QUANTIFYING THE IONIZING FLUX ESCAPING FROM THE ULTRA-LUMINOUS HII-REGIONS IN 4 SPIRAL GALAXIES

There are two principal problems in explaining, in terms of the emissi on from OB stars, the ionisation of gas which emits diffuse H alpha in spiral galaxies. One is the long pathlength which the ionising photon s need to traverse to reach their objectives, the other is whether suf ficient photons can escape from within the H II regions which surround the hot stars. Here we treat the second point, assuming that the H II regions above a certain threshold luminosity are density bounded. We calculate the escaping Lyman continuum (Lyc) fluxes from the density b ounded regions in four galaxies, and show that in each of them this is easily sufficient to produce the measured diffuse H alpha emission.