  The known galaxies most dominated by dark matter (Draco, Ursa Minor and
Andromeda IX) are satellites of the Milky Way and the Andromeda galaxies. They
are members of a class of faint galaxies, devoid of gas, known as dwarf
spheroidals, and have by far the highest ratio of dark to luminous matter. None
of the models proposed to unravel their origin 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.
