Under the Firelight: Stellar Tracers of the Local Dark Matter Velocity
  Distribution in the Milky Way
  The Gaia era opens new possibilities for discovering the remnants of
disrupted satellite galaxies in the Solar neighborhood. If the population of
local accreted stars is correlated with the dark matter sourced by the same
mergers, one can then map the dark matter distribution directly. Using two
cosmological zoom-in hydrodynamic simulations of Milky Way-mass galaxies from
the Latte suite of Fire-2 simulations, we find a strong correlation between the
velocity distribution of stars and dark matter at the solar circle that were
accreted from luminous satellites. This correspondence holds for dark matter
that is either relaxed or in kinematic substructure called debris flow, and is
consistent between two simulated hosts with different merger histories. The
correspondence is more problematic for streams because of possible spatial
offsets between the dark matter and stars. We demonstrate how to reconstruct
the dark matter velocity distribution from the observed properties of the
accreted stellar population by properly accounting for the ratio of stars to
dark matter contributed by individual mergers. After demonstrating this method
using the Fire-2 simulations, we apply it to the Milky Way and use it to
recover the dark matter velocity distribution associated with the recently
discovered stellar debris field in the Solar neighborhood. Based on results
from Gaia, we estimate that $42 ^{+26}_{-22}\%$ of the local dark matter that
is accreted from luminous mergers is in debris flow.
