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 discovered1,2 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 scales3,4 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 galaxies6,7 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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We thank K. Johnston and S. White for discussion on this paper, and J. Strader for help in the acquisition of data for the Willman 1 satellite.
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Strigari, L., Bullock, J., Kaplinghat, M. et al. A common mass scale for satellite galaxies of the Milky Way. Nature 454, 1096–1097 (2008). https://doi.org/10.1038/nature07222
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