The Metal-Poor Stellar Halo in RAVE-TGAS and its Implications for the
  Velocity Distribution of Dark Matter
  The local velocity distribution of dark matter plays an integral role in
interpreting the results from direct detection experiments. We previously
showed that metal-poor halo stars serve as excellent tracers of the virialized
dark matter velocity distribution using a high-resolution hydrodynamic
simulation of a Milky Way--like halo. In this paper, we take advantage of the
first \textit{Gaia} data release, coupled with spectroscopic measurements from
the RAdial Velocity Experiment (RAVE), to study the kinematics of stars
belonging to the metal-poor halo within an average distance of $\sim 5$ kpc of
the Sun. We study stars with iron abundances [Fe/H]$ < -1.5$ and $-1.8$ that
are located more than $1.5$ kpc from the Galactic plane. Using a Gaussian
mixture model analysis, we identify the stars that belong to the halo
population, as well as some kinematic outliers. We find that both metallicity
samples have similar velocity distributions for the halo component, within
uncertainties. Assuming that the stellar halo velocities adequately trace the
virialized dark matter, we study the implications for direct detection
experiments. The Standard Halo Model, which is typically assumed for dark
matter, is discrepant with the empirical distribution by $\sim6\sigma$ and
predicts fewer high-speed particles. As a result, the Standard Halo Model
overpredicts the nuclear scattering rate for dark matter masses below $\sim 10$
GeV. The kinematic outliers that we identify may potentially be correlated with
dark matter substructure, though further study is needed to establish this
correspondence.
