Figure Rotation of IllustrisTNG Halos

We use the TNG50 and TNG50 dark matter (DM)-only simulations from the IllustrisTNG simulation suite to conduct an updated survey of halo figure rotation in the presence of baryons. We develop a novel methodology to detect coherent figure rotation about an arbitrary axis and for arbitrary durations and apply it to a catalog of 1,577 DM halos from the DM-only run and 1,396 DM halos from the DM+baryons (DM+B) run that are free of major mergers. Figure rotation was detected in $94\%$ of DM-only halos and $82\%$ of the DM+B halos. The pattern speeds of rotations lasting $\gtrsim 1h^{-1}$ Gyr were log-normally distributed with medians of $0.25~h$ km s$^{-1}$ kpc$^{-1}$ for DM-only in agreement with past results, but $14\%$ higher at $0.29~h$ km s$^{-1}$ kpc$^{-1}$ in the DM+B halos. We find that rotation axes are typically aligned with the halo minor or major axis, in $57\%$ of DM-only halos and in $62\%$ of DM+B halos. The remaining rotation axes were not strongly aligned with any principal axis but typically lay in the plane containing the halo minor and major axes. Longer-lived rotations showed greater alignment with the halo minor axis in both simulations. Our results show that in the presence of baryons, figure rotation is marginally less common, shorter-lived, faster, and better aligned with the minor axis than in DM-only halos. This updated understanding will be consequential for future efforts to constrain figure rotation in the Milky Way dark halo using the morphology and kinematics of tidal streams.