Figure Rotation of IllustrisTNG Halos

TL;DR

This study uses advanced simulations to analyze the rotation of dark matter halos, revealing that baryons influence the frequency, speed, and alignment of halo figure rotation, which impacts understanding of the Milky Way's dark halo.

Contribution

Developed a new method to detect halo figure rotation and provided an updated comparison of rotation characteristics in dark matter-only and baryon-including simulations.

Findings

Figure rotation is common in halos, detected in 94% of DM-only and 82% of DM+B halos.

Baryons increase the median pattern speed of halo rotation.

Rotations tend to align with the halo's minor axis, especially for longer-lived rotations.

Abstract

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.