Galaxy modelling II. Multi-wavelength faint counts from a semi-analytic model of galaxy formation

This paper predicts self-consistent faint galaxy counts from the UV to the submm wavelength range. The STARDUST spectral energy distributions describe d in Devriendt et al. (1999) (Paper I) are embedded within the explicit cos mological framework of a simple semi-analytic model of galaxy formation and evolution. We begin with a description of the non-dissipative and dissipat ive collapses of primordial perturbations, and plug in standard recipes for star formation, stellar evolution and feedback. We also model the absorpti on of starlight by dust and its re-processing in the IR and submm. We then build a class of models which capture the luminosity budget of the universe through faint galaxy counts and redshift distributions in the whole wavele ngth range spanned by our spectra. In contrast with a rather stable behavio ur in the optical and even in the far-IR, the submm counts are dramatically sensitive to variations in the cosmological parameters and changes in the star formation history. Faint submm counts are more easily accommodated wit hin an open universe with a low value of Omega (0), or a flat universe with a non-zero cosmological constant. We confirm the suggestion of Guiderdoni et al. (1998) that matching the current multi-wavelength data requires a po pulation of heavily-extinguished, massive galaxies with large star formatio n rates (similar to 500 M. yr(-1)) at intermediate and high redshift (z gre ater than or equal to 1.5). Such a population of objects probably is the co nsequence of an increase of interaction and merging activity at high redshi ft, but a realistic quantitative description can only be obtained through m ore detailed modelling of such processes. This study illustrates the implem entation of multi-wavelength spectra into a semi-analytic model. In spite o f its simplicity, it already provides fair fits of the current data of fain t counts, and a physically motivated way of interpolating and extrapolating these data to other wavelengths and fainter flux levels.