A dark matter disc in the Milky Way
  Predicting the local flux of dark matter particles is vital for dark matter
direct detection experiments. To date, such predictions have been based on
simulations that model the dark matter alone. Here we include the influence of
the baryonic matter for the first time. We use two different approaches.
Firstly, we use dark matter only simulations to estimate the expected merger
history for a Milky Way mass galaxy, and then add a thin stellar disc to
measure its effect. Secondly, we use three cosmological hydrodynamic
simulations of Milky Way mass galaxies. In both cases, we find that a
stellar/gas disc at high redshift (z~1) causes merging satellites to be
preferentially dragged towards the disc plane. This results in an accreted dark
matter disc that contributes ~0.25 - 1 times the non-rotating halo density at
the solar position. An associated thick stellar disc forms with the dark disc
and shares a similar velocity distribution. If these accreted stars can be
separated from those that formed in situ, future astronomical surveys will be
able to infer the properties of the dark disc from these stars. The dark disc,
unlike dark matter streams, is an equilibrium structure that must exist in disc
galaxies that form in a hierarchical cosmology. Its low rotation lag with
respect to the Earth significantly boosts WIMP capture in the Earth and Sun,
increases the likelihood of direct detection at low recoil energy, boosts the
annual modulation signal, and leads to distinct variations in the flux as a
function of recoil energy that allow the WIMP mass to be determined (see
contribution from T. Bruch this volume).
