Nearest Neighbor: The Low-Mass Milky Way Satellite Tucana III
  We present Magellan/IMACS spectroscopy of the recently discovered Milky Way
satellite Tucana III (Tuc 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^+0.7_-0.1 km/s, and a mean
metallicity of [Fe/H] = -2.42^+0.07_-0.08. The upper limit on the velocity
dispersion is sigma < 1.5 km/s at 95.5% confidence, and the corresponding upper
limit on the mass within the half-light radius of Tuc III is 9.0 x 10^4 Msun.
We cannot rule out mass-to-light ratios as large as 240 Msun/Lsun 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 0.19 dex 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. Its metallicity is also much lower than those of the
clusters with similar luminosity. We therefore tentatively suggest that Tuc III
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 Tuc III 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 Tuc III are quite bright, including one star at V=15.7 that
is the brightest known member star of an ultra-faint satellite.
