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 (ν_los, median random error δ_νlos= 1.4 km s^-1), effective temperature (T_eff, Δ _Teff K), surface gravity (loh 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^+1.0km s^-1 about a mean of 〈ν_los〉 = 64.3_-1.2^+1.2 km s^-1 in the solar rest frame ( ∼ -90.9 km s^-1 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_hσνlos;²/(2G) = 2.4_-0.8^+1.4 × 10⁵ M_⊙enclosed within projected halflight radius R_h ∼32 pc, with mass-to-light ratio ≈2M (R_h)/L_V = 467_-168⁺²⁸⁶in solar units.