Dark matter at the end of the Galaxy

Abstract

Dark matter density profiles based upon $\ensuremath{\Lambda}\mathrm{CDM}$ cosmology motivate an ansatz velocity distribution function with fewer high-velocity particles than the Maxwell-Boltzmann distribution or proposed variants. The high-velocity tail of the distribution is determined by the outer slope of the dark matter halo---the large radius behavior of the Galactic dark matter density. $N$-body simulations of Galactic halos reproduce the high-velocity behavior of this ansatz. Predictions for direct detection rates are dramatically affected for models where the threshold scattering velocity is within 30% of the escape velocity.