The Evolution of Stellar Populations in the Outer Disks of Spiral
  Galaxies
  We investigate recent star formation in the extended ultraviolet (XUV) disks
of five nearby galaxies (NGC 0628, NGC 2090, NGC 2841, NGC 3621, and NGC 5055)
using a long wavelength baseline comprised of ultraviolet and mid-infrared
imaging from the Galaxy Evolution Explorer and the Spitzer Infrared Array
Camera. We identify 229 unresolved stellar complexes across targeted portions
of their XUV disks and utilize spectral energy distribution fitting to measure
their stellar ages and masses through comparison with Starburst99 population
synthesis models of instantaneous burst populations. We find that the median
age of outer disk associations in our sample is ~100 Myr with a large
dispersion that spans the entire range of our models (1 Myr-1 Gyr). This
relatively evolved state for most associations addresses the observed dearth of
Halpha emission in some outer disks, as Halpha can only be observed in star
forming regions younger than ~10 Myr. The large age dispersion is robust
against variations in extinction (in the range E(B-V)=0-0.3 mag) and variations
in the upper end of the stellar Initial Mass Function (IMF). In particular, we
demonstrate that the age dispersion is insensitive to steepening of the IMF, up
to extreme slopes.
