Magellan/IMACS Spectroscopy of Grus. I. A Low Metallicity Ultra-faint Dwarf Galaxy

We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (R similar to 11,000) Magellan/IMACS spectra of its individual member stars. We identify eight confirmed members of Grus I, based on their low metallicities and coherent radial velocities, and four candidate members for which only velocities are derived. In contrast to previous work, we find that Grus I has a very low mean metallicity of <[Fe/H]> = -2.62 +/- 0.11 dex, making it one of the most metal-poor UFDs. Grus I has a systemic radial velocity of -143.5 +/- 1.2 km s(-1) and a velocity dispersion of sigma(rv) = 2.5(-0.8)(+1.3) km s(-1), which results in a dynamical mass of M(1/2()r(h)) = 8(-4)(+12) 2 x 10(5) M-circle dot and a mass-to-light ratio of M/L-V = 440(-250)(+650) M-circle dot/L-circle dot. Under the assumption of dynamical equilibrium, our analysis confirms that Grus I is a dark-matter-dominated UFD (M/L> 80 M-circle dot/L-circle dot). However, we do not resolve a metallicity dispersion (sigma([Fe/H]) < 0.44 dex). Our results indicate that Grus I is a fairly typical UFD with parameters that agree with mass-metallicity and metallicity-luminosity trends for faint galaxies. This agreement suggests that Grus I has not lost an especially significant amount of mass from tidal encounters with the Milky Way, in line with its orbital parameters. Intriguingly, Grus I has among the lowest central densities ( rho(1/2) similar to 3.5(-2 1)(+5.7) x 10(7) M-circle dot kpc(-3))of the UFDs that are not known to be tidally disrupting. Models of the formation and evolution of UFDs will need to explain the diversity of these central densities, in addition to any diversity in the outer regions of these relic galaxies.