A Keck DEIMOS Kinematic Study of Andromeda IX: Dark Matter on the Smallest Galactic Scales

Abstract

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 () found to date. Using Keck DEIMOS M p 8.3 V spectroscopic data, we have measured its velocity relative to M31, its velocity dispersion, and its metallicity. It exhibits a significant velocity dispersion km s 1 , which coupled with the low luminosity implies a 3.0 j p 6.8 2.0 v very high mass-to-V-band light ratio, (at 99% confidence). Unless strong 120 M/L ∼ 93 M /L M/L 1 17 M /L 50 , , , , tidal forces have perturbed this system, this smallest of galaxies is a highly dark matter-dominated system.