MAGELLAN/M2FS SPECTROSCOPY OF TUCANA 2 AND GRUS 1

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 × 10 6 ?> M ⊙ ?> enclosed within the central ∼160 pc, and dynamical mass-to-light ratio 1913 − 950 + 2234 ?> M ⊙ / L V , ⊙ ?> . 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 a distance of ≲ 300 kpc ?> . Moreover, their proximity to each other in this space arises naturally if both objects are trailing the Large Magellanic Cloud.