Modeling the Pan–Spectral Energy Distribution of Starburst Galaxies. II. Control of the H II Region Parameters

We examine, from a theoretical viewpoint, how the physical parameters of H II regions are controlled in both normal galaxies and in starburst environments. These parameters are the H II region luminosity function, the time-dependent size, the covering fraction of molecular clouds, the pressure in the ionized gas, and the ionization parameter. The factors that control them are the initial mass function (IMF) of the exciting stars, the cluster mass function, the metallicity, and the mean pressure in the surrounding interstellar medium. We investigate the sensitivity of the Hα luminosity to the IMF, and find that this can translate to more than a factor 2 variation in derived star formation rates. The molecular cloud dissipation timescale is estimated from a case study of M17 to be ±1 Myr for this object. Based on H II luminosity function fitting for nearby galaxies, we suggest that the H II region cluster mass function is fitted by a lognormal form peaking at ±100 MΘ. The cluster mass function continues the stellar IMF to a higher mass regime. The pressure in the H II regions is controlled by the mechanical luminosity flux from the central cluster. Since this is closely related to the ionizing photon flux, we show that the ionization parameter is not a free variable, and that the diffuse ionized medium may be composed of many large, faint, and old H II regions. Finally, we derive theoretical probability distributions for the ionization parameter as a function of metallicity and compare these to those derived for SDSS galaxies