On the nature of star formation at large galactic radii

Abstract

We have compared far-ultraviolet (FUV), near-ultraviolet (NUV) and Hα measurements for star-forming regions in 21 galaxies, in order to characterize the properties of their discs at radii beyond the main optical radius (R25).In our representative sample of extended and non-extended ultraviolet (UV) discs, we find that half of the extended UV discs also exhibit extended Hα emission. We find that extended UV discs fall into two categories: those with a sharp truncation in the Hα disc close to the optical edge (R25), and those with extended emission in Hα as well as in the UV. Although most galaxies with strong Hα truncations near R25 show a significant corresponding falloff in UV emission (a factor of 10-100), the transition tends to be much smoother than in Hα, and significant UV emission often extends well beyond this radius, confirming earlier results by Thilker et al. and others.After correcting for dust attenuation the median fraction of total FUV emission from regions outside of R25 is 1.7 per cent, but it can be as high as 35 per cent in the most extreme cases. The corresponding fractions of Hα emission are approximately half as large on average. This difference reflects both a slightly lower ratio of Hα to UV emission in the H ii regions in the outer discs and a lower fraction of star clusters showing H ii regions. Most H ii regions in the extended disc have fluxes consistent with small numbers of ionizing O-type stars, and this poor sampling of the upper initial mass function (IMF) in small clusters can probably account for the differences in the emission properties, consistent with earlier conclusions by Zaritsky & Christlein, without needing to invoke a significant change in the stellar IMF itself. Consistent Hα/FUV ratios and brightest H ii region to total Hα 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. © 2010 The Authors. Journal compilation © 2010 RAS.