Discovery of Isolated, Quenched, and Likely Backsplash Dwarf Galaxies near M101

Abstract

I report the discovery of three faint, semi-resolved quiescent dwarf galaxies, two of which are strong backsplash candidates associated with the nearby satellite-sparse spiral M101 (D ~ 6.7 Mpc). The galaxies lie within the magnitude range MV ~ -7.70 to -8.2 and half-light radii rh ~ 110 to 204 pc. Shapiro DG-I (Sha DG-I/MAGE1412+5650) is a concurrently discovered and isolated galaxy. Shapiro DG-II (Sha DG-II) is a fainter dwarf and potential companion to the SMC-mass galaxy NGC 5585 (D ~ 6.84 Mpc). Shapiro DG-III (Sha DG-III) is an isolated dwarf on the edge of the ultra-faint regime. Hydrodynamical simulations suggest a backsplash population of galaxies that have been environmentally stripped by interactions with a host and ejected from the system, though they have not yet been definitively observed in the local universe. Considering their quenched stellar populations, indicated by the lack of coincident GALEX emission, relative distances, and fitting models, the galaxies are consistent with a backsplash origin and are pending follow-up. Analysis is performed to test whether the system's population can be explained by standard cosmology. A potential correlation is found between satellite abundances and halo masses calibrated by tidal indices in a sample of nearby MW-like galaxies, suggesting the importance of environmental density in the formation of dark matter subhaloes, though a larger sample is required. M101's sparse satellite system fits well in the relation and is in agreement with {\Lambda}CDM expectations.