2008ApJ...683L..13B

The outer regions of disk galaxies show a drop-off in optical and Hα emission, suggesting a star formation threshold radius, assumed to owe to a critical surface density below which star formation does not take place. Signs of filamentary star formation beyond this threshold radius have been observed in individual galaxies in the Hα, and recent GALEX surveys have discovered that 30% of disk galaxies show UV emission out to 2-3 times the optical radius of the galaxy. We run smooth particle hydrodynamics simulations of disk galaxies with constant-density extended gas disks to test whether overdensities owing to spiral structure in the outer disk can reproduce the observed star formation. We indeed find that spiral density waves from the inner disk propagate into the outer gas disk and raise local gas regions above the star formation density threshold, yielding features similar to those observed. Because the amount of star formation is low, we expect to see little optical emission in outer disks, as observed. Our results indicate that XUV disks can be simulated simply by adding an extended gas disk with a surface density near the threshold density to an isolated galaxy and evolving it with fiducial star formation parameters.