Magellan/M2FS Spectroscopy of Tucana 2 and Grus 1

Abstract

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 $1900_{-900}^{+2200}$ ${\mathrm{M}}_{\odot }/L_{V,\odot }$. 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 $\le 300$ kpc. Moreover, their proximity to each other in this space arises naturally if both objects are trailing the Large Magellanic Cloud.

replaced with ApJ-accepted version, all spectra and data products (including samples from posterior PDFs) are available at http://www.andrew.cmu.edu/user/mgwalker/tuc2gru1_dataproducts.tar.gz

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