Available flux for ionizing the diffuse medium

In Rozas et al. (1997, these proceedings), we apply our hypothesis of density bounding to estimate numerically the flux of ionizing photons which leak from the luminous H II regions and are available to ionize the diffuse gas in the disc and the halo. This is done by extrapolating the luminosity function below the break to values above the luminosity of the break, and subtracting off the observed LF above the break. As a clear example, we can estimate that an H II region with observed H luminosity of 510 erg s should be emitting a few times 10 erg s in escaping Lyc photons. Just half a dozen of these per galaxy (and generally there are a few tens of regions with luminosities above the critical value) would, at least in energy terms, give the ionizing luminosity required to ionize the diffuse medium. Regions of this luminosity have diameters of a couple of hundred parsecs, at least, which implies that a good fraction of the escaping photons are emitted at a sufficient distance from the galactic plane to escape readily from their immediate surroundings, both into the halo and across the disc. There are a number of further rather direct observational tests which can be carried out in order to check the overall picture, which include a geometrical correlation between the positions of the density bounded regions and the isophotes of the observed diffuse H, and these are in progress.

© Copyright Astronomical Society of Australia 1997