The dependence of velocity and clustering statistics on galaxy properties

We use a combination of N-body simulations of the hierarchical clustering of dark matter and semi-analytic modelling of the physics of galaxy formation to probe the relationship between the galaxy distribution and the mass distribution in a flat, cold dark matter universe with mean density Ohm(0) = 0.3 (Lambda CDM). We find that the statistical properties of the galaxy distribution in the model, as quantified by pairwise velocity dispersions and clustering strength, can be quite different from those displayed by the dark matter. The pairwise line-of-sight velocity dispersion of galaxies is sensitive to the number of galaxies present in haloes of different mass. In our model, which is consistent with the observed galaxy number distribution, the galaxy velocity dispersion is similar to 40 per cent lower than that of the dark matter and is in reasonable agreement with the values measured in the Las Campanas redshift survey by Jing et al. over two decades in pair separation. The origin of this offset is statistical rather than dynamical, and depends upon the relative efficiency of galaxy formation in dark matter haloes of different mass. Although the model galaxies and the dark matter have markedly different correlation functions in real space, such biases conspire to cause the redshift-space correlation functions to be remarkably similar to each other. Thus, although genuinely biased relative to the dark matter on small scales, the distribution of galaxies as seen in redshift space appears unbiased. The predicted redshift-space galaxy correlation function agrees well with observations. We find no evidence in the model for a dependence of clustering strength on intrinsic galaxy luminosity, unless extremely bright galaxies, 2 mag brighter than L-*, are considered. However, there are significant differences when model galaxies are selected either by morphology or by colour. Early-type or red galaxies show a much stronger clustering amplitude than late-type or blue galaxies, particularly on small scales, again in good agreement with observations.