Modelling galaxy spectra in presence of interstellar dust - I. The model of interstellar medium and the library of dusty single stellar populations

The advent of modern infrared astronomy has brought into evidence the role played by the interstellar dust in galaxy formation and evolution. Therefore, to fully exploit modern data, realistic spectrophotometric models of galaxies must include this important component of the interstellar medium (ISM). In this paper, the first of a series of two devoted to modelling the spectra of galaxies of different morphological type in the presence of dust, we present our description of the dust both in the diffuse ISM and in the molecular clouds (MCs). Our galaxy model contains three interacting components: the diffuse ISM, made of gas and dust, the large complexes of MCs in which active star formation occurs and, finally, the populations of stars that are no longer embedded in the dusty environment of their parental MCs. Our model for the dust takes into account three components, i.e. graphite, silicates and polycyclic aromatic hydrocarbons (PAHs). We consider and adapt to our aims two prescriptions for the size distribution of the dust grains and two models for the emission of the dusty ISM. We cross-check the emission and extinction models of the ISM by calculating the extinction curves and the emission for the typical environments of the Milky Way (MW) and the Large and Small Magellanic Clouds (LMC and SMC) and by comparing the results with the observational data. The final model we have adopted is a hybrid one which stems from combining the analysis of Guhathakurta & Draine for the emission of graphite and silicates and Puget, Leger & Boulanger for the PAH emission, and using the distribution law of Weingartner & Draine and the ionization model for PAHs of Weingartner & Draine. We apply the model to calculate the spectral energy distribution (SED) of single stellar populations (SSPs) of different age and chemical composition, which may be severely affected by dust at least in two types of stars: the young, massive stars while they are still embedded in their parental MCs and the intermediate- and low-mass asymptotic giant branch (AGB) stars when they form their own dust shell around. We use the 'ray-tracing' method to solve the problem of radiative transfer and to calculate extended libraries of SSP SEDs. Particular care is taken to model the contribution from PAHs, introducing different abundances of C in the population of very small carbonaceous grains (VSGs) and different ionization states in PAHs. The SEDs of young SSPs are then compared with observational data of star-forming regions of four local galaxies successfully reproducing their SEDs from the ultraviolet (UV)-optical regions to the mid- and far-infrared region (MIR and FIR, respectively).