Ursa Major: A Missing Low-Mass CDM Halo?

Abstract

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 = -52.45 ± 4.27 km s-1 and a central line-of-sight velocity dispersion of v21/2 = 9.3 km s-1, with 95% confidence that v21/2 > 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 mass-to-light ratio of M/L ~ 500 M☉/L☉ and, based on our studies of other dSphs, may possess a much larger total 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.