2015ApJ...808..108W

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 (νi_los, median random error δ_νlos = 1.4 km/s), effective temperature (T_eff, δ_Tef = 478 K), surface gravity (log g, δ_log g_ = 0.63 dex), and iron abundance ([Fe/H], δ_[Fe/H] = 0.47 dex). We use these results to confirm 17 stars as members of Ret 2. From the member sample we estimate a velocity dispersion of σ_νlos = 3.6_–0.7^+1.0 km/s about a mean of <ν_los> = 64.3_1.2^+1.2 km/s in the solar rest frame (~-90.9 km/s in the Galactic rest frame), and a metallicity dispersion of σ_[Fe/H] = 0.49_–0.14^+0.19 dex about a mean of <[Fe/H]_> = -2.58_–0.33^+0.34. 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 M(R_h)≈5R_hσ_νlos²/(2G) = 2.4_–0.8^+1.4 x 10⁵M_☉ enclosed within projected halflight radius R_h ∼ 32 pc pc, with mass-to-light ratio ≈ 2M(R_h)/L_V = 467_–168⁺²⁸⁶ in solar units.