The Impact of Stellar Migration on Disk Outskirts

Stellar migration, whether due to trapping by transient spirals (churning), or to scattering by non-axisymmetric perturbations, has been proposed to explain the presence of stars in outer disks. After a review of the basic theory, we present compelling, but not yet conclusive, evidence that churning has been important in the outer disks of galaxies with Type II (down-bending) profiles, while scattering has produced the outer disks of Type III (up-bending) galaxies. In contrast, field galaxies with Type I (pure exponential) profiles appear to not have experienced substantial migration. We conclude by suggesting work that would improve our understanding of the origin of outer disks.