Member stars in the MW satellite Tucana III

We identify 26 member stars in Tuc III from which we measure a mean radial velocity of v_hel_=-102.3+/-0.4(stat.)+/-2.0(sys.)km/s, a velocity dispersion of 0.1_-0.1_^+0.7^km/s, and a mean metallicity of [Fe/H]=-2.42_-0.08_^+0.07^. The upper limit on the velocity dispersion is {sigma}<1.5km/s at 95.5% confidence, and the corresponding upper limit on the mass within the half-light radius of Tuc III is 9.0x10^4^M_{sun}_.

We cannot rule out mass-to-light ratios as large as 240M_{sun}_/L_{sun}_ for Tuc III, 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 Tuc III of 0.19dex at 95.5% confidence. Tuc III 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.