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 ^ { -1 } .
The 6 galaxies with absolute magnitudes M _ { V } < -4 are highly dark matter-dominated , with mass-to-light ratios approaching 1000 M _ { \odot } / L _ { \odot,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 .
We also measure the metallicities of the observed stars and find that several of the new dwarfs have mean metallicities as low as [ Fe/H ] = -2.3 ; these galaxies represent some of the most metal-poor known stellar systems .
The 6 brightest of the ultra-faint dwarfs extend the luminosity-metallicity relationship followed by more luminous dwarfs by \sim 2 orders of magnitude in luminosity .
We detect metallicity spreads of 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 \sim 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 .