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arXiv · 1510.01326

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

Nicolas F. Martin·Marla Geha·Rodrigo A. Ibata·Michelle L. M. Collins·Benjamin P. M. Laevens·Eric F. Bell·Hans-Walter Rix·Annette M. N. Ferguson·Kenneth C. Chambers·Richard J. Wainscoat·Christopher Waters
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Abstract

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 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 9 member stars at a systemic heliocentric velocity =-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 σ_{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.

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Nicolas F. Martin, Marla Geha, Rodrigo A. Ibata, Michelle L. M. Collins, Benjamin P. M. Laevens, Eric F. Bell, Hans-Walter Rix, Annette M. N. Ferguson, Kenneth C. Chambers, Richard J. Wainscoat, Christopher Waters. 2016-01-18. Is Draco II one of the faintest dwarf galaxies? First study from Keck/DEIMOS spectroscopy. https://doi.org/10.1093/mnrasl%2Fslw013

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