Andromeda X, a New Dwarf Spheroidal Satellite of M31: Photometry

We report the discovery of Andromeda X, a new dwarf spheroidal satellite of M31, based on stellar photometry from the Sloan Digital Sky Survey. Using follow-up imaging data we have estimated its distance and other physical properties. We find that Andromeda X has a dereddened central surface brightness of μ_V,0~26.7 mag arcsec^-2 and a total apparent magnitude of V_tot~16.1, which at the derived distance modulus, (m-M)₀~24.12-24.34, yields an absolute magnitude of M_V~-8.1+/-0.5 these values are quite comparable to those of Andromeda IX, a previously discovered low-luminosity M31 satellite. The discoveries of Andromeda X and of numerous other extremely faint satellites around M31 and the Milky Way in the past few years suggest that such objects may be plentiful in the Local Group.