Modeling galaxy formation with high-resolution N-body simulations

We model the galaxy formation in a series of high-resolution N-body simulations using the semi-analytical approach. Unlike many earlier investigations based on semi-analytical models, we make use of the subhalos resolved in the N -body simulations to follow the mergers of galaxies in dark halos, and we show that this is pivotal in modeling correctly the galaxy luminosity function at the bright end and the bimodal nature of galaxy color distribution. Merger of galaxies based on subhalos also results in many more bright red galaxies at high z. The semi-analytical model we adopt is similar to those used in earlier semi-analytical studies, except that we consider the effect of a prolonged cooling in small halos and that we explicitly follow the chemical enrichment in the interstellar medium. We use our model to make predictions for the properties of the galaxy population at low redshift and compare them with various current observations. We find that our model prediction can match the luminosity functions of galaxies in various wavebands. The shape of the luminosity function at bright end is well reproduced if galaxy mergers are modeled with the merger trees of subhalos and the steep faint-end slope can be moderated if the gas cooling time in low-mass halos is comparable to the age of the universe. The same model can also match the observed bimodal distribution of galaxy color, the color-magnitude relation for elliptical galaxies in clusters, the metallicity-luminosity relation and metallicityrotation velocity relation of spiral galaxies, and the gas fraction in present-day spiral galaxies. We also identify areas where further improvement of the model is required.