Outer Disk Star Formation in HI selected Galaxies
  The HI in galaxies often extends past their conventionally defined optical
extent. I report results from our team which has been probing low intensity
star formation in outer disks using imaging in H-alpha and ultraviolet. Using a
sample of hundreds of HI selected galaxies, we confirm that outer disk HII
regions and extended UV disks are common. Hence outer disks are not dormant but
are dimly forming stars. Although the ultraviolet light in galaxies is more
centrally concentrated than the HI, the UV/HI ratio (the Star Formation
Efficiency) is nearly constant, with a slight dependency on surface brightness.
This result is well accounted for in a model where disks maintain a constant
stability parameter Q. This model also accounts for how the ISM and star
formation are distributed in the bright parts of galaxies, and how HI appears
to trace the distribution of dark matter in galaxy outskirts.
