Galaxy formation using halo merger histories taken from N-body simulations

We develop a hybrid galaxy formation model that uses outputs from an N -body simulation to follow the merger histories (or 'merger trees') of dark matter haloes and treats baryonic processes, such as the cooling of gas within haloes and subsequent star formation, using the semi-analytic model of Cole et al. We compare this hybrid model with an otherwise identical model that utilizes merger-tree realizations generated using a Monte Carlo algorithm and find that, apart from the limited mass resolution imposed by the N -body particle mass, the only significant differences between the models are caused by the known discrepancy between the distribution of halo progenitor masses predicted by the extended Press-Schechter theory and that found in N -body simulations. We investigate the effect of limited mass resolution on the hybrid model by comparing with a purely semi-analytic model that has greatly improved mass resolution. We find that the mass resolution of the simulation we use, which has a particle mass of 1.4 x 10(10) h (-1) M-., is insufficient to produce a reasonable luminosity function for galaxies with magnitudes in the b(J) band fainter than -17.