1. Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697-4574, USA
2. Department of Astronomy, California Institute of Technology, 1200 East California Boulevard, MS105-24, Pasadena, California 91125, USA
3. Department of Astronomy, Yale University, PO Box 208101, New Haven, Connecticut 06520-8101, USA
4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
5. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Correspondence to: Louis E. Strigari¹ Correspondence and requests for materials should be addressed to L.E.S. (Email: lstrigar@uci.edu).

Abstract

The Milky Way has at least twenty-three known satellite galaxies that shine with luminosities ranging from about a thousand to a billion times that of the Sun. Half of these galaxies were discovered^{1, 2} in the past few years in the Sloan Digital Sky Survey, and they are among the least luminous galaxies in the known Universe. A determination of the mass of these galaxies provides a test of galaxy formation at the smallest scales^{3, 4} and probes the nature of the dark matter that dominates the mass density of the Universe⁵. Here we use new measurements of the velocities of the stars in these galaxies^{6, 7} to show that they are consistent with them having a common mass of about 10⁷ within their central 300 parsecs. This result demonstrates that the faintest of the Milky Way satellites are the most dark-matter-dominated galaxies known, and could be a hint of a new scale in galaxy formation or a characteristic scale for the clustering of dark matter.

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