Willman 1—A Probable Dwarf Galaxy with an Irregular Kinematic Distribution

Creators: Willman, Beth; Geha, Marla; Strader, Jay; Strigari, Louis E.; Simon, Joshua D.; Kirby, Evan; Ho, Nhung; Warres, Alex

Abstract

We investigate the kinematic properties and stellar population of the Galactic satellite Willman 1 (Wil 1) by combining Keck/DEIMOS spectroscopy with Kitt Peak National Observatory mosaic camera imaging. Wil 1, also known as SDSS J1049+5103, is a nearby, ultra-low luminosity Milky Way companion. This object lies in a region of size-luminosity space (M_V ~ –2 mag, d ~ 38 kpc, r_(half) ~ 20 pc) also occupied by the Galactic satellites Boötes II and Segue 1 and 2, but no other known old stellar system. We use kinematic and color-magnitude criteria to identify 45 stars as possible members of Wil 1. With a systemic velocity of v_(helio) = –12.8 ± 1.0 km s^(–1), Wil 1 stars have velocities similar to those of foreground Milky Way stars. Informed by Monte Carlo simulations, we identify 5 of the 45 candidate member stars as likely foreground contaminants, with a small number possibly remaining at faint apparent magnitudes. These contaminants could have mimicked a large velocity dispersion and abundance spread in previous work. The significant spread in the [Fe/H] of the highly likely Wil 1 red giant branch members ([Fe/H] = –1.73 ± 0.12 and –2.65 ± 0.12) supports the scenario that Wil 1 is an ultra-low luminosity dwarf galaxy, or the remnants thereof, rather than a star cluster. However, Wil 1's innermost stars move with radial velocities offset by 8 km s^(–1) from its outer stars and have a velocity dispersion consistent with 0 km s^(–1), suggesting that Wil 1 may not be in dynamical equilibrium. The combination of the foreground contamination and unusual kinematic distribution make it difficult to robustly determine the dark matter mass of Wil 1. As a result, X-ray or gamma-ray observations of Wil 1 that attempt to constrain models of particle dark matter using an equilibrium mass model are strongly affected by the systematics in the observations presented here. We conclude that, despite the unusual features in the Wil 1 kinematic distribution, evidence indicates that this object is, or at least once was, a dwarf galaxy.

Additional Information

© 2011 American Astronomical Society. Received 2010 July 21; accepted 2011 July 30; published 2011 September 14. B.W. acknowledges support from NSF AST-0908193 and thanks Ewa Lokas, Anil Seth, Joe Wolf, and Gail Gutowkski for interesting and helpful conversations during the preparation of this paper. We also thank the anonymous referee for providing thoughtful suggestions that resulted in substantial improvement of this manuscript. M.G. acknowledges support from NSF AST-9008752. E.N.K., J. Strader, and L.E.S. acknowledge support provided by NASA through Hubble Fellowship grants HST-HF-01233.01, HST-HF-51237.01, and HST-HF-01225.01, respectively, awarded 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. We thank David W. Hogg and Morad Masjedi for obtaining the Wil 1 observations at KPNO in 2005. Some of 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 the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This research has also made use of NASA's Astrophysics Data System Bibliographic Services.

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