Is Draco II one of the faintest dwarf galaxies? First study from
  Keck/DEIMOS spectroscopy
  We present the first spectroscopic analysis of the faint and compact stellar
system Draco II (Dra II, M_V=-2.9+/-0.8, r_h=19^{+8}_{-6} pc), recently
discovered in the Pan-STARRS1 3{\pi} survey. The observations, conducted with
DEIMOS on the Keck II telescope, establish some of its basic characteristics:
the velocity data reveal a narrow peak with 9 member stars at a systemic
heliocentric velocity <v_r>=-347.6^{+1.7}_{-1.8} km/s, thereby confirming Dra
II is a satellite of the Milky Way; we infer a velocity dispersion with
\sigma_{vr}=2.9+/-2.1 km/s (<8.4 km/s at the 95% confidence level), which
implies log_{10}(M_{1/2})=5.5^{+0.4}_{-0.6} and
log_{10}((M/L)_{1/2})=2.7^{+0.5}_{-0.8}, in Solar units; furthermore, very weak
Calcium triplet lines in the spectra of the high signal-to-noise member stars
imply [Fe/H]<-2.1, whilst variations in the line strengths of two stars with
similar colours and magnitudes suggest a metallicity spread in Dra II. These
new data cannot clearly discriminate whether Draco II is a star cluster or
amongst the faintest, most compact, and closest dwarf galaxies. However, the
sum of the three --- individually inconclusive --- pieces of evidence presented
here, seems to favour the dwarf galaxy interpretation.
