The effect of radial migration on galactic disks
  We study the radial migration of stars driven by recurring multi-arm spiral
features in an exponential disk embedded in a dark matter halo. The spiral
perturbations redistribute angular momentum within the disk and lead to
substantial radial displacements of individual stars, in a manner that largely
preserves the circularity of their orbits and that results, after 5 Gyr (~40
full rotations at the disk scalelength), in little radial heating and no
appreciable changes to the vertical or radial structure of the disk. Our
results clarify a number of issues related to the spatial distribution and
kinematics of migrators. In particular, we find that migrators are a heavily
biased subset of stars with preferentially low vertical velocity dispersions.
This "provenance bias" for migrators is not surprising in hindsight, for stars
with small vertical excursions spend more time near the disk plane and thus
respond more readily to non-axisymmetric perturbations. We also find that the
vertical velocity dispersion of outward migrators always decreases, whereas the
opposite holds for inward migrators. To first order, newly arrived migrators
simply replace stars that have migrated off to other radii, thus inheriting the
vertical bias of the latter. Extreme migrators might therefore be recognized,
if present, by the unexpectedly small amplitude of their vertical excursions.
Our results show that migration, understood as changes in angular momentum that
preserve circularity, can affect strongly the thin disk, but cast doubts on
models that envision the Galactic thick disk as a relic of radial migration.
