The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem

We present Keck/DEIMOS spectroscopy of stars in 8 of the newly discovered ultra-faint dwarf galaxies around the Milky Way. We measure the velocity dispersions of Canes Venatici I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, Leo T, Ursa Major I, and Ursa Major II from the velocities of 18− 214 stars in each galaxy and find dispersions ranging from 3.3 to 7.6 km s. The 6 galaxies with absolute magnitudes MV < −4 are highly dark matter-dominated, with mass-tolight ratios approaching 1000 M⊙/L⊙,V . For the fainter galaxies, Ursa Major II and (to a lesser extent) Coma Berenices, we find tentative evidence for tidal disruption, which for UMa II is strongly supported by previous studies. If these 2 galaxies are also dark matter-dominated, they have extremely large mass-to-light ratios. The measured velocity dispersions of the ultra-faint dwarf galaxies are correlated with their luminosities, indicating that a minimum mass for luminous galactic systems may not yet have been reached. The 6 brightest of the ultra-faint dwarfs extend the luminosity-metallicity relationship followed by normal dwarfs by ∼ 2 orders of magnitude in luminosity; several of these objects have mean metallicities as low as [Fe/H] =−2.3 and therefore represent some of the most metalpoor known stellar systems. We detect metallicity spreads up to 0.5 dex in several objects, suggesting multiple star formation epochs. UMa II and Com, despite their exceptionally low luminosities, have higher metallicities that suggest they may once have been much more massive. Having established the masses of the ultra-faint dwarfs, we re-examine the missing satellite problem. After correcting for the sky coverage of the Sloan Digital Sky Survey, we find that the ultra-faint dwarfs substantially alleviate the discrepancy between the predicted and observed numbers of satellites around the Milky Way, but there are still a factor of ∼ 4 too few dwarf galaxies over a significant range of masses. We show that if galaxy formation in low-mass dark matter halos is strongly suppressed after reionization, the simulated circular velocity function of CDM subhalos can be brought into approximate agreement with the observed circular velocity function of Milky Way satellite galaxies. Subject headings: dark matter — galaxies: dwarf — galaxies: kinematics and dynamics — Local Group — techniques: radial velocities