THE DARK DISK OF THE MILKY WAY

Abstract

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