We present Magellan/M2FS , VLT/GIRAFFE , and Gemini South/GMOS spectroscopy of the newly discovered Milky Way satellite Reticulum II .
Based on the spectra of 25 Ret II member stars selected from Dark Energy Survey imaging , we measure a mean heliocentric velocity of 62.8 \pm 0.5 \mathrm { km } \mathrm { s } ^ { -1 } and a velocity dispersion of 3.3 \pm 0.7 \mathrm { km } \mathrm { s } ^ { -1 } .
The mass-to-light ratio of Ret II within its half-light radius is 470 \pm 210 \mathrm { M _ { \odot } } / \mathrm { L _ { \odot } } , demonstrating that it is a strongly dark matter-dominated system .
Despite its spatial proximity to the Magellanic Clouds , the radial velocity of Ret II differs from that of the LMC and SMC by 199 and 83 \mathrm { km } \mathrm { s } ^ { -1 } , respectively , suggesting that it is not gravitationally bound to the Magellanic system .
The likely member stars of Ret II span 1.3 dex in metallicity , with a dispersion of 0.28 \pm 0.09 dex , and we identify several extremely metal-poor stars with \mbox { [ Fe / H ] } < -3 .
In combination with its luminosity , size , and ellipticity , these results confirm that Ret II is an ultra-faint dwarf galaxy .
With a mean metallicity of \mbox { [ Fe / H ] } = -2.65 \pm 0.07 , Ret II matches Segue 1 as the most metal-poor galaxy known .
Although Ret II is the third-closest dwarf galaxy to the Milky Way , the line-of-sight integral of the dark matter density squared is \log _ { 10 } ( J ) = 18.8 \pm 0.6 \mathrm { GeV } ^ { 2 } \mathrm { cm } ^ { -5 } within 0.2 \degr , indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies .