THE CLUSTER DISTRIBUTION AS A TEST OF DARK-MATTER MODELS .3. THE CLUSTER VELOCITY-FIELD

We study the large-scale velocity fields traced by galaxy clusters in numerical simulations of a box of side 960 h(-1) h(-1), and compare th em with available data on real clusters. In order to test the reliabil ity of the simulations, which are based on an optimized version of the Zel'dovich approximation, we compare their cluster velocities with th ose of 'exact' N-body simulations, and find a remarkable agreement bet ween the two according to a variety of statistical tests. We analyse c old dark matter (CDM) models with density parameter in the range 0.2 l ess than or equal to Ohm(0) less than or equal to 1, both with and wit hout the cosmological constant term to provide a flat geometry. We als o simulate a cold+hot dark matter (CHDM) model, with 30 per cent provi ded by the hot component. Comparison with real data is performed by ap plying tests based on the cumulative velocity frequency distribution ( CVFD) and bulk flow statistics. For the CVFD, we use observational vel ocity data from different authors, and find that results based on diff erent data sets are contradictory. In particular, the recent infrared Tully-Fisher (IRTF) data of Giovanelli yield smaller velocities with s maller errors than both the IRTF and D-n-sigma data of Hudson. It turn s out that the Giovanelli data are only only consistent with the open Ohm(0) = 0.4 and the flat Omega(0) = 0.2 models, while the Hudson data , though less discriminatory because of their larger errors, appear to exclude open models with Ohm(0) less than or equal to 0.4 and hat mod els with Ohm(0) = 0.2. This latter conclusion also holds if one pools all the data into a single sample regardless of the systematic differe nces in the two different sources. Furthermore, CVFD and bulk flow ana lyses of the Branchini et al. reconstructed velocity data again disfav our precisely those models accepted on the grounds of Giovanelli's sam ple. Finally, we confirm that the Lauer & Postman reported bulk flow d etermination would be a rare event in the cosmological models we have analysed.