Faint galaxies in semi–analytic models: how robust are the predictions?

In spite of their overall success, semi-analytic models of galaxy formation and evolution predict slopes of luminosity functions which are steeper than the observed ones. This discrepancy has generally been explained by subtle surface brightness effects acting on the observational samples. In this paper, we explicitly implement the computation of surface brightness in a simple semi– analytic model (with standard CDM), and we estimate the effect of observational surface brightness thresholds on the predicted luminosity functions. The crucial free parameter in this computation is the efficiency ǫ of supernova feedback which is responsible for the triggering of galactic winds. With the classical formalism for this process, it is difficult to reproduce simultaneously the Tully–Fisher relation and the flat slope of the observational luminosity function with the same value of ǫ. This suggests that the triggering of galactic winds is a complex phenomenon. The highly uncertain formalism for supernova feedback that is used by semi–analytic models produces large uncertainties in the results. However, once a value of ǫ has been chosen, the various luminosity functions observed in different wavebands (B, r, K) and at different surface brightness thresholds, are consistently reproduced with the surface brightness thresholds quoted by the observers. This seems to show that these observations do see subsamples of the same underlying populations of “sub–L⋆” and dwarf galaxies. The conclusion of this heuristic paper is that a more realistic description of SN feedback is needed, and that surface brightness effects should not be neglected in the modelling of galaxy formation.