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 ($v_{los}$, median random error ${\delta }_{v_{los}}=1.4$ km s⁻¹), effective temperature ($T_{eff}$, ${\delta }_{T_{eff}}=478$ K), surface gravity ($logg$, ${\delta }_{logg}=0.63$ dex), and iron abundance ($\left[Fe/H\right]$, ${\delta }_{\left[Fe/H\right]}=0.47$ dex). We use these results to confirm 17 stars as members of Ret 2. From the member sample we estimate a velocity dispersion of ${\sigma }_{v_{los}}=$ ${3.6}_{-0.7}^{+1.0}$ km s⁻¹ about a mean of $⟨v_{los}⟩=$ ${64.3}_{-1.2}^{+1.2}$ km s⁻¹ in the solar rest frame ($\sim -90.9$ km s⁻¹ in the Galactic rest frame), and a metallicity dispersion of ${\sigma }_{\left[Fe/H\right]}=$ ${0.49}_{-0.14}^{+0.19}$ dex about a mean of $⟨\left[Fe/H\right]⟩=$ $-{2.58}_{-0.33}^{+0.34}$. 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 $M\left(R_h\right)\approx 5R_h{\sigma }_{v_{los}}{}^2/\left(2G\right)=$ ${2.4}_{-0.8}^{+1.4}\times {10}^5$ $M_{\odot }$ enclosed within projected halflight radius $R_h\sim 32$ pc, with mass-to-light ratio $\approx 2M\left(R_h\right)/L_V=$ ${467}_{-168}^{+286}$ in solar units.