Tightly correlated H I and FUV emission in the outskirts of M83

We compare sensitive H I data from The H I Nearby Galaxy Survey and deep far-ultraviolet (FUV) data from the Galaxy Evolution Explorer in the outer disk of M83. The FUV and H I maps show a stunning spatial correlation out to almost 4 optical radii (r 25), roughly the extent of our maps. This underscores that H I traces the gas reservoir for outer-disk star formation (SF), and it implies that massive (at least low level) SF proceeds almost everywhere that H I is observed. Whereas the average FUV intensity steadily decreases with increasing radius before leveling off at ~1.7 r 25, the decline in H I surface density is more subtle. Low H I columns (lsim2 M sun 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 H I, inferred from the ratio of H I to FUV intensity, rises with increasing radius before leveling off at ~100 Gyr, i.e., many Hubble times, near ~1.7 r 25. Assuming that the relatively short H2 depletion times observed in the inner parts of galaxies hold in outer disks, the conversion of H I into bound, molecular clouds seems to limit SF in outer galaxy disks. The long consumption times suggest that most of the extended H I 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