Young star clusters in the outer disks of LITTLE THINGS dwarf irregular
  galaxies
  We examine FUV images of the LITTLE THINGS sample of nearby dwarf irregular
(dIrr) and Blue Compact Dwarf (BCD) galaxies to identify distinct young regions
in their far outer disks. We use these data, obtained with the Galaxy Evolution
Explorer satellite, to determine the furthest radius at which in situ star
formation can currently be identified. The FUV knots are found at distances
from the center of the galaxies of 1 to 8 disk scale lengths and have ages of
<20 Myrs and masses of 20 to 1E5 Msolar. The presence of young clusters and OB
associations in the outer disks of dwarf galaxies shows that dIrrs do have star
formation taking place there in spite of the extreme nature of the environment.
Most regions are found where the HI surface density is ~1 Msolar per pc2,
although both the HI and dispersed old stars go out much further. This limiting
density suggests a cutoff in the ability to form distinct OB associations and
perhaps even stars. We compare the star formation rates in the FUV regions to
the average rates expected at their radii and beyond from the observed gas,
using the conventional correlation for gas-rich regions. The localized rates
are typically 10% of the expected average rates for the outer disks. Either
star formation in dIrrs at surface densities <1 Msolar per pc2 occurs without
forming distinct associations, or the Kennicutt-Schmidt relation over-predicts
the rate beyond this point. In the latter case, the stellar disks in the
far-outer parts of dIrrs result from scattering of stars from the inner disk.
