Clustering of galaxies in a hierarchical universe - III. Mock redshift surveys

This is the third paper in a series which combines N-body simulations and s emi-analytic modelling to provide a fully spatially resolved simulation of the galaxy formation and clustering processes. Here we extract mock redshif t surveys from our simulations: a cold dark matter model with either Omega( 0) = 1 (tau CDM) or Omega(0) = 0.3 and Lambda = 0.7 (Lambda CDM). We compar e the mock catalogues with the northern region (CfA2N) of the Center for As trophysics (CfA) Redshift Surveys. We study the properties of galaxy groups and clusters identified using standard observational techniques, and also the relation of these groups to real virialized systems. Most features of C fA2N groups are reproduced quite well by both models with no obvious depend ence on Omega(0). Redshift-space correlations and pairwise velocities are a lso similar in the two cosmologies. The luminosity functions predicted by o ur galaxy formation models depend sensitively on the treatment of star form ation and feedback. For the particular choices of Paper I they agree poorly with the CfA survey. To isolate the effect of this discrepancy on our mock redshift surveys, we modify galaxy luminosities in our simulations to repr oduce the CfA luminosity function exactly. This adjustment improves agreeme nt with the observed abundance of groups, which depends primarily on the ga laxy luminosity density, but other statistics, connected mon closely with t he underlying mass distribution, remain unaffected. Regardless of the lumin osity function adopted, modest differences with observation remain. These c an be attributed to the presence of the 'Great Wall' in the CfA2N. It is un clear whether the greater coherence of the real structure is a result of co smic variance, given the relatively small region studied, or reflects a phy sical deficiency of the models.