The evolution of actively star-forming galaxies in the mid-infrared

In this paper we analyze the evolution of actively star-forming galaxies in the mid-infrared (MIR). This spectral region, characterized by continuum emission by hot dust and by the presence of strong emission features generally ascribed to polycyclic aromatic hydrocarbon (PAH) molecules, is the most strongly affected by the heating processes associated with star formation and/or active galactic nuclei (AGNs). Following the detailed observational characterization of galaxies in the MIR by the Infrared Space Observatory (ISO), we have updated the modelling of this spectral region in our spectrophotometric model GRASIL. In the diffuse component we have updated the treatment of PAHs according to the model by Li & Draine. As for the dense phase of the interstellar medium associated with the star-forming regions, the molecular clouds, we strongly decrease the abundance of PAHs as compared to that in the cirrus, based on the observational evidence of the lack or weakness of PAH bands close to the newly formed stars, possibly due to the destruction of the molecules in strong ultraviolet fields. The robustness of the model is checked by fitting near-infrared to radio broad-band spectra and the corresponding detailed MIR spectra of a large sample of galaxies, at once. With this model, we have analyzed the larger sample of actively star-forming galaxies by Dale et al. We show that the observed trends of galaxies in the ISO-IRAS-radio colour-colour plots can be interpreted in terms of the different evolutionary phases of star formation activity, and the consequent different dominance in the spectral energy distribution of the diffuse or dense phase of the ISM. We find that the observed colours indicate a surprising homogeneity of the starburst phenomenon, allowing only a limited variation of the most important physical parameters, such as the optical depth of the molecular clouds, the time-scale of the escape of young stars from their for mation sites, and the gas consumption time-scale. In this paper we do not attempt to reproduce the far-infrared coolest region in the colour-colour plots, as we concentrate on models meant to reproduce active star-forming galaxies, but we discuss possible requirements of a more complex modelling for the coldest objects.