We develop a hybrid galaxy formation model which uses outputs from an N-body simulation to follow the merger histories ( or “ merger trees ” ) of dark matter halos and treats baryonic processes , such as the cooling of gas within halos and subsequent star formation , using the semi-analytic model of Cole et al .
We compare this hybrid model to an otherwise identical model which utilises merger tree realisations generated by 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 due to the known discrepancy between the distribution of halo progenitor masses predicted by 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 to a purely semi-analytic model with greatly improved mass resolution .
We find that the mass resolution of the simulation we use , which has a particle mass of 1.4 \times 10 ^ { 10 } \it { h } ^ { -1 } M _ { \odot } , is insufficient to produce a reasonable luminosity function for galaxies with magnitudes in the { b _ { J } } band fainter than -17 .