A dark matter disc in the Milky Way

TL;DR

This paper demonstrates that a dark matter disc, formed through hierarchical galaxy formation processes, significantly influences local dark matter detection signals and can be inferred from associated stellar populations.

Contribution

It introduces the first inclusion of baryonic matter effects in simulations predicting the dark matter distribution in the Milky Way, revealing a dark disc's impact on detection experiments.

Findings

Dark matter disc contributes 0.25-1 times the halo density at the solar position.

A dark disc shares a similar velocity distribution with a thick stellar disc.

The dark disc enhances WIMP detection signals and modulates annual detection rates.

Abstract

Predicting the local flux of dark matter particles is vital for dark matter direct detection experiments. To date, such predictions have been based on simulations that model the dark matter alone. Here we include the influence of the baryonic matter for the first time. We use two different approaches. Firstly, we use dark matter only simulations to estimate the expected merger history for a Milky Way mass galaxy, and then add a thin stellar disc to measure its effect. Secondly, we use three cosmological hydrodynamic simulations of Milky Way mass galaxies. In both cases, we find that a stellar/gas disc at high redshift (z~1) causes merging satellites to be preferentially dragged towards the disc plane. This results in an accreted dark matter disc that contributes ~0.25 - 1 times the non-rotating halo density at the solar position. An associated thick stellar disc forms with the dark disc and shares a similar velocity distribution. If these accreted stars can be separated from those that formed in situ, future astronomical surveys will be able to infer the properties of the dark disc from these stars. The dark disc, unlike dark matter streams, is an equilibrium structure that must exist in disc galaxies that form in a hierarchical cosmology. Its low rotation lag with respect to the Earth significantly boosts WIMP capture in the Earth and Sun, increases the likelihood of direct detection at low recoil energy, boosts the annual modulation signal, and leads to distinct variations in the flux as a function of recoil energy that allow the WIMP mass to be determined (see contribution from T. Bruch this volume).