The low abundance and insignificance of dark discs in simulated Milky Way galaxies

TL;DR

This study analyzes 24 simulated Milky Way galaxies and finds dark discs are rare and have minimal impact on local dark matter density and velocity distribution, suggesting they are unlikely to influence direct detection experiments.

Contribution

The paper provides the first comprehensive analysis of dark disc presence in a large sample of simulated Milky Way analogues, showing their rarity and limited effect.

Findings

Only one galaxy had a detectable dark disc.

Dark discs increase local dark matter density by at most 35%.

Dark discs affect the high energy tail of velocity distribution by less than 1%.

Abstract

We investigate the presence and importance of dark matter discs in a sample of 24 simulated Milky Way galaxies in the APOSTLE project, part of the EAGLE programme of hydrodynamic simulations in Lambda-CDM cosmology. It has been suggested that a dark disc in the Milky Way may boost the dark matter density and modify the velocity modulus relative to a smooth halo at the position of the Sun, with ramifications for direct detection experiments. From a kinematic decomposition of the dark matter and a real space analysis of all 24 halos, we find that only one of the simulated Milky Way analogues has a detectable dark disc component. This unique event was caused by a merger at late time with an LMC-mass satellite at very low grazing angle. Considering that even this rare scenario only enhances the dark matter density at the solar radius by 35% and affects the high energy tail of the dark matter velocity distribution by less than 1%, we conclude that the presence of a dark disc in the Milky Way is unlikely, and is very unlikely to have a significant effect on direct detection experiments.