Dwarf Spheroidal Galaxies in the M101 Group and Behind it

Abstract

We use images from the Hubble Space Telescope to determine the distances to a dozen objects with low surface brightness recently observed around the bright nearby spiral M101. Only two dwarf galaxies, M101-DwA and M101-Dw9, turn out to be actual satellites of M101 at distances of about 7 Mpc. The other objects are probably members of a distant group around the S0-galaxies NGC5485/5473. Based on the radial velocities and projected separations of the 9 satellites, we obtain an estimate of (8.5 ± 3.0) 10¹¹M_⨀ for the total mass of M101, which is consistent with a ratio 16±6 for the total mass to the stellar mass of the galaxy.