Inferred Evidence For Dark Matter Kinematic Substructure with SDSS-Gaia
  We use the distribution of accreted stars in SDSS-Gaia DR2 to demonstrate
that a non-trivial fraction of the dark matter halo within Galactocentric radii
of 7.5-10 kpc and $|z| > 2.5$ kpc is in substructure, and thus may not be in
equilibrium. Using a mixture likelihood analysis, we separate the contributions
of an old, isotropic stellar halo and a younger anisotropic population. The
latter dominates and is uniform within the region studied. It can be explained
as the tidal debris of a disrupted massive satellite on a highly radial orbit,
and is consistent with mounting evidence from recent studies. Simulations that
track the tidal debris from such mergers find that the dark matter traces the
kinematics of its stellar counterpart. If so, our results indicate that a
component of the nearby dark matter halo that is sourced by luminous satellites
is in kinematic substructure referred to as debris flow. These results
challenge the Standard Halo Model, which is discrepant with the distribution
recovered from the stellar data, and have important ramifications for the
interpretation of direct detection experiments.
