A tilted dark halo origin of the Galactic disk warp and flare

The outer disk of the Milky Way Galaxy is warped and flared. Several mechanisms have been proposed to explain these phenomena, but none have quantitatively reproduced both features. Recent work has demonstrated that the Galactic stellar halo is tilted with respect to the disk plane, suggesting that at least some component of the dark matter halo may also be tilted. Here we show that a dark halo tilted in the same direction as the stellar halo can induce a warp and flare in the Galactic disk at the same amplitude and orientation as the data. In our model, the warp is visible in both the gas and stars of all ages, which is consistent with the breadth of observational tracers of the warp. These results, in combination with data in the stellar halo, provide compelling evidence that our Galaxy is embedded in a tilted dark matter halo. This misalignment of the dark halo and the disk holds clues to the formation history of the Galaxy and represents the next step in the dynamical modelling of the Galactic potential. Han, Conroy and Hernquist propose a solution to an old problem: the origin of the warp in the Galactic disk. Adopting a dark halo model that is tilted with respect to the disk, the authors reproduce the warp and flare of the disk in the observed direction and magnitude.