The Dark Matter at the End of the Galaxy

Dark matter density profiles based upon Lambda-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.

💡 Research Summary

The paper revisits the standard assumption that the local dark‑matter velocity distribution follows a Maxwell‑Boltzmann (MB) form, an assumption that underlies most direct‑detection analyses. Starting from the ΛCDM prediction for the radial density profile of a Milky Way‑like halo, the authors argue that the outer slope of the halo – the way the density falls off at large radii – directly controls the high‑velocity tail of the velocity distribution. They propose an “ansatz” distribution that explicitly incorporates the outer slope as a parameter α. In this formulation the distribution scales as f(v)∝v²