Anisotropies in the redshift-space correlations of galaxy groups and clusters -- II. Analysis of observational cluster samples

Abstract

We study the redshift-space correlation function of galaxy clusters for observational samples constructed in different surveys. We explore correlation amplitudes, pairwise velocity distributions and bias factors. Systematic effects in cluster identification procedures are the main source of biased estimates of the correlation amplitude and inferred velocity dispersions. We find that the large elongations along the line of sight in the Abell catalogue cannot be explained solely in terms of the errors in distance measurement originating from using a small number of galaxies. The inclusion of a significant fraction of galaxies and systems not physically bound to the clusters is responsible for this large systematic effect. We also find a significant dependence of the redshift-space distortion of the correlation function on the cluster Bautz-Morgan (BM) type, an effect that may rely on the fact that, owing to the regular appearance of low Bautz-Morgan type clusters, a sample of such objects would be less contaminated. We confirm that the effect of a low number of redshift measurements, n z , is to increase the redshift-space correlation length and bias factor. The results are very stable for n z > 10. We also test the effects of different n z in the catalogue of groups/clusters derived from the Updated Zwicky Catalogue and find that, for cluster samples identified in redshift surveys, even a low number of redshifts n z ∼ 5 is sufficient to provide reliable results. By comparing our results with those of numerical simulations we explore the strong influence on the clustering distortion pattern in redshift space from effects associated with the cluster identification procedure from two-dimensional surveys. The identification of clusters in X-ray surveys improves this situation, although there are still systematic effects, which are probably due to identification of optical sources in the determination of cluster redshifts. These systematic effects are particularly strong for the most luminous X-ray-selected clusters in the Extended Bright Cluster Survey, which exhibits very large anisotropies, comparable to those present in the Abell catalogue. Our results demonstrate that forthcoming large redshift surveys will be extremely important for the construction of new samples of groups and clusters, as well as improving the determination of optical and X-ray-selected cluster distances, essential for reliable analyses of the large-scale structure. High-resolution plots and information are available at http://star-www.dur.ac.uk/∼nelsonp/ anisotropies.