Building out the census of faint and ultra-faint satellites of Milky Way-mass galaxies: new satellites-of-satellites in the M81 group

The satellites of the Milky Way and Andromeda form the observational bedrock upon which our understanding of small-scale cosmology rests. In order to understand if this insight generalizes, it is imperative to build a census of faint satellite galaxies in nearby galaxy groups. While diffuse light searches are sensitive to some classical dwarfs in nearby galaxy groups and have been used to amass samples of hundreds of nearby satellites, resolved star techniques are necessary to discover and characterize fainter and ultra-diffuse satellite galaxies. With current datasets, such searches are limited to nearby systems, within 5Mpc of the Milky Way. We describe our search for ultra-faint and ultra-diffuse galaxies in the M81 group using deep ground-based resolved star datasets from Subaru’s Hyper Suprime-Cam. We present new satellite galaxies in the M81 group and identify additional candidate companions that — if confirmed — would reach deep into the ultra-faint regime at M_V~-6.5 (similar to current limits for Andromeda satellites). Intriguingly, these candidates (and all but one of the recent dwarf discoveries) are spatially clustered around NGC 3077, which is itself an M81 group satellite in an advanced state of tidal disruption. This is somewhat surprising, as M81 itself and its largest satellite M82 are both substantially more massive than NGC 3077, and by virtue of their greater masses would have been expected to host as many or more ultra-faint candidates. This lends considerable support to the idea that satellites-of-satellites are an unexpectedly important contribution to the growth of satellite populations around Milky Way-massed galaxies.