Dark Matter, Dwarf Galaxies, and Massive Failures in the Halo of the Milky Way


The favored dark energy plus cold dark matter (LCDM) model of
cosmology predicts that the Milky Way should be surrounded by
thousands of dark matter satellite clumps, in great excess of the
observed count of Galactic dwarf satellite galaxies. This mismatch is
known as the "missing satellite problem". Recent discoveries in the
Sloan Digital Sky Survey have revealed a new population of ultra-faint
dwarf satellites, motivating excitement within the community that some
"missing" LCDM satellites are finally being found. Unfortunately for
the theory, the situation is not quite so rosy once the dynamical
masses of the known satellites are considered. Specifically, the
majority of the most massive dark matter satellites predicted to exist
are too dense to host any of the bright satellite galaxies of the
Milky Way. This poses a serious puzzle theoretically: either galaxy
formation becomes effectively stochastic on scales smaller than ~0.1
L* or the central densities of dark matter subhalos are significantly
lower than predicted in dissipationless simulations. I discuss some
possible solutions to this puzzle from the standpoint of baryonic
physics and non-standard dark matter physics.