Modelling the Pan-spectral Energy Distribution of Starburst Galaxies: Iii. Emission Line Diagnostics of Ensembles of Evolving H Ii Regions

We build, as far as theory will permit, self consistent model H II regions around central clusters of aging stars. These produce strong emission line diagnostics applicable to either individual H II regions in galaxies, or to the integrated emission line spectra of disk or starburst galaxies. The models assume that the expansion and internal pressure of individual H II regions is driven by the net input of mechanical energy from the central cluster, be it through winds or supernova events. This eliminates the ionization parameter as a free variable, replacing it with a parameter which depends on the ratio of the cluster mass to the pressure in the surrounding interstellar medium. These models explain why H II regions with low abundances have high excitation, and demonstrate that at least part of the warm ionized medium is the result of overlapping faint, old, large, and low pressure H II regions. We present line ratios (at both optical and IR wavelengths) which provide reliable abundance diagnostics for both single H II regions or for integrated galaxy spectra, and we find a number that can be used to estimate the mean age of the cluster stars exciting individual H II regions. Subject headings: galaxies: general – galaxies: star formation rates – galaxies: abundances – galaxies: starburst –ISM: H II –ISM: abundances-