TIGHTLY CORRELATED Hi AND FUV EMISSION IN THE OUTSKIRTS OF M83

We compare sensitive Hi data from The Hi Nearby Galaxy Survey and deep far-ultraviolet (FUV) data from the Galaxy Evolution Explorer in the outer disk of M83. The FUV and Hi maps show a stunning spatial correlation out to almost 4 optical radii (r(25)), roughly the extent of our maps. This underscores that Hi traces the gas reservoir for outer-disk star formation (SF), and it implies that massive (at least low level) SF proceeds almost everywhere that Hi is observed. Whereas the average FUV intensity steadily decreases with increasing radius before leveling off at similar to 1.7 r(25), the decline in Hi surface density is more subtle. Low Hi columns (less than or similar to 2 M(circle dot) pc(-2)) contribute most of the mass in the outer disk, which is not the case within r(25). The time for SF to consume the available Hi, inferred from the ratio of Hi to FUV intensity, rises with increasing radius before leveling off at similar to 100 Gyr, i.e., many Hubble times, near similar to 1.7 r(25). Assuming that the relatively short H(2) depletion times observed in the inner parts of galaxies hold in outer disks, the conversion of Hi into bound, molecular clouds seems to limit SF in outer galaxy disks. The long consumption times suggest that most of the extended Hi observed in M83 will not be consumed by in situ SF. However, even these low SF rates are enough for moderate chemical enrichment in a closed outer disk to be expected.