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 (vlos, median random error 1.4 vlos d = km s ), effective temperature (Teff , 478 Teff d = K), surface gravity ( g log , 0.63 g log d = dex), and iron abundance ([Fe H], 0.47 [Fe H] d = dex). We use these results to confirm 17 stars as members of Ret 2. From the member sample we estimate a velocity dispersion of vlos s = 3.6 0.7 1.0 + km s about a mean of vlos á ñ = 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 [Fe H] s = 0.49 0.14 0.19 + dex about a mean of [Fe H] á ñ = 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 R G ( ) 5 (2 ) v h h 2 los s » = 2.4 10 0.8 1.4 5 ́ + M enclosed within projected halflight radius R 32 h ~ pc, with mass-to-light ratio M R L 2 ( ) V h » = 467 168 286 + in solar units.