Is Draco II one of the faintest dwarf galaxies? First study from Keck/DEIMOS spectroscopy

Abstract We present the first spectroscopic analysis of the faint and compact stellar system Draco II (Dra II, MV = −2. 9 ± 0. 8, r ₇=19^+8-₆ \, pc), recently discovered in the Panoramic Survey Telescope and Rapid Response System 1 3π 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 v ₑ =-347. 6^+1. 7-₁. ₈ \, km\, s^{-1}, thereby confirming Dra II is a satellite of the Milky Way; we infer a velocity dispersion with σvr = 2. 9 ± 2. 1 km s−1 (8. 4 km s−1 at the 95 per cent confidence level), which implies ₁₀ (M₁/₂) =5. 5^+0. 4-₀. ₆ and 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.