The Dark Disk of the Milky Way

TL;DR

This paper investigates the presence and properties of a dark matter disk in the Milky Way resulting from satellite accretion, using high-resolution simulations to compare with observed galactic features and assess implications for dark matter detection.

Contribution

It provides the first detailed simulation-based analysis of the Milky Way's dark disk, constraining its size, rotation, and impact on detection experiments.

Findings

The Milky Way's dark disk is likely small, about 20% or less of local dark matter density.

Co-rotating dark matter near the Sun is increased by up to 30% compared to standard models.

Dark disks could contribute significantly to indirect detection signals, up to 25% of the annihilation flux.

Abstract

Massive satellite accretions onto early galactic disks can lead to the deposition of dark matter in disk-like configurations that co-rotate with the galaxy. This phenomenon has potentially dramatic consequences for dark matter detection experiments. We utilize focused, high-resolution simulations of accretion events onto disks designed to be Galaxy analogues, and compare the resultant disks to the morphological and kinematic properties of the Milky Way's thick disk in order to bracket the range of co-rotating accreted dark matter. We find that the Milky Way's merger history must have been unusually quiescent compared to median LCDM expectations and therefore its dark disk must be relatively small: the fraction of accreted dark disk material near the Sun is about 20% of the host halo density or smaller and the co-rotating dark matter fraction near the Sun, defined as particles moving with a rotational velocity lag less than 50 km/s, is enhanced by about 30% or less compared to a standard halo model. Such a dark disk could contribute dominantly to the low energy (of order keV for a dark matter particle with mass 100 GeV) nuclear recoil event rate of direct dectection experiments, but it will not change the likelihood of detection significantly. These dark disks provide testable predictions of weakly-interacting massive particle dark matter models and should be considered in detailed comparisons to experimental data. Our findings suggest that the dark disk of the Milky Way may provide a detectable signal for indirect detection experiments, contributing up to about 25% of the dark matter self-annihilation signal in the direction of the center of the Galaxy, lending the signal a noticeably oblate morphology.