We present the results of a spectroscopic survey of the recently discovered faint Milky Way satellites Boötes , Ursa Major I , Ursa Major II and Willman 1 .
Using the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope , we have obtained samples that contain from \sim 15 to \sim 85 probable members of these satellites for which we derive radial velocities precise to a few { km s ^ { -1 } } down to i \sim 21 - 22 .
About half of these stars are observed with a high enough S/N to estimate their metallicity to within \pm 0.2 dex .
The characteristics of all the observed stars are made available , along with those of the Canes Venatici I dwarf galaxy that have been analyzed in a companion paper . From this dataset , we show that Ursa Major II is the only object that does not show a clear radial velocity peak .
However , the measured systemic radial velocity ( v _ { r } = 115 \pm 5 { km s ^ { -1 } } ) is in good agreement with recent simulations in which this object is the progenitor of the recently discovered Orphan Stream .
The three other satellites show velocity dispersions that make them highly dark-matter dominated systems ( under the usual assumptions of symmetry and virial equilibrium ) .
In particular , we show that despite its small size and faintness , the Willman 1 object is not a globular cluster given its metallicity scatter over -2.0 \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 % pt \hbox { $ \mathchar 316 $ } } { [ Fe / H ] } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { % $ \mathchar 536 $ } \hss } \raise 2.0 pt \hbox { $ \mathchar 316 $ } } -1.0 and is therefore almost certainly a dwarf galaxy or dwarf galaxy remnant .
We measure a radial velocity dispersion of only 4.3 _ { -1.3 } ^ { +2.3 } { km s ^ { -1 } } around a systemic velocity of -12.3 \pm 2.3 { km s ^ { -1 } } which implies a mass-to-light ratio of \sim 700 and a total mass of \sim 5 \times 10 ^ { 5 } { M _ { \odot } } for this satellite , making it the least massive satellite galaxy known to date .
Such a low mass could mean that the 10 ^ { 7 } { M _ { \odot } } limit that had until now never been crossed for Milky Way and Andromeda satellite galaxies may only be an observational limit and that fainter , less massive systems exist within the Local Group .
However , more modeling and an extended search for potential extra-tidal stars are required to rule out the possibility that these systems have not been significantly heated by tidal interaction .