Spiral arm crossings inferred from ridges in Gaia stellar velocity
  distributions
  The solar neighborhood contains disc stars that have recently crossed spiral
arms in the Galaxy. We propose that boundaries in local velocity distributions
separate stars that have recently crossed or been perturbed by a particular arm
from those that haven't. Ridges in the stellar velocity distributions
constructed from the second Gaia data release trace orbits that could have
touched nearby spiral arms at apocentre or pericentre. The multiple ridges and
arcs seen in local velocity distributions are consistent with the presence of
multiple spiral features and different pattern speeds and imply that the outer
Galaxy is flocculent rather than grand design.
