Nearest neighbor : the low-mass Milky Way satellite Tucana III

Abstract

We present Magellan/IMACS spectroscopy of the recently discovered Milky Way satellite TucanaIII (Tuc III). We identify 26 member stars in TucIII from which we measure a mean radial velocity of vhel=−102.3±0.4 (stat.)±2.0(sys.) km s-1, a velocity dispersion of - 0.1+0.1 0.7 km s-1, and a mean metallicity of = - - [Fe/H] 2.42+0.08 0.07. The upper limit on the velocity dispersion is σ<1.5 km s-1 at 95.5%confidence, and the corresponding upper limit on the mass within the half-light radius of TucIII is 9.0×104Me. We cannot rule out mass-to-light ratios as large as 240Me/Le for TucIII, but much lower mass-to-light ratios that would leave the system baryon-dominated are also allowed. We measure an upper limit on the metallicity spread of the stars in TucIII of 0.19dex at 95.5% confidence. TucIII has a smaller metallicity dispersion and likely a smaller velocity dispersion than any known dwarf galaxy, but a larger size and lower surface brightness than any known globular cluster. Its metallicity is also much lower than those of the clusters with similar luminosity. We therefore tentatively suggest that TucIII is the tidally stripped remnant of a dark matter-dominated dwarf galaxy, but additional precise velocity and metallicity measurements will be necessary for a definitive classification. If TucIII is indeed a dwarf galaxy, it is one of the closest external galaxies to the Sun. Because of its proximity, the most luminous stars in TucIII are quite bright, including one star at V=15.7 that is the brightest known member star of an ultra-faint satellite.