2017ApJ...843L...6S

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 distance of 3.98_–0.43^+0.39 Mpc, establishing group membership. We derive an absolute V-band magnitude, from stellar isochrone fitting, of M_V=-7.94_–0.50^+0.38, with a half-light radius of r_h=188_–41⁺³⁹ 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.