Quantifying the ionising flux escaping from the ultra-luminous HII regions in four spiral galaxies

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