Empirical Determination of Dark Matter Velocities using Metal-Poor Stars

Jonah Herzog-Arbeitman, Mariangela Lisanti, Piero Madau, Lina Necib

Published 2017-04-14Version 1

The Milky Way dark matter halo is formed from the accretion of smaller subhalos. These sub-units also harbor stars--typically old and metal-poor--that are deposited in the Galactic inner regions by disruption events. In this Letter, we show that the dark matter and metal-poor stars in the Solar neighborhood share similar kinematics due to their common origin. Using the high-resolution Eris simulation, which traces the evolution of both the dark matter and baryons in a realistic Milky Way analog galaxy, we demonstrate that metal-poor stars are indeed effective tracers for the local, virialized dark matter velocity distribution. The dark matter velocities in the Solar neighborhood can therefore be inferred from observations of the smooth inner halo made by the Sloan Digital Sky Survey. This empirical distribution has a lower peak speed and smaller dispersion than what is typically assumed in the Standard Halo Model, affecting the interpretation of direct detection experiments. Specifically, the bounds on the spin-independent scattering cross section are weakened by nearly an order of magnitude for masses below ~10 GeV. Upcoming data from Gaia will allow us to further refine the expected distribution for the smooth dark matter component, and to test for the presence of local substructure.