The infrared side of galaxy formation. I. The local universe in the semianalytical framework

We present a new evolutionary model for predicting the far-uv-to-submillime ter properties of the galaxy population. This combines a semianalytic galax y formation model based on hierarchical clustering (GALFORM) with a spectro photometric code that includes dust reprocessing (GRASIL). The former provi des the star formation and metal enrichment histories, together with the ga s mass and various geometrical parameters, for a representative sample of g alaxies formed in different density environments. These quantities, togethe r with a few other assumptions concerning the spatial distribution of dust and its optical properties, allow us to model the spectral energy distribut ions (SEDs) of galaxies, taking into account stellar emission and also dust extinction (absorption plus scattering) and reemission. In the spectrophot ometric code, dust is considered only in the disk, but the general radiatio n field is contributed by both the disk and the bulge components with their own distinct age and metallicity distributions. Two phases are considered for the dust: molecular cloud complexes, where stars are assumed to be born , and the diffuse interstellar medium. The model includes both galaxies for ming stars quiescently in disks and starbursts triggered by galaxy mergers. We test our models against the observed spectrophotometric properties of g alaxies in the local universe, assuming a cold dark matter cosmology with O mega (0) = 0.3 and Lambda (0) = 0.7. The models reproduce fairly well the S EDs of normal spirals and starbursts from the far-UV to the submillimeter a nd their internal extinction properties. The starbursts follow the observed relationship between the far-IR-to-UV luminosity ratio and the slope of th e UV continuum. They also reproduce the observed starburst attenuation law. This result is remarkable because we use a dust mixture that reproduces th e Milky Way extinction law. It suggests that the observed attenuation law i s closely related to the geometry of the stars and dust. We compute galaxy luminosity functions over a wide range of wavelengths, which turn out to be in good agreement with observational data in the UV (2000 Angstrom), in th e B and K bands, and in the IR (12-100 mum). Finally, we investigate the re liability of some star formation indicators that are based on the propertie s of the continuum SEDs of galaxies. The UV continuum turns out to be a poo r star formation indicator for our models, while the infrared luminosity is much more reliable.