2010/07/31 by Joshua D. Simon, Marla Geha, Quinn E. Minor +11 · 11 citations
Physics and Astronomy · #Astronomy #Astronomy and Astrophysical Research #Astrophysics #Dwarf galaxy #Galaxies: Formation, Evolution, Phenomena #Galaxy #Globular cluster #Metallicity #Milky Way #Physics #Satellite galaxy #Stars #Stellar kinematics #Stellar, planetary, and galactic studies #Velocity dispersion #astro-ph.CO #astro-ph.GA
paper · pdf · doi:10.1088/0004-637x/733/1/46
24 pages, 4 tables, 11 figures (10 in color). Submitted for publication in ApJ. V3 revised according to comments from the referee
arxiv created 2011/02/22 · openalex publication_date 2011/05/03 · arxiv updated 2015/05/19 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We present the results of a comprehensive Keck/DEIMOS spectroscopic survey of the ultra-faint Milky Way satellite galaxy Segue 1. We have obtained velocity measurements for 98.2% of the stars within 67 pc (10', or 2.3 half-light radii) of the center of Segue 1 that have colors and magnitudes consistent with membership, down to a magnitude limit of r = 21.7. Based on photometric, kinematic, and metallicity information, we identify 71 stars as probable Segue 1 members, including some as far out as 87 pc. After correcting for the influence of binary stars using repeated velocity measurements, we determine a velocity dispersion of 3.7 +1.4 − 1.1 km s −1 . The mass within the half-light radius is 5.8 +8.2 − 3.1 × 10 5 M ☉ . The stellar kinematics of Segue 1 require very high mass-to-light ratios unless the system is far from dynamical equilibrium, even if the period distribution of unresolved binary stars is skewed toward implausibly short periods. With a total luminosity less than that of a single bright red giant and a V -band mass-to-light ratio of 3400 M ☉ / L ☉ , Segue 1 is the darkest galaxy currently known. We critically re-examine recent claims that Segue 1 is a tidally disrupting star cluster and that kinematic samples are contaminated by the Sagittarius stream. The extremely low metallicities ([Fe/H] < −3) of two Segue 1 stars and the large metallicity spread among the members demonstrate conclusively that Segue 1 is a dwarf galaxy, and we find no evidence in favor of tidal effects. We also show that contamination by the Sagittarius stream has been overestimated. Segue 1 has the highest estimated dark matter density of any known galaxy and will therefore be a prime testing ground for dark matter physics and galaxy formation on small scales.