The Cluster Distribution as a Test of Dark Matter Models. III. the Cluster Velocity Field

Abstract

We study the large-scale velocity fields traced by galaxy clusters in numerical simulations of a box of side 960 $h^{-1}$ Mpc, and compare them with available data on real clusters. In order to test the reliability of the simulations, which are based on an optimized version of the Zel'dovich approximation, we compare their cluster velocities with those of "exact" N-body simulations, and find a remarkable agreement between the two according to a variety of statistical tests. We analyse Cold Dark Matter models with density parameter in the range $0.2\le Ω_0\le 1$, both with and without the cosmological constant term to provide a flat geometry. We also simulate a Cold+Hot Dark Matter model, with 30% provided by the hot component. Comparison with real data is performed by applying tests based on the cumulative velocity frequency distribution (CVFD) and bulk flow statistics. For the CVFD, we use observational velocity data from different authors. By merging all available data in a combined sample, the CVFD test is able to exclude only open models with $Ω_0\le 0.4$ and flat models with $Ω_0=0.2$. However, the analysis of individual observational samples gives contradictory results; in particular the recent Giovanelli data, which imply much lower velocities (with significantly smaller uncertainties) than the other data, are actually only consistent with the open $Ω_0=0.4$ and the flat $Ω_0=0.2$ models, i.e. the models which are excluded by the other data. Furthermore, CVFD and bulk flow analyses of the Branchini et al. reconstructed velocity data disfavour precisely those models accepted on the grounds of Giovanelli's sample. Finally, we confirm that the Lauer & Postman bulk flow determination is an extremely rare event in the cosmological models we have analysed.