Bending waves excited by irregular gas inflow along warps

TL;DR

This study uses simulations to show that irregular gas inflow along warps excites persistent bending waves in galactic discs, influencing stellar kinematics and matching Gaia observations of the Milky Way.

Contribution

It demonstrates that misaligned gas accretion continuously excites bending waves, explaining observed vertical motions in the Milky Way's disc.

Findings

Warped model shows larger amplitude bending waves.

Prograde bending waves are stronger in the warped model.

Bending waves cause correlations matching Gaia data.

Abstract

Gaia has revealed clear evidence of bending waves in the vertical kinematics of stars in the Solar Neighbourhood. We study bending waves in two simulations, one warped, with the warp due to misaligned gas inflow, and the other unwarped. We find slow, retrograde bending waves in both models, with the ones in the warped model having larger amplitudes. We also find fast, prograde bending waves. Prograde bending waves in the unwarped model are very weak, in agreement with the expectation that these waves should decay on short, ~ crossing, timescales, due to strong winding. However, prograde bending waves are much stronger for the duration of the warped model, pointing to irregular gas inflow along the warp as a continuous source of excitation. We demonstrate that large amplitude bending waves that propagate through the Solar Neighbourhood give rise to a correlation between the mean vertical velocity and the angular momentum, with a slope consistent with that found by Gaia. The bending waves affect populations of all ages, but the sharpest features are found in the young populations, hinting that short wavelength waves are not supported by the older, kinematically hotter, populations. Our results demonstrate the importance of misaligned gas accretion as a recurrent source of vertical perturbations of disc galaxies, including in the Milky Way.