Orientations of DM Halos in FIRE-2 Milky Way-mass Galaxies by Jay Baptista et al. on Wednesday 30 November

The shape and orientation of dark matter (DM) halos are sensitive to the

micro-physics of the DM particle, 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 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 (LMC) analog, and when varying the DM model. In three of

the four CDM 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--90 degrees depending on the

individual halo's formation history. In identical simulations using a model of

self-interacting DM with $\sigma = 1 \, \mathrm{cm}^2 \, \mathrm{g}^{-1}$, the

halo 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; $M \gtrsim 3.3 \times 10^{10} \, \rm{M_\odot}$ 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 SIDM if the effects of massive satellites can be accounted for.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.16382v1