Modeling the Pan‐Spectral Energy Distribution of Starburst Galaxies. III. Emission Line Diagnostics of Ensembles of Evolving H <scp>ii</scp> Regions

Abstract

We build, as far as theory will permit, self consistent model HII regions\naround central clusters of aging stars. These produce strong emission line\ndiagnostics applicable to either individual HII regions in galaxies, or to the\nintegrated emission line spectra of disk or starburst galaxies. The models\nassume that the expansion and internal pressure of individual HII regions is\ndriven by the net input of mechanical energy from the central cluster, be it\nthrough winds or supernova events. This eliminates the ionization parameter as\na free variable, replacing it with a parameter which depends on the ratio of\nthe cluster mass to the pressure in the surrounding interstellar medium. These\nmodels explain why HII regions with low abundances have high excitation, and\ndemonstrate that at least part of the warm ionized medium is the result of\noverlapping faint, old, large, and low pressure HII regions. We present line\nratios (at both optical and IR wavelengths) which provide reliable abundance\ndiagnostics for both single HII regions or for integrated galaxy spectra, and\nwe find a number that can be used to estimate the mean age of the cluster stars\nexciting individual HII regions.\n