Ursa Major: A Missing Low-Mass CDM Halo?

The recently discovered Ursa Major dwarf spheroidal (dSph) galaxy candidateis about five to eight times less luminous than the faintest previously knowndSphs And IX, Draco, and UMi. In this Letter, we present velocity measurementsof seven color-magnitude selected UMajor candidate stars. Two of them areapparent non-members based on metallicity and velocity, and the remaining fivestars yield a systemic heliocentric velocity of v=-52.45 +/-4.27 km/s and acentral 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 indynamical equilibrium, it is clearly dark matter dominated, and cannot be apurely stellar system like a globular cluster. It has an inferred centralmass-to-light ratio of M/L~500 and, based on our studies of other dSphs, maypossess a much larger total mass to light ratio. UMajor is unexpectedly massivefor its low luminosity -- indeed, UMajor appears to be the most dark-matterdominated galaxy yet discovered. The presence of so much dark matter in UMajorimmediately suggests that it may be a member of the missing population oflow-mass galaxies predicted by the Cold Dark Matter (CDM) paradigm. Given theweak correlation between dSph mass and luminosity, it is entirely likely that apopulation of dark dwarfs surrounds our Galaxy.Comment: 8 pages, 3 figures; submitted to ApJ Letters - minor corrections to first versio