Discovery of Isolated, Quenched Candidate Backsplash Dwarf Galaxies near M101

Abstract

I report the discovery of three faint, semi-resolved quiescent candidate dwarf galaxies, which are potential backsplash galaxies associated with the nearby spiral M101 ($D \sim 6.7 \,\mathrm{Mpc}$). The galaxies are cataloged as Shapiro DG-I/MAGE1412+5650, Shapiro DG-II, and Shapiro DG-III. Sha DG-I is concurrently discovered in ID-MAGE. The backsplash candidates lie in a magnitude range $M_V\sim-7.3$ to $-8.1$, and half-light radii $r_h\sim110-140\,\mathrm{pc}$. Hydrodynamical simulations suggest a population of backsplash 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. Initial distance estimates are determined using surface brightness fluctuations in S\'ersic model-subtracted CFHTLS and HSC archival images. The galaxies are not detected in GALEX, or H$\alpha$ for Sha DG-I/MAGE1412+5650, providing upper limits on their SFR history. The provisional distances and quiescence of the dwarf candidates are consistent with a potential backsplash nature, though HST/JWST follow-up are necessary for stronger constraints, and to probabilistically distinguish from cosmic web stripping. Sha DG-II is alternatively a possible satellite of NGC 5585. Abundance models of satellites based on halo masses, which can be extended to model backsplash galaxies, will enable important consistency tests of $\Lambda\mathrm{CDM}$ with upcoming surveys, e.g., LSST. This is briefly explored in the context of the M101 group.