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

Abstract

Abstract We present the first spectroscopic analysis of the faint and compact stellar system Draco II (Dra II, MV = −2.9 ± 0.8, $r_{\rm h}=19^{+8}_{-6}{\rm \,pc}$), 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 $\langle v_{\rm r}\rangle =-347.6^{+1.7}_{-1.8}{\rm \,km\,s^{-1}}$, 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 $\log _{10}\left(M_{1/2}\right)=5.5^{+0.4}_{-0.6}$ and $\log _{10}\left((\text{M/L})_{1/2}\right)=2.7^{+0.5}_{-0.8}$, 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.