A Complete Spectroscopic Survey of the Milky Way Satellite Segue 1: The Darkest Galaxy

Creators: Simon, Joshua D.; Geha, Marla; Minor, Quinn E.; Martinez, Gregory D.; Kirby, Evan N.; Bullock, James S.; Kaplinghat, Manoj; Strigari, Louis E.; Willman, Beth; Choi, Philip I.; Tollerud, Erik J.; Wolf, Joe

Abstract

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.

Additional Information

© 2011 The American Astronomical Society. Received 2010 July 26; accepted 2011 March 10; published 2011 May 3. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. J.D.S. gratefully acknowledges the support of a Vera Rubin Fellowship provided by the Carnegie Institution of Washington. M.G. acknowledges support from NSF grant AST-0908752. Work at UCI was supported by NSF grant PHY-0855462 and NASA grant NNX09AD09G. Support for this work was also provided by NASA through Hubble Fellowship grant HST-HF-01233.01 awarded to E.N.K. by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. B.W. acknowledges support from NSF grant AST-0908193. This paper benefitted from the workshop "Shedding Light on the Nature of Dark Matter" held by the Keck Institute of Space Studies. We appreciate the contributions of the referee, who helped us clarify the paper. We thank Vasily Belokurov, Michael Cooper, Gerry Gilmore, Juna Kollmeier, Mark Krumholz, David Law, and George Preston for helpful conversations, and Matt Walker for providing his EM code.We also thank Alan McConnachie and Pat Côté for sharing a draft of their work on binary stars prior to publication. The analysis pipeline used to reduce the DEIMOS data was developed at UC Berkeley with support from NSF grant AST-0071048. This research has also made use of NASA's Astrophysics Data System Bibliographic Services. Facilities: Keck:II (DEIMOS)

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