Galaxy modelling I. Spectral energy distributions from far-UV to sub-mm wavelengths

We present STARDUST, a new self-consistent modelling of the spectral energy distributions (SEDs) of galaxies from far-UV to radio wavelengths. In orde r to derive the SEDs in this broad spectral range, we first couple spectrop hotometric and (closed-box) chemical evolutions to account for metallicity effects on the spectra of synthetic stellar populations. We briefly compare the UV/visible/near-IR colours and magnitudes predicted by our code with t hose of other codes available in the literature and we find an overall agre ement, in spite of differences in the stellar data. We then use a phenomeno logical fit for the metal-dependent extinction curve and a simple geometric distribution of the dust to compute the optical depth of galaxies and the corresponding obscuration curve. This enables us to calculate the fraction of stellar light reprocessed in the infrared range. In a final step, we def ine a dust model with various components and we fix the weights of these co mponents in order to reproduce the IRAS correlation of IR colours with tota l IR luminosities. This allows us to compute far-IR SEDs that phenomenologi cally mimic observed trends. We are able to predict the spectral evolution of galaxies in a broad wavelength range, and we can reproduce the observed SEDs of local spirals, starbursts, luminous infrared galaxies (LIRGs) and u ltra luminous infrared galaxies (ULIRCs). This modelling is so far kept as simple as possible and depends on a small number of free parameters, namely the initial mass function (IMF), star formation rate (SFR) time scale, gas density, and galaxy age, as well as on more refined assumptions on dust pr operties and the presence (or absence) of gas inflows/outflows. However, th ese SEDs will be subsequently implemented in a semi-analytic approach of ga laxy formation, where most of the free parameters can be consistently compu ted from more general assumptions for the physical processes ruling galaxy formation and evolution.