Overmerging and mass-to-light ratios in phenomenological galaxy formation models

We show that the discrepancy between the Tully-Fisher relation and the lumi nosity function predicted by most phenomenological galaxy formation models is mainly caused by overmerging of galaxy haloes. We circumvent this overme rging problem, which is inherent in both the Press-Schechter formalism and dissipationless N-body simulations, by including a specific galaxy halo for mation recipe into an otherwise standard N-body code. This numerical techni que provides the merger trees which, together with simplified gas dynamics and star formation physics, constitute our implementation of a phenomenolog ical galaxy formation model. Resolving the overmerging problem provides us with the means to match both the I-band Tully-Fisher relation and the B- an d K-band luminosity functions within an Omega = 1 sCDM structure formation scenario. It also allows us to include models for chemical evolution and st arbursts, which improves the match to observational data and renders the mo delling more realistic. We show that the inclusion of chemical evolution in to the modelling requires a significant fraction of stars to be formed in s hort bursts triggered by merging events.