Simulating galaxy clusters - II. Global star formation histories and the galaxy populations

We performed N-body + hydrodynamical simulations of the formation and evolution of galaxy groups and clusters in a A cold dark matter cosmology. The simulations invoke star formation, chemical evolution with non-instantaneous recycling, metal-dependent radiative cooling, strong starbursts and (optionally) active galactic nucleus (AGN) driven galactic superwinds, effects of a meta-galactic ultraviolet field and thermal conduction. The properties of the galaxy populations in two clusters, one Virgo-like (T similar to 3 keV) and one (sub)Coma-like (T similar to 6 keV), are discussed. The global star formation rates of the cluster galaxies are found to decrease very significantly from redshift z = 2 to 0, in agreement with observations. The total K-band luminosity of the cluster galaxies correlates tightly with total cluster mass, and for models without additional AGN feedback, the zero-point of the relation matches the observed one fairly well. Compared to the observed galaxy luminosity function (LF), the simulations nicely match the number of intermediate-mass galaxies (-20 less than or similar to MB less than or similar to - 17, smaller galaxies being affected by resolution limits) but they show a deficiency of bright galaxies in favour of an overgrown central dominant (cD) galaxy. High-resolution tests indicate that this deficiency is not simply due to numerical 'overmerging'. The redshift evolution of the LFs from z = I to 0 is mainly driven by luminosity evolution, but also by merging of bright galaxies with the cD. The colour-magnitude relation of the cluster galaxies matches the observed 'red sequence', although with a large scatter, and on average galaxy metallicity increases with luminosity. As the brighter galaxies are essentially coeval. The colour-magnitude relation results from metallicity rather than age effects, as observed. On the whole, a top-heavy initial mass function appears to be preferably required to reproduce also the observed colours and metallicities of the stellar populations.