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The shape and orientation of dark matter (DM) halos are sensitive to themicro-physics of the DM particle, yet in many mass models, the symmetry axes ofthe Milky Way's DM halo are often assumed to be aligned with the symmetry axesof the stellar disk. This is well-motivated for the inner DM halo but not forthe outer halo. We use zoomed cosmological-baryonic simulations from the Lattesuite of FIRE-2 Milky Way-mass galaxies to explore the evolution of the DMhalo's orientation with radius and time, with or without a major merger with aLarge Magellanic Cloud (LMC) analog, and when varying the DM model. In three ofthe four CDM halos we examine, the orientation of the halo minor axis divergesfrom the stellar disk vector by more than 20 degrees beyond about 30galactocentric kpc, reaching a maximum of 30--90 degrees depending on theindividual halo's formation history. In identical simulations using a model ofself-interacting DM with $\sigma = 1 \, \mathrm{cm}^2 \, \mathrm{g}^{-1}$, thehalo remains aligned with the stellar disk out to $\sim$200--400 kpc.Interactions with massive satellites ($M \gtrsim 4 \times 10^{10} \,\rm{M_\odot}$ at pericenter; $M \gtrsim 3.3 \times 10^{10} \, \rm{M_\odot}$ atinfall) affect the orientation of the halo significantly, aligning the halo'smajor axis with the satellite galaxy from the disk to the virial radius. Therelative orientation of the halo and disk beyond 30 kpc is a potentialdiagnostic of SIDM if the effects of massive satellites can be accounted for.

Orientations of DM Halos in FIRE-2 Milky Way-mass Galaxies