Orientations of Dark Matter Halos in FIRE-2 Milky Way-mass Galaxies

The shape and orientation of dark matter (DM) halos are sensitive to the microphysics of the DM particles, yet in many mass models, the symmetry axes of the Milky Way's DM halo are often assumed to be aligned with the symmetry axes of the stellar disk. This is well motivated for the inner DM halo, but not for the outer halo. We use zoomed-in cosmological baryonic simulations from the Latte suite of FIRE-2 Milky Way-mass galaxies to explore the evolution of the DM halo's orientation with radius and time, with or without a major merger with a Large Magellanic Cloud analog, and when varying the DM model. In three of the four cold DM halos we examine, the orientation of the halo minor axis diverges from the stellar disk vector by more than 20 degrees beyond about 30 galactocentric kpc, reaching a maximum of 30 degrees-90 degrees, depending on the individual halo's formation history. In identical simulations using a model of self-interacting DM with sigma = 1 cm2 g-1, the halo remains aligned with the stellar disk out to similar to 200-400 kpc. Interactions with massive satellites (M greater than or similar to 4 x 1010 M circle dot at pericenter; M greater than or similar to 3.3 x 1010 M circle dot at infall) affect the orientation of the halo significantly, aligning the halo's major axis with the satellite galaxy from the disk to the virial radius. The relative orientation of the halo and disk beyond 30 kpc is a potential diagnostic of self-interacting DM, if the effects of massive satellites can be accounted for.