Properties of Extended UV Disk (XUV-disk) Galaxies Discovered by GALEX

2005, AAS, 207, 20202; We have examined a sample of nearby spiral galaxies observed in the UV by GALEX for which Halpha imaging is also available. Our study focused on the radial variation of star formation rate and surface covering factor of UV-bright complexes in the outer galactic disk, in the regime where star formation threshold mechanisms are vitally important. We suggest a classification scheme for identifying extended UV disks (XUV-disks, eg. Thilker et al. and Gil de paz et al.) in our sample, and use this method to constrain salient ensemble characteristics of this galaxy population. Approximately 20-30% of the galaxies in our sample exhibit XUV-disk emission, most frequently in mid-to-early type giant spirals and dwarf Magellanic type galaxies. Some of the XUV disks in our sample appear to have similarly extended Halpha emission originating from sparsely distributed HII regions. We suggest that optically-identified, anti-truncated S0-Sb galaxies may represent the evolved (or presently quiescent) counterparts of the massive XUV-disk population. Finally, we utilize multiwavelength optical imaging in conjunction with our GALEX data for a few XUV-disk galaxies to constrain the stellar mass, bolometric luminosity, and age of individual stellar complexes in the outer disk environment.