Modelling the pan-spectral energy distribution of starburst galaxies: I. The role of the ISM pressure and the molecular cloud dissipation timescale

In this paper, we combine the stellar spectral synthesis code STARBURST99, the nebular modeling code MAPPINGS IIIq, a one-dimensional dynamical evolution model of H II regions around massive clusters of young stars, and a simplified model of synchrotron emissivity to produce purely theoretical self-consistent synthetic spectral energy distributions (SEDs) for (solar metallicity) starbursts lasting ± 10⁸ yr. These SEDs extend from the Lyman limit to beyond 21 cm. We find that two ISM parameters control the form of the SED: the pressure in the diffuse phase of the ISM (or, equivalently, its density), and the molecular cloud dissipation timescale. In particular, the shape of the far-infrared (dust re-emission) bump is strongly dependent on the mean pressure in the star-forming or starburst galaxy. This can explain the range of far-infrared (FIR) colors seen in starburst galaxies. In the case of objects of composite excitation, such diagrams potentially provide a means of estimating the fraction of the FIR emission that is contributed by an active nucleus. We present detailed SED fits to Arp 220 and NGC 6240, and we give the predicted colors for starburst galaxies derived from our models for the IRAS and the Spitzer Space Telescope MIPS and IRAC instruments. Our models reproduce the spread in observed colors of starburst galaxies. From both the SED fits and the color : color diagrams, we infer the presence of a population of compact and ultracompact H II regions around single OB stars or small OB clusters. Finally, we present absolute calibrations to convert observed fluxes into star formation rates in the UV (GALEX), at optical wavelengths (H), and in the IR (IRAS or Spitzer). We show that 25 m fluxes are particularly valuable as star formation indicators, since they largely eliminate one of the parameters controlling the IR SED