The Coupling of Galactic Dark Matter Halos with Stellar Bars

Resonant torques couple stellar bars to dark matter halos. Here we use high-resolution numerical simulations to demonstrate long-term angular momentum transfer between stellar bars and dark matter orbits of varying orientation. We show that bar-driven reversals of dark matter orbit orientations can play a surprisingly large role in the evolution of the bar pattern speed. In predominantly prograde (corotating) halos, dark matter orbits become trapped in the stellar bar, forming a parallel dark matter bar. This dark matter bar reaches more than double the vertical height of the stellar bar. In halos dominated by retrograde orbits, a dark matter wake forms oriented perpendicular to the stellar bar. These dark matter overdensities provide a novel space to look for dark matter annihilation or decay signals. We predict that the Milky Way hosts a dark matter bar aligned with the stellar bar as well as a dark matter wake, the near side of which should extend from the Galactic center to a galactic longitude of l ≍ 323°.