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 (NGC0628, NGC2090, NGC2841, NGC3621, and NGC5055) 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 100Myr with a large dispersion that spans the entire range of our models (1Myr to 1Gyr). This relatively evolved state for most associations addresses the observed dearth of Hα emission in some outer disks, as Hα can only be observed in star-forming regions younger than 10Myr. The large age dispersion is robust against variations in extinction (in the range E(B - V) = 0-0.3mag) 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.