A Keck deimos kinematic study of Andromeda IX: Dark matter on the smallest galactic scales

We present the results of a kinematic survey of the dwarf spheroidal satellite of M31, And IX, which appears to be the lowest surface brightness and also the faintest galaxy (M-V = -8.3) found to date. Using Keck DEIMOS spectroscopic data, we have measured its velocity relative to M31, its velocity dispersion, and its metallicity. It exhibits a significant velocity dispersion sigma(v) = 6.8(-2.0)(+3.0) km s(-1), which coupled with the low luminosity implies a very high mass-to-V-band light ratio, M/L similar to 93(-50)(+120) M circle dot/L circle dot (M/L > 17 M circle dot/L circle dot at 99% confidence). Unless strong tidal forces have perturbed this system, this smallest of galaxies is a highly dark matter-dominated system.