The Dark Disk of the Milky Way
  Massive satellite accretions onto early galactic disks can lead to the
deposition of dark matter in disk-like configurations that co-rotate with the
galaxy. This phenomenon has potentially dramatic consequences for dark matter
detection experiments. We utilize focused, high-resolution simulations of
accretion events onto disks designed to be Galaxy analogues, and compare the
resultant disks to the morphological and kinematic properties of the Milky
Way's thick disk in order to bracket the range of co-rotating accreted dark
matter. We find that the Milky Way's merger history must have been unusually
quiescent compared to median LCDM expectations and therefore its dark disk must
be relatively small: the fraction of accreted dark disk material near the Sun
is about 20% of the host halo density or smaller and the co-rotating dark
matter fraction near the Sun, defined as particles moving with a rotational
velocity lag less than 50 km/s, is enhanced by about 30% or less compared to a
standard halo model. Such a dark disk could contribute dominantly to the low
energy (of order keV for a dark matter particle with mass 100 GeV) nuclear
recoil event rate of direct dectection experiments, but it will not change the
likelihood of detection significantly. These dark disks provide testable
predictions of weakly-interacting massive particle dark matter models and
should be considered in detailed comparisons to experimental data. Our findings
suggest that the dark disk of the Milky Way may provide a detectable signal for
indirect detection experiments, contributing up to about 25% of the dark matter
self-annihilation signal in the direction of the center of the Galaxy, lending
the signal a noticeably oblate morphology.
