  Segue 2, discovered by Belokurov et al. (2009), is a galaxy with a luminosity
of only 900 L_sun. We present Keck/DEIMOS spectroscopy of 25 members of Segue
2--a threefold increase in spectroscopic sample size. The velocity dispersion
is too small to be measured with our data. The upper limit with 90% (95%)
confidence is sigma_v < 2.2 (2.6) km/s, the most stringent limit for any
galaxy. The corresponding limit on the mass within the 3-D half-light radius
(46 pc) is M_1/2 < 1.5 (2.1) x 10^5 M_sun. Segue 2 is the least massive galaxy
known. We identify Segue 2 as a galaxy rather than a star cluster based the
wide dispersion in [Fe/H] (from -2.85 to -1.33) among the member stars. The
stars' [alpha/Fe] ratios decline with increasing [Fe/H], indicating that Segue
2 retained Type Ia supernova ejecta despite its presently small mass and that
star formation lasted for at least 100 Myr. The mean metallicity, <[Fe/H]> =
-2.22 +/- 0.13 (about the same as the Ursa Minor galaxy, 330 times more
luminous than Segue 2), is higher than expected from the luminosity-metallicity
relation defined by more luminous dwarf galaxy satellites of the Milky Way.
Segue 2 may be the barest remnant of a tidally stripped, Ursa Minor-sized
galaxy. If so, it is the best example of an ultra-faint dwarf galaxy that came
to be ultra-faint through tidal stripping. Alternatively, Segue 2 could have
been born in a very low-mass dark matter subhalo (v_max < 10 km/s), below the
atomic hydrogen cooling limit.
