutions erties is almost entirely confined to the Local Group. 2 Smercina et al. Characterization of satellite populations around other STARRS1 (Magnier et al. 2013). An aggressive back- Local Group analogs is crucial if we are to obtain a com- ground subtraction using a 32 pixel region for determin- plete description of low-mass galaxy formation. ing the background was used. Objects are detected in Propelled by the advent of wide-field imagers on i band and forced photometry is performed in g and r. large telescopes, discovery and characterization of faint The average FWHM in M81 Field 2 (in which d1005+68 00 `classical dwarfs' (MV < −10) has become possible was discovered) is ∼0: 7 in all bands, giving limiting in nearby galaxy groups and clusters using large area 5σ point-source magnitudes of g ∼ 27, r ∼ 26:5, and (approaching 100 deg2) diffuse light surveys (e.g., Chi- i ∼ 26. All magnitudes use the SDSS photometric sys- boucas et al. 2009; M¨ulleret al. 2015; Mu~nozet al. tem, corrected for foreground Galactic extinction using 2015; Ferrarese et al. 2016). Observationally expensive, the Schlegel et al.(1998) maps as calibrated by Schlafly smaller area deep surveys of resolved stellar populations & Finkbeiner(2011). in nearby galaxy groups are bringing even fainter dwarf As the dwarf galaxies of interest are low surface galaxies within reach (e.g., Sand et al. 2015; Carlin et al. brightness and possess little diffuse emission, we detect 2016; Crnojevi´cet al. 2016; Toloba et al. 2016). dwarf candidates by resolving them into individual stars. In this Letter, we present the discovery of a faint At the distance of M81 (3.6 Mpc; Radburn-Smith et al. dwarf spheroidal galaxy in the M81 group, d1005+68 2011), only stars in the top ∼25%, or tip of the RGB (following the naming convention of Chiboucas et al. (TRGB), are visible. TRGB stars are relatively numer- 2013), detected as an overdensity of stars in observations ous, and as they trace the old stellar population of galax- taken with the Subaru Hyper Suprime-Cam. At MV = ies, their number can be scaled to a total luminosity with −7:9 (see § 3), d1005+68 is one of the faintest confirmed modest uncertainty (Harmsen et al. 2017). galaxies discovered outside of the Local Group. At our survey depths, contaminants { high- redshift background galaxies { dominate. The majority of these galaxy contaminants must be removed in or- Table 1. d1005+68 Parameters der to reach the surface brightness sensitivity necessary 2 to detect faint dwarf satellites (µV . 28 mag arcsec ). Parameter Value We reject galaxies using a combined morphology and color cut; such a process sacrifices completeness in or- α (J2000) 10h05m31s:82 ± 1s:1 der to dramatically suppress contamination (this will be δ (J2000) +68◦1401900: 56 ± 500: 95 revisited in § 3). To be defined as a star, a source must +0:39 00 DTRGB 3:98−0:43 Mpc satisfy two criteria: (1) FWHM 6 0:6 across all three a +0:38 MV −7:94−0:50 bands (we will consider less stringent cuts later), and (2) 00 00 rh 9: 7 ± 2: 0 consistent with the g − r vs. r − i stellar locus within +39 rh 188−41 pc σg−r (the photometric uncertainty) + 0.2 mag (intrin- b +0:22 log10(M∗=M ) 5:40−0:16 sic scatter; High et al. 2009). Next, we locate stars on [Fe/H] c −1:90 ± 0:24 the RGB from the g − r vs. r color-magnitude diagram (CMD) and divide them into three metallicity bins using simple polygonal boundaries (see Figure1). Note|a Isochrone fitting, assuming b d1005+68 stands out as a significant overden- DTRGB. Current stellar mass, assuming 40% mass loss. c Metallicity of best-fit sity of metal-poor stars in the sparse, metallicity-binned isochrone, assuming [α/Fe] = 0.25. RGB star map of M81's stellar halo (Figure1), with nine RGB stars visible in a 10 × 10 region centered on d1005+68. To quantify the prominence of this over- density against the surrounding diffuse stellar halo, we 2. DETECTION extract 500 10 × 10 (independent) regions from a 0.14 We use observations taken with the Subaru Hy- deg2 region south of d1005+68, away from the stellar per Suprime-Cam (HSC; Miyazaki et al. 2012) through debris associated with the tidal disruption of NGC 3077. NOAO Gemini-Subaru exchange time (PI: Bell, 2015A- We compute the discrete probability distribution of the 0281). The observations consist of two pointings for a number of RGB stars returned in each region and fit survey footprint area of ∼ 3:5 deg2, in three filters: g, it to a Poisson distribution, p(Njλ). From the best- r, and i, with ∼ 3600 s per filter per pointing. The fit Poisson distribution, we take a mean background of data were reduced using the HSC pipeline (Bosch et al. λ = 0:38 ± 0:03 RGB stars arcmin−2. Integrating over 2017, in preparation), which was developed from the the best-fit distribution, and correcting for the number LSST Pipeline (Axelrod et al.