Age dissection of the vertical breathing motions in Gaia DR2: evidence
  for spiral driving
  Gaia DR2 has revealed breathing motions in the Milky Way, with stars on both
sides of the Galactic mid-plane moving coherently towards or away from it. The
generating mechanism of these breathing motions is thought to be spiral density
waves. Here we test this hypothesis. Using a self-consistent, high-resolution
simulation with star formation, and which hosts prominent spirals, we first
study the signatures of breathing motions excited by spirals. In the model, the
breathing motions induced by the spiral structure have an increasing amplitude
with distance from the mid-plane, pointing to an internal cause for them. We
then show that, at fixed height, the breathing motion amplitude decreases with
age. Next, we investigate the signature of the breathing motions in the Gaia
DR2 dataset. We demonstrate that, at the location with a consistently large
breathing motion, the corresponding amplitude increases monotonically with
distance from the mid-plane, in agreement with the model. Furthermore, we show
that at the same location, the breathing motion amplitude decreases with age,
again similar to what we find in the model. This strengthens the case that the
observed breathing motions are driven by spiral density waves.
