Journal
ASTROPHYSICAL JOURNAL
Volume 733, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/733/1/46
Keywords
dark matter; galaxies: dwarf; galaxies: individual (Segue 1); galaxies: kinematics and dynamics; Local Group
Categories
Funding
- W. M. Keck Foundation
- Carnegie Institution of Washington
- NSF [AST-0908752, PHY-0855462, AST-0908193, AST-0071048]
- NASA [NNX09AD09G, HST-HF-01233.01]
- Space Telescope Science Institute [NAS 5-26555]
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [0908193] Funding Source: National Science Foundation
- Division Of Physics
- Direct For Mathematical & Physical Scien [0855462] Funding Source: National Science Foundation
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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(-1.1)(+1.4) km s(-1). The mass within the half-light radius is 5.8(-3.1)(+8.2) x 10(5) M-circle dot. 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 3400M(circle dot)/L-circle dot, 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.
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