On the Nature of Star Formation at Large Galactic Radii
  We have compared far-ultraviolet (FUV), near-ultraviolet (NUV), and Halpha
measurements for star forming regions in 21 galaxies, in order to characterise
the properties of their discs at radii beyond the main optical radius (R25). In
our representative sample of extended and non-extended UV discs we find that
half of the extended UV discs also exhibit extended Halpha emission. We find
that extended UV discs fall into two categories, those with a sharp truncation
in the Halpha disc close to the optical edge (R25), and those with extended
emission in Halpha as well as in the ultraviolet. Although most galaxies with
strong Halpha truncations near R25 show a significant corresponding falloff in
UV emission (factor 10--100), the transition tends to be much smoother than in
Halpha, and significant UV emission often extends well beyond this radius,
confirming earlier results by Thilker et al. (2007) and others. After
correcting for dust attenuation the median fraction of total FUV emission from
regions outside of R25 is 1.7%, but it can be as high as 35% in the most
extreme cases. The corresponding fractions of Halpha emission are approximately
half as large on average. This difference reflects both a slightly lower ratio
of Halpha to UV emission in the HII regions in the outer discs, as well as a
lower fraction of star clusters showing HII regions. Most HII regions in the
extended disc have fluxes consistent with small numbers of ionising O-type
stars, and this poor sampling of the upper initial mass function in small
clusters can probably account for the differences in the emission properties,
consistent with earlier conclusions by Zaritsky & Christlein (2007), without
needing to invoke a significant change in the stellar IMF itself. Consistent
Ha/FUV ratios and brightest HII region to total Halpha fluxes in the inner and
extended discs across our whole galaxy sample demonstrate no evidence for a
change in the cluster luminosity function or the IMF in the low gas density
outer disc.
