Outer-Disk Populations in NGC 7793: Evidence for Stellar Radial
  Migration
  We analyzed the radial surface brightness profile of the spiral galaxy NGC
7793 using HST/ACS images from the GHOSTS survey and a new HST/WFC3 image
across the disk break. We used the photometry of resolved stars to select
distinct populations covering a wide range of stellar ages. We found breaks in
the radial profiles of all stellar populations at 280" (~5.1 kpc). Beyond this
disk break, the profiles become steeper for younger populations. This same
trend is seen in numerical simulations where the outer disk is formed almost
entirely by radial migration. We also found that the older stars of NGC 7793
extend significantly farther than the underlying HI disk. They are thus
unlikely to have formed entirely at their current radii, unless the gas disk
was substantially larger in the past. These observations thus provide evidence
for substantial stellar radial migration in late-type disks.
