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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_h=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_rrangle =-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 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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https://hal-insu.archives-ouvertes.fr/insu-03707889
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Submitted on : Wednesday, June 29, 2022 - 11:58:26 AM
Last modification on : Wednesday, June 29, 2022 - 11:58:27 AM

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Nicolas F. Martin, Marla Geha, Rodrigo A. Ibata, Michelle L. M. Collins, Benjamin P. M. Laevens, et al.. Is Draco II one of the faintest dwarf galaxies? First study from Keck/DEIMOS spectroscopy. Monthly Notices of the Royal Astronomical Society: Letters, 2016, 458, pp.L59-L63. ⟨10.1093/mnrasl/slw013⟩. ⟨insu-03707889⟩

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