We present results from spectroscopic observations with the Michigan/Magellan Fiber System ( M2FS ) of 147 stellar targets along the line of sight to the newly-discovered ‘ ultrafaint ’ stellar systems Tucana 2 ( Tuc 2 ) and Grus 1 ( Gru 1 ) .
Based on simultaneous estimates of line-of-sight velocity and stellar-atmospheric parameters , we identify 8 and 7 stars as probable members of Tuc 2 and and Gru 1 , respectively .
Our sample for Tuc 2 is sufficient to resolve an internal velocity dispersion of 8.6 _ { -2.7 } ^ { +4.4 } km s ^ { -1 } about a mean of -129.1 _ { -3.5 } ^ { +3.5 } km s ^ { -1 } ( solar rest frame ) , and to estimate a mean metallicity of [ Fe/H ] = -2.23 _ { -0.12 } ^ { +0.18 } .
These results place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies , suggesting a dominant dark matter component with dynamical mass 2.7 _ { -1.3 } ^ { +3.1 } \times 10 ^ { 6 } \mathrm { M } _ { \odot } enclosed within the central \sim 160 pc , and dynamical mass-to-light ratio 1913 _ { -950 } ^ { +2234 } \mathrm { M } _ { \odot } / L _ { V, \odot } .
For Gru 1 we estimate a mean velocity of -140.5 _ { -1.6 } ^ { +2.4 } km s ^ { -1 } and a mean metallicity of [ Fe/H ] = -1.42 _ { -0.42 } ^ { +0.55 } but our sample does not resolve Gru 1 ’ s velocity dispersion .
The radial coordinates of Tuc 2 and Gru 1 in Galactic phase space suggest that their orbits are among the most energetic within distance \lesssim 300 kpc .
Moreover , their proximity to each other in this space arises naturally if both objects are trailing the Large Magellanic Cloud .