1. Institute for Theoretical Physics, University of Zurich, Winterthurestrasse 190, CH-8057 Zürich, Switzerland
2. Institut für Astronomie, Department of Physics, ETH Zürich, Wolfgang-Pauli-Strasse 16, CH-8093 Zürich, Switzerland
3. Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics, Stanford University, MS 29, Stanford, California 94309, USA
4. Kavli Institute for Cosmological Physics, Department of Astronomy and Astrophysics, The University of Chicago, Illinois 60637, USA
5. Universitäts Sternwarte München, Scheinerstrasse 1, D-81679 München, Germany
6. Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada

Correspondence to: L. Mayer^1,2S. Kazantzidis^3,4 Correspondence and requests for materials should be addressed to L.M. (Email: lucio@phys.ethz.ch) or S.K. (Email: stelios@kicp.uchicago.edu).

Abstract

The known galaxies most dominated by dark matter (Draco, Ursa Minor and Andromeda IX) are satellites of the Milky Way and the Andromeda galaxies^{1, 2, 3, 4}. They are members of a class of faint galaxies, devoid of gas, known as dwarf spheroidals^{3, 4, 5}, and have by far the highest ratio of dark to luminous matter^{3, 6}. None of the models proposed to unravel their origin^{7, 8, 9, 10} can simultaneously explain their exceptional dark matter content and their proximity to a much larger galaxy. Here we report simulations showing that the progenitors of these galaxies were probably gas-dominated dwarf galaxies that became satellites of a larger galaxy earlier than the other dwarf spheroidals. We find that a combination of tidal shocks and ram pressure swept away the entire gas content of such progenitors about ten billion years ago because heating by the cosmic ultraviolet background kept the gas loosely bound: a tiny stellar component embedded in a relatively massive dark halo survived until today. All luminous galaxies should be surrounded by a few extremely dark-matter-dominated dwarf spheroidal satellites, and these should have the shortest orbital periods among dwarf spheroidals because they were accreted early.

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