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-submillimeter properties of the galaxy population. This combines a semianalytic galaxy formation model based on hierarchical clustering (GALFORM) with a spectrophotometric code that includes dust reprocessing (GRASIL). The former provides the star formation and metal enrichment histories, together with the gas mass and various geometrical parameters, for a representative sample of galaxies formed in different density environments. These quantities, together with a few other assumptions concerning the spatial distribution of dust and its optical properties, allow us to model the spectral energy distributions (SEDs) of galaxies, taking into account stellar emission and also dust extinction (absorption plus scattering) and reemission. In the spectrophotometric code, dust is considered only in the disk, but the general radiation 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 forming stars quiescently in disks and starbursts triggered by galaxy mergers. We test our models against the observed spectrophotometric properties of galaxies in the local universe, assuming a cold dark matter cosmology with Omega (0) = 0.3 and Lambda (0) = 0.7. The models reproduce fairly well the SEDs of normal spirals and starbursts from the far-UV to the submillimeter and their internal extinction properties. The starbursts follow the observed relationship between the far-IR-to-UV luminosity ratio and the slope of the UV continuum. They also reproduce the observed starburst attenuation law. This result is remarkable because we use a dust mixture that reproduces the Milky Way extinction law. It suggests that the observed attenuation law is 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 the B and K bands, and in the IR (12-100 mum). Finally, we investigate the reliability of some star formation indicators that are based on the properties of the continuum SEDs of galaxies. The UV continuum turns out to be a poor star formation indicator for our models, while the infrared luminosity is much more reliable.