2019ApJ...884..128O

We use Hyper Suprime-Cam on the Subaru Telescope to investigate the structural and photometric properties of early-type dwarf galaxies and young stellar systems at the center of the M81 Group. We have mapped resolved stars to ∼2 mag below the tip of the red giant branch (RGB) over almost 6.5 square degrees, corresponding to a projected area of 160 x 160 kpc at the distance of M81. The resulting stellar catalog enables a homogeneous analysis of the member galaxies with unprecedented sensitivity to low surface brightness emission. The radial profiles of the dwarf galaxies are well described by Sersic and King profiles, and show no obvious signatures of tidal disruption. The measured radii for most of these systems are larger than the existing literature values, and we find the total luminosity of IKN (M_V,0 = -14.29) to be almost 3 mag brighter than previously thought. We identify new dwarf satellite candidates, d1006+69 and d1009+68, which we estimate to lie at a distance of 4.3 ± 0.2 Mpc and 3.5 ± 0.5 Mpc. With M_V,0 = -8.91 ± 0.40 and [M/H] = -1.83 ± 0.28, d1006+69 is one of the faintest and most metal-poor dwarf satellites currently known in the M81 Group. The luminosity functions of young stellar systems in the outlying tidal neutral hydrogen (H I) debris imply continuous star formation in the recent past and the existence of populations as young as 30 Myr old. We find no evidence for old RGB stars coincident with the young MS/cHeB stars that define these objects, supporting the idea that they are genuinely new stellar systems resulting from triggered star formation in gaseous tidal debris.