A deep Subaru Hyper Suprime Cam i-band image of the newly-found dwarf M81 group member d1005+68. The concentric green circles correspond to apertures with 1 and 2× the half-light radius. High confidence member stars are shown in red and lower confidence members shown in blue.
We have discovered a new faint member of the M81 group. With MV~-8, it is one of the faintest ever galaxies found outside the Local Group. In a recently published Letter to the Astrophysical Journal, Adam Smercina and our collaborators discovered this galaxy in deep wide-field imaging from Subaru’s Hyper Suprime Cam by resolving it into individual red giant branch stars. Galaxies at this faintness have extremely low surface brightness and are typically invisible in their diffuse light.
The stars in d1005+68 have brightnesses and colors suggesting a distance of 4+/-0.4 Mpc, consistent with d1005+68 being a group member. It ha a half-light radius of ~200pc, and a metallicity of [Fe/H]~-1.9. These properties are similar to known satellites in the Local Group.
d1005+68 has a projected separation from nearby M81 satellite BK5N of only 5 kpc. As this is well within BK5N’s virial radius, we speculate that d1005+68 may be a satellite of BK5N. If confirmed, this would make d1005+68 one of the first detected satellites-of-a-satellite.