Ursa Major: a missing low-mass CDM halo?

The recently discovered Ursa Major dwarf spheroidal (dSph) galaxy candidate is about five to eight times less luminous than the faintest previously known dSphs And IX, Draco, and UMi. In this Letter, we present velocity measurements of seven color-magnitude selected UMajor candidate stars. Two of them are apparent non-members based on metallicity and velocity, and the remaining five stars yield a systemic heliocentric velocity of v=-52.45 +/-4.27 km/s and a central line of sight velocity dispersion of 9.3 (+11.7 -1.2) km/s, with 95% confidence that the dispersion is >6.5 km/s. Assuming that UMajor is in dynamical equilibrium, it is clearly dark matter dominated, and cannot be a purely stellar system like a globular cluster. It has an inferred central mass-to-light ratio of M/L~500 and, based on our studies of other dSphs, may possess a much larger total mass to light ratio. UMajor is unexpectedly massive for its low luminosity -- indeed, UMajor appears to be the most dark-matter dominated galaxy yet discovered. The presence of so much dark matter in UMajor immediately suggests that it may be a member of the missing population of low-mass galaxies predicted by the Cold Dark Matter (CDM) paradigm. Given the weak correlation between dSph mass and luminosity, it is entirely likely that a population of dark dwarfs surrounds our Galaxy.