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We present Keck/DEIMOS spectroscopy of stars in 8 of the newly discoveredultra-faint dwarf galaxies around the Milky Way. We measure the velocitydispersions of Canes Venatici I and II, Ursa Major I and II, Coma Berenices,Hercules, Leo IV and Leo T from the velocities of 18 - 214 stars in each galaxyand find dispersions ranging from 3.3 to 7.6 km/s. The 6 galaxies with absolutemagnitudes M_V < -4 are highly dark matter-dominated, with mass-to-light ratiosapproaching 1000. The measured velocity dispersions are inversely correlatedwith their luminosities, indicating that a minimum mass for luminous galacticsystems may not yet have been reached. We also measure the metallicities of theobserved stars and find that the 6 brightest of the ultra-faint dwarfs extendthe luminosity-metallicity relationship followed by brighter dwarfs by 2 ordersof magnitude in luminosity; several of these objects have mean metallicities aslow as [Fe/H] = -2.3 and therefore represent some of the most metal-poor knownstellar systems. We detect metallicity spreads of up to 0.5 dex in severalobjects, suggesting multiple star formation epochs. Having established themasses of the ultra-faint dwarfs, we re-examine the missing satellite problem.After correcting for the sky coverage of the SDSS, we find that the ultra-faintdwarfs substantially alleviate the discrepancy between the predicted andobserved numbers of satellites around the Milky Way, but there are still afactor of ~4 too few dwarf galaxies over a significant range of masses. We showthat if galaxy formation in low-mass dark matter halos is strongly suppressedafter reionization, the simulated circular velocity function of CDM subhaloscan be brought into approximate agreement with the observed circular velocityfunction of Milky Way satellite galaxies. [slightly abridged]

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