The imprint of radial migration on the vertical structure of galaxy
  disks
  We use numerical simulations to examine the effects of radial migration on
the vertical structure of galaxy disks. The simulations follow three
exponential disks of different mass but similar circular velocity, radial
scalelength, and (constant) scale height. The disks develop different
non-axisymmetric patterns, ranging from feeble, long-lived multiple arms to
strong, rapidly-evolving few-armed spirals. These fluctuations induce radial
migration through secular changes in the angular momentum of disk particles,
mixing the disk radially and blurring pre-existing gradients. Migration affects
primarily stars with small vertical excursions, regardless of spiral pattern.
This "provenance bias" largely determines the vertical structure of migrating
stars: inward migrators thin down as they move in, whereas outward migrators do
not thicken up but rather preserve the disk scale height at destination.
Migrators of equal birth radius thus develop a strong scale-height gradient,
not by flaring out as commonly assumed, but by thinning down as they spread
inward. Similar gradients have been observed for low-[$\alpha$/Fe]
mono-abundance populations (MAPs) in the Galaxy but our results argue against
interpreting them as a consequence of radial migration. This is because outward
migration does not lead to thickening, implying that the maximum scale height
of any population should reflect its value at birth. In contrast, Galactic MAPs
have scale heights that increase monotonically outwards, reaching values that
greatly exceed those at their presumed birth radii. Given the strong vertical
bias affecting migration, a proper assessment of the importance of radial
migration in the Galaxy should take carefully into account the strong radial
dependence of the scale heights of the various stellar populations.
