We use the Mitchell Spectrograph (formerly VIRUS-P) to observe 12 nearby disk
galaxies. We successfully measure ages in the outer disk in six systems. In
three cases (NGC 2684, NGC 6155, and NGC 7437), we find that a downward break
in the disk surface brightness profile corresponds with a change in the
dominant stellar population with the interior being dominated by active star
formation and the exterior having older stellar populations that are best-fit
with star formation histories that decline with time. The observed increase in
average stellar ages beyond a profile break is similar to theoretical models
that predict surface brightness breaks are caused by stellar migration, with
the outer disk being populated from scattered old interior stars. In three more
cases (IC 1132, NGC 4904, and NGC 6691), we find no significant change in the
stellar population as one crosses the break radius. In these galaxies, both the
inner and outer disks are dominated by active star formation and younger
stellar populations. While radial migration can contribute to the stellar
populations beyond the break, it appears more than one mechanism is required to
explain all of our observed stellar profile breaks.Comment: 18 pages, 21 figures. Accepted for publication in Ap
