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(-6)(+8) pc), recently discovered in the Panoramic Survey Telescope and Rapid Response System 1 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 nine member stars at a systemic heliocentric velocity < vr > = -347.6(-1.8)(+1.7) km s(-1), thereby confirming Dra II is a satellite of the Milky Way; we infer a velocity dispersion with sigma(vr) = 2.9 +/- 2.1 kms(-1) (<8.4 kms(-1) at the 95 per cent confidence level), which implies log(10) (M-1/2) = 5.5(-0.6)(+0.4) and log(10) ((M/L)(1/2)) = 2.7(-0.8)(+0.5), 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.