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 _ { \mathrm { los } } , median random error \delta _ { v _ { \mathrm { los } } } = 1.3 km s ^ { -1 } ) , effective temperature ( T _ { \mathrm { eff } } , \delta _ { T _ { \mathrm { eff } } } = 464 K ) , surface gravity ( \log g , \delta _ { \log g } = 0.54 dex ) and iron abundance ( \mathrm { [ 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 _ { \mathrm { los } } } = 3.6 _ { -0.6 } ^ { +0.9 } km s ^ { -1 } about a mean of \langle v _ { \mathrm { 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 _ { \mathrm { [ 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 _ { h } ) \approx 5 R _ { h } \sigma _ { v _ { \mathrm { los } } } ^ { 2 } / ( 2 G ) = 2.4 _ { -0.8 } ^ { +1.3 } \times 10 ^ { 5 } M _ { \odot } enclosed within projected halflight radius R _ { h } \sim 32 pc , with mass-to-light ratio \approx 2 M ( R _ { h } ) / L _ { V } = 462 _ { -157 } ^ { +264 } in solar units .