2024ApJ...977..179G

In the last decade, we have been able to probe further down the galaxy luminosity function than ever before and expand into the regime of ultra-faint dwarfs (UFDs), which are some of the best probes we have of small-scale cosmology and galaxy formation. Digital sky surveys have enabled the discovery and study of these incredibly low-mass, highly dark-matter-dominated systems around the Local Group, but it is critical that we expand the satellite census further out to understand if Milky Way and M31 satellites are representative of dwarf populations in the local Universe. Using data from the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS), we present updated characterization of four satellite systems in the M81 group. These systems-D1005+68, D1006+69, DWJ0954+6821, and D1009+68-were previously discovered using ground-based Subaru Hyper Suprime-Cam data as overdensities in M81's halo, and are now confirmed with HST/ACS by this work. These are all faint (M_V ≥ -7.9) and consistent with old (∼13 Gyr), metal-poor ([M/H] < -1.5) populations. Each system possesses relatively unusual features-including one of the most concentrated satellite galaxies with a Sérsic index of n ∼ 5, one of the most elliptical galaxies outside the Local Group with an ϵ ∼ 0.6, and one of the most compact galaxies for its magnitude. Two of the satellites have very low surface brightness, lower than most known galaxies in this absolute magnitude range. This work previews the scientific promise of the upcoming Rubin Observatory and Roman Telescope for illuminating the diversity of UFDs in the Local Volume and beyond.