Discriminating among theories of spiral structure using Gaia DR2
  We compare the distribution in position and velocity of nearby stars from the
Gaia DR2 radial velocity sample with predictions of current theories for
spirals in disc galaxies. Although the rich substructure in velocity space
contains the same information, we find it more revealing to reproject the data
into action-angle variables, and we describe why resonant scattering would be
more readily identifiable in these variables. We compute the predicted changes
to the phase space density, in multiple different projections, that would be
caused by a simplified isolated spiral pattern, finding widely differing
predictions from each theory. We conclude that the phase space structure
present in the Gaia data shares many of the qualitative features expected in
the transient spiral mode model. We argue that the popular picture of
apparently swing-amplified spirals results from the superposition of a few
underlying spiral modes.
