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 to the
newly-discovered `ultrafaint' object Reticulum 2 (Ret 2). For 38 of these
targets, the spectra are sufficient to provide simultaneous estimates of
line-of-sight velocity ($v_{\rm los}$, median random error $\delta_{v_{\rm
los}}=1.3$ km s$^{-1}$), effective temperature ($T_{\rm eff}$, $\delta_{T_{\rm
eff}}=464$ K), surface gravity ($\log g$, $\delta_{\rm logg}=0.54$ dex) and
iron abundance ([Fe/H], $\delta_{\mathrm{[Fe/H]}}=0.45$ dex). We use these
results to confirm 18 stars as members of Ret 2. From the member sample we
estimate a velocity dispersion of $\sigma_{v_{\rm los}}=3.6_{-0.6}^{+0.9}$ km
s$^{-1}$ about a mean of $\langle v_{\rm los}\rangle =64.8_{-1.0}^{+1.1}$ km
s$^{-1}$ in the solar rest frame ($\sim -90.9$ km s$^{-1}$ in the Galactic rest
frame), and a metallicity dispersion of $\sigma_{\rm
[Fe/H]}=0.50_{-0.13}^{+0.17}$ dex about a mean of $\langle \mathrm{[Fe/H]}
\rangle =-2.67_{-0.34}^{+0.34}$. 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 $M(R_{\rm h})\approx 5R_{\rm
h}\sigma_{v_{\rm los}}^2/(2G)=2.4_{-0.8}^{+1.3}\times 10^5$ $M_{\odot}$
enclosed within projected halflight radius $R_{\rm h}\sim 32$ pc, with
mass-to-light ratio $\approx 2M(R_{\rm h})/L_{\rm V}=462_{-157}^{+264}$ in
solar units.
