Cluster correlations in redshift space

Abstract

We test an analytic model for the two-point correlations of galaxy clusters in redshift space using the Hubble Volume N-body simulations. The correlation function of clusters shows no enhancement along the line of sight, due to the lack of any virialised structures in the cluster distribution. However, the distortion of the clustering pattern due to coherent bulk motions is clearly visible. The distribution of cluster peculiar motions is well described by a Gaussian, except in the extreme high velocity tails. The simulations produce a small but significant number of clusters with large peculiar motions. The form of the redshift space power spectrum is strongly influenced by errors in measured cluster redshifts in extant surveys. When these errors are taken into account, the model reproduces the power spectrum recovered from the simulation to an accuracy of 15% or better over a decade in wavenumber. We compare our analytic predictions with the power spectrum measured from t! he! APM cluster redshift survey. The cluster power spectrum constrains the amplitude of density fluctuations, as measured by the linear rms variance in spheres of radius 8 h^(-1) Mpc, denoted by sigma_8. When combined with the constraints on sigma_8 and the density parameter Omega derived from the local abundance of clusters, we find a best fitting cold dark matter model with sigma_8~1.25 and Omega~0.2, for a power spectrum shape that matches that measured for galaxies. However, for the best fitting value of Omega and given the value of Hubble's constant from recent measurements, the assumed shape of the power spectrum is incompatible with the most readily motivated predictions from the cold dark matter paradigm.