Spiral-induced Star Formation in the Outer Disks of Galaxies

The outer regions of galactic disks have received increased attention since ultraviolet observations with Galaxy Evolution Explorer demonstrated that nearly 30% of galaxies have UV emission beyond their optical extents, indicating star formation activity. These galaxies have been termed extended UV (XUV) disks. Here, we address whether these observations contradict the gas surface density threshold for star formation inferred from Ha radial profiles of galaxies. We run smoothed particle hydrodynamic simulations of isolated disk galaxies with fiducial star formation prescriptions and show that over-densities owing to the presence of spiral structure can induce star formation in extended gas disks. For direct comparison with observations, we use the three-dimensional radiative transfer code Sunrise to create simulated FUV and K(s)-band images. We find that galaxies classified as Type I XUV disks are a natural consequence of spiral patterns, but we are unable to reproduce Type II XUV disks. We also compare our results to studies of the Kennicutt-Schmidt relation in outer disks.