  The outer regions of disk galaxies show a drop-off in optical and Halpha
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 Halpha 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
over-densities 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.
