Modelling the extinction properties of galaxies

Recently (Granato, Lacey, Silva et al., 2000, astro-ph/0001308) we have com bined our spectrophotometric galaxy evolution code which includes dust repr ocessing (GRASIL, Silva et al., 1998) with semi-analytical galaxy formation models (GALFORM, Cole et al., 1999). One of the most characteristic featur es of the former is that the dust is divided in two main phases: molecular cloud complexes, where stars are assumed to be born, and the diffuse inters tellar medium. As a consequence, stellar populations of different ages have different geometrical relationships with the two phases, which is essentia l in understanding several observed properties of galaxies, in particular t hose undergoing major episodes of star formation at any redshift. Indeed, o ur merged GRASIL+GAL-FORM model reproduces fairly well the SEDs of normal s pirals and starbursts from the far-UV to the sub-mm and their internal exti nction properties. In particular in the model the observed starburst attenu ation law (Calzetti, 1999) is accounted for as an effect of geometry of sta rs and dust, and has nothing to do with the optical properties of dust grai ns.