Bending waves excited by irregular gas inflow along warps
  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.
