Stellar-gas kinematic misalignments in eagle: lifetimes and longevity of misaligned galaxies

This is the authors' abstract. We don't add key points for this paper.

The dominant processes by which galaxies replenish their cold gas reservoirs remain disputed, especially in massive galaxies. Stellar-gas kinematic misalignments offer an opportunity to study these replenishment processes. However, observed distributions of these misalignments conflict with current models of gas replenishment in early-type galaxies, with longer relaxation time-scales suggested as a possible solution. We use the eagle simulation to explore the relaxation of unstable misaligned gas in galaxies with masses of M* 109.5 M_ in the range $0< z< 1$. We extract misalignments from formation to relaxation providing a sample of ~ 3200 relaxations. We find relaxation time-scales tend to be of short-duration, with median lifetimes of ~ 0.5 Gyr, though with a notable population of unstable misalignments lasting 1 Gyr. Relaxation time distributions show a log-linear relationship, with ≈ 20~\ per\ cent of unstable misalignments persisting for 3 torquing times. Long-lived unstable misalignments are predominantly found in galaxies with higher stellar masses, lower star-forming gas fractions, higher ongoing gas inflow, and in galaxies that reside in the centres of dense environments. Mergers only cause ≈ 10~\ per\ cent of unstable misalignments among galaxies at $z< 0.35$, and ≈21 per cent at $0.35< c z< 1.0$ in eagle. We conclude that, at least in eagle, unstable kinematic misalignments are not predominantly driven by gas-rich minor mergers at any redshift probed. Additionally, processes that significantly extend relaxation times are not dominant in the galaxy population. Instead, we see a diverse formation pathway for misalignments such as through hot halo cooling.