Starburst galaxies in the far-infrared. Modelling the line, PAH and dust continuum emission

Using the codes STARBURST 99, and MAPPINGS 3r, we construct a set of theoretical spectral energy distributions (SEDs) of starburst galaxies, which include both the stellar and the dust continuum emission, as well as the line and PAH emission. The SEDs are presented in a form which allows fitting to observations and the extraction of the fundamental parameters of the starburst. These models extend the results of Dopita et al. (2005) (see also Dopita et al. 2004a,b) to cover a wider range in initial parameters, separate out the contribution of the older stars, and explicitly consider the contribution of compact and ultracompact (UC) HII regions to the theoretical SEDs for starbursts galaxies. These SEDs extend from the Lyman Limit to roughly one millimetre. We find that UC HII regions are important contributors to the SED in the 10-3 5μm spectral region currently accessible to spectroscopy with the Spitzer Space Observatory, and that their contribution in this waveband strongly increases with the pressure in the ISM. This pressure is confirmed as the most important parameter determining the overall shape of the SED of starburst galaxies. We show that for one galaxy for which Spitzer Space Observatory spectoscopy has been published, NGC7714, the line, PAH, and dust continuum spectrum is well fit by a combination of compact and ultracompact HII regions, massive evolving clusters, and an older stellar population. Together with the models for normal spiral galaxies of Popescu et al. (2000), the models presented here will allow physical parameters to be routinely extracted from the FIR SEDs of the large numbers of gas-rich galaxies which will be observed with the new generation of infrared space observatories.