Modelling the extinction properties of galaxies

Recently (Granato, Lacey, Silva et al., 2000, astro-ph/0001308) we have combined our spectrophotometric galaxy evolution code which includes dust reprocessing (GRASIL, Silva et al., 1998) with semi-analytical galaxy formation models (GALFORM, Cole et al., 1999). One of the most characteristic features 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 interstellar medium. As a consequence, stellar populations of different ages have different geometrical relationships with the two phases, which is essential in understanding several observed properties of galaxies, in particular those undergoing major episodes of star formation at any redshift. Indeed, our merged GRASIL+GAL-FORM model reproduces fairly well the SEDs of normal spirals and starbursts from the far-UV to the sub-mm and their internal extinction properties. In particular in the model the observed starburst attenuation law (Calzetti, 1999) is accounted for as an effect of geometry of stars and dust, and has nothing to do with the optical properties of dust grains.