Anisotropies in the redshift-space correlations of galaxy groups and clusters – I. Simulated catalogues

We analyse the correlation function of mock galaxy clusters in redshift space. We have constructed several mock catalogues designed to mimic the selection biases inherent in a variety of observational surveys. We explore different effects that contribute to the distortion of the clustering pattern: the pairwise velocity distribution of galaxy systems, coherent bulk motions, redshift errors and systematic effects in cluster identification. Our tests show that the redshift-space clustering pattern of galaxy systems is highly influenced by effects associated with the identification procedure from two-dimensional surveys. These systems show a spuriously large correlation amplitude, an effect that is present and even stronger in a subsample where the angular positions coincide with three-dimensional identified clusters. The effect of a small number of redshift measurements is also that of increasing the correlation amplitude. In a similar fashion, the bias parameter inferred from cluster samples is subject to these observational problems, which induce variations of up to a factor of 2 in such determinations. Also, we find that the estimated mean pairwise velocity dispersion can be up to an order of magnitude larger than the actual value. Errors in the estimated cluster redshift, originating from the use of too few redshift measurements per cluster, have a smaller impact on the measured correlation function. We show that an angular incompleteness in redshift surveys, such as that present in the 2dF Galaxy Redshift Survey 100 k public release, has no significant effect on the results. We suggest that the nature of projection effects arises mainly from structures along the line of sight in the filamentary large-scale clustering pattern. Thus, spectroscopic surveys are the only means of providing unbiased cluster samples.

High-resolution plots and information are available at http://star-www.dur.ac.uk/~nelsonp/anisotropies.