The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem

Creators: Simon, Joshua D.; Geha, Marla

Abstract

We present Keck DEIMOS spectroscopy of stars in eight of the newly discovered ultra-faint dwarf galaxies around the Milky Way. We measure the velocity dispersions of Canes Venatici I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, Leo T, Ursa Major I, and Ursa Major II from the velocities of 18-214 stars in each galaxy and find dispersions ranging from 3.3 to 7.6 km s^(-1). The six galaxies with absolute magnitudes M_V < -4 are highly dark matter dominated, with mass-to-light ratios approaching 1000 M_☉/L_(☉,V). For the fainter galaxies we find tentative evidence for tidal disruption. The measured velocity dispersions of the ultra-faint dwarfs are correlated with their luminosities, indicating that a minimum mass for luminous galactic systems may not yet have been reached. We also measure the metallicities of the observed stars and find that the new dwarfs have mean metallicities of [Fe/H] = -2.0 to -2.3; these galaxies represent some of the most metal-poor stellar systems known. The six brightest of the ultra-faint dwarfs extend the luminosity-metallicity relationship followed by more luminous dwarfs by a factor of ~30 in luminosity. We detect metallicity spreads of up to 0.5 dex in several objects, suggesting multiple star formation epochs. UMa II and Com, despite their exceptionally low luminosities, have higher metallicities that suggest they may once have been much more massive. Having established the masses of the ultra-faint dwarfs, we re-examine the missing satellite problem. After correcting for the sky coverage of the Sloan Digital Sky Survey, we find that the ultra-faint dwarfs substantially alleviate the discrepancy between the predicted and observed numbers of satellites around the Milky Way, but there are still a factor of ~4 too few dwarf galaxies over a significant range of masses. We show that if galaxy formation in low-mass dark matter halos is strongly suppressed after reionization, the simulated circular velocity function of CDM subhalos can be brought into approximate agreement with the observed circular velocity function of Milky Way satellite galaxies.

Additional Information

© 2007 American Astronomical Society. Print publication: Issue 1 (2007 November 20); received 2007 May 18; accepted for publication 2007 July 21. 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. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. J. D. S. gratefully acknowledges the support of a Millikan Fellowship provided by Caltech and M. G. acknowledges support from a Plaskett Research Fellowship at the Herzberg Institute of Astrophysics of the National Research Council of Canada. We thank George Djorgovski, Vasily Belokurov, Leo Blitz, James Bullock, Judy Cohen, Pat Cote, Jurg Diemand, Gerry Gilmore, Raja Guhathakurta, Nicolas Martin, Emma Ryan-Weber,Wal Sargent, Peter Stetson, Louie Strigari, Beth Willman, and Dan Zucker for helpful conversations, and we acknowledge the useful suggestions of the anonymous referee. We also thank Michael Cooper and the DEEP2 team for their hard work on the DEIMOS data reduction pipeline. The analysis pipeline used to reduce the DEIMOS data was developed at UC Berkeley with support from NSF grantAST-0071048. This research hasmade use of NASA's Astrophysics Data System Bibliographic Services, the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and the SIMBAD database, operated at CDS, Strasbourg, France. Facilities: Keck:II (DEIMOS).

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