Clusters and groups of galaxies in the simulated local universe

We compare the properties of galaxy groups extracted from the Updated Zwicky Catalogue (UZC) with those of groups extracted from N-body simulations of the local Universe, in a A cold dark matter (COM) and a τCDM cosmology. In the simulations, the initial conditions of the dark matter density field are set to reproduce the present time distribution of the galaxies within 80 h ^-1 Mpc from the Milky Way. These initial conditions minimize the uncertainty originated by cosmic variance, which has affected previous analyses of this small volume of the Universe. The simulations also model the evolution of the photometric properties of the galaxy population with semi-analytic prescriptions. The models yield a galaxy luminosity function sensibly different from that of the UZC and are unable to reproduce the distribution of groups and their luminosity content. The discrepancy between the model and the UZC reduces substantially, if we redistribute the luminosity among the galaxies in the simulation according to the UZC luminosity function while preserving the galaxy luminosity rank. The modified ACDM model provides the best match to the UZC: the abundances of groups by harmonic radius, velocity dispersion, mass and luminosity are consistent with observations. We find that this model also reproduces the halo occupation number of groups and clusters. However, the large-scale distribution of groups is marginally consistent with the UZC and the redshift-space correlation function of galaxies on scales larger than 6 h ^-1 Mpc is still more than 3σ smaller than observed. We conclude that reproducing the properties of the observed groups certainly requires a more sophisticated treatment of galaxy formation, and possibly an improvement of the dark matter model.