Global properties of the population of HII regions in NGC 7479 from photometric H alpha imaging

We present a new high quality continuum-subtracted Her image of NGC 7479. U sing a novel semiautomated technique we have determined the positions, angu lar sizes, and absolute fluxes of over 1000 HII regions and have constructe d the luminosity function for the regions over the whole galaxy showing tha t its slope is within the published range for spirals of the same morpholog ical type. However NGC 7479 is notable in that its bar shows unusually stro ng star formation along the whole structure. This prompted us to undertake separate analyses of the HII regions in the bar and in the disc. The disc l uminosity function (LF) shows clean bi-linear behaviour as found previously in late-type spirals, with a break at log L-H alpha = 38.6 erg s(-1), wher eas the bar LF shows a much less regular form. The difference is not due to the small numbers of HII regions in the bar, but reflects a physical diffe rence between the bar and the disc in the properties of their sets of regio ns; We show separate plots of the parameters for HII regions in the bar and the disc selected for their regular shapes and absence of blending. We have derived galaxy-wide relations for the HII region set: the diameter distribution function, luminosity versus volume and number versus luminosit y Versus diameter, and deduced the global Her surface density distribution and disc scale length. Measuring the integrated diffuse Her flux for the ga laxy, we compare this with the computed ionizing flux escaping from the pop ulation of density-bounded HII regions, finding that both in terms of energ y balance and of geometry, the hypothesis that this escaping flux gives ris e to the diffuse H alpha is well borne out in NGC 7479, and that a signific ant fraction of this flux escapes completely into the intergalactic medium.