We present the first spectroscopic analysis of the faint and compact stellar system Draco II ( Dra II , M _ { V } = -2.9 \pm 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 \langle v _ { r } \rangle = -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 \sigma _ { vr } = 2.9 \pm 2.1 { km s ^ { -1 } } ( < 8.4 { km s ^ { -1 } } at the 95 % confidence level ) , which implies \log _ { 10 } \left ( M _ { 1 / 2 } \right ) = 5.5 ^ { +0.4 } _ { -0.6 } and \log _ { 10 } \left ( ( 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 can not 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 .