Magellan/M2fs Spectroscopy of the Reticulum 2 Dwarf Spheroidal Galaxy

We present results from spectroscopic observations with the Michigan/ Magellan Fiber System (M2FS) of 182 stellar targets along the line of sight (LOS) to the newly discovered "ultrafaint" object Reticulum 2 (Ret 2). For 37 of these targets, the spectra are sufficient to provide simultaneous estimates of LOS velocity ( , median random error km s −1 ), effective temperature ( , K), surface gravity ( , dex), and iron abundance ( , dex). We use these results to confirm 17 stars as members of Ret 2. From the member sample we estimate a velocity dispersion of km s −1 about a mean of km s −1 in the solar rest frame ( km s −1 in the Galactic rest frame), and a metallicity dispersion of dex about a mean of . These estimates marginalize over possible velocity and metallicity gradients, which are consistent with zero. Our results place Ret 2 on chemodynamical scaling relations followed by the Milky Way's dwarf-galactic satellites. Under assumptions of dynamic equilibrium and negligible contamination from binary stars—both of which must be checked with deeper imaging and repeat spectroscopic observations—the estimated velocity dispersion suggests a dynamical mass of enclosed within projected halflight radius pc, with mass-to-light ratio in solar units.