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

We present a new high quality continuum-subtracted Hα image, of NGC 7479. Using a novel semi-automated technique we have determined the positions, angular sizes, and absolute fluxes of over 1000 HII regions and have constructed the luminosity function for the regions over the whole galaxy, showing that its slope is within the published range for spirals of the same morphological type. However NGC 7479 is notable in that its bar shows unusually strong 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 luminosity function (LF) shows clean bi-linear behaviour as found previously in late-type spirals, with a break at log L_Hα = 38.6 erg s^-1, whereas 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 difference between the bar and the disc in the properties of their sets of regions. 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 luminosity versus diameter, and deduced the global Hα surface density distribution and disc scale length. Measuring the integrated diffuse Hα flux for the galaxy, we compare this with the computed ionizing flux escaping from the population of density-bounded HII regions, finding that both in terms of energy balance and of geometry, the hypothesis that this escaping flux gives rise to the diffuse Hα is well borne out in NGC 7479, and that a significant fraction of this flux escapes completely into the intergalactic medium.