Ursa Major: A missing low-mass CDM halo?

The recently discovered Ursa Major dwarf spheroidal ( dSph) galaxy candidate is about 5 - 8 times less luminous than the faintest previously known dSphs, And IX, Draco, and Ursa Minor. In this Letter, we present velocity measurements of seven color- magnitude - selected Ursa Major candidate stars. Two of them are apparent nonmembers based on metallicity and velocity, and the remaining five stars yield a systemic heliocentric velocity of (v) over bar = -52.45 +/- 4.27 km s(-1) and a central line- of- sight velocity dispersion of < v(2)>(1/2) = 9.3 (+11.7)(-1.2) km s(-1), with 95% confidence that < v(2)>(1/2). > 6.5 km s(-1) Assuming that UMa 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 masstolight ratio of and, based on our studies of other dSphs, may possess a much larger total M/ L similar to 500 M-circle dot/ L-circle dot mass- to- light ratio. UMa is unexpectedly massive for its low luminosity - indeed, UMa appears to be the most dark matter - dominated galaxy yet discovered. The presence of so much dark matter in UMa 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.