Laevens et al . recently discovered Triangulum II , a satellite of the Milky Way .
Its Galactocentric distance is 36 kpc , and its luminosity is only 450 L _ { \sun } .
Using Keck/DEIMOS , we measured the radial velocities of six member stars within 1.2′ of the center of Triangulum II , and we found a velocity dispersion of \sigma _ { v } = 5.1 _ { -1.4 } ^ { +4.0 } km s ^ { -1 } .
We also measured the metallicities of three stars and found a range of 0.8 dex in [ Fe/H ] .
The velocity and metallicity dispersions identify Triangulum II as a dark matter-dominated galaxy .
The galaxy is moving very quickly toward the Galactic center ( v _ { GSR } = -262 km s ^ { -1 } ) .
Although it might be in the process of being tidally disrupted as it approaches pericenter , there is no strong evidence for disruption in our data set .
The ellipticity is low , and the mean velocity , \langle v _ { helio } \rangle = -382.1 \pm 2.9 km s ^ { -1 } , rules out an association with the Triangulum–Andromeda substructure or the Pan-Andromeda Archaeological Survey ( PAndAS ) stellar stream .
If Triangulum II is in dynamical equilibrium , then it would have a mass-to-light ratio of 3600 _ { -2100 } ^ { +3500 } ~ { } M _ { \sun } ~ { } L _ { \sun } ^ { -1 } , the highest of any non-disrupting galaxy ( those for which dynamical mass estimates are reliable ) .
The density within the 3-D half-light radius would be 4.8 _ { -3.5 } ^ { +8.1 } ~ { } M _ { \sun } ~ { } { pc } ^ { -3 } , even higher than Segue 1 .
Hence , Triangulum II is an excellent candidate for the indirect detection of dark matter annihilation .