The Richness-Dependent Cluster Correlation Function: Early SDSS Data

The cluster correlation function and its richness dependence are determined from 1108 clusters of galaxies -- the largest sample of clusters studied so far -- found in 379 deg^2 of Sloan Digital Sky Survey early data. The results are compared with previous samples of optically and X-ray selected clusters. The richness-dependent correlation function increases monotonically from an average correlation scale of ~ 12 h^{-1} Mpc for poor clusters to ~ 25 h^{-1} Mpc for the richer, more massive clusters with a mean separation of ~ 90 h^{-1} Mpc. X-ray selected clusters suggest slightly stronger correlations than optically selected clusters (~ 2-\sigma). The results are compared with large-scale cosmological simulations. The observed richness-dependent cluster correlation function is well represented by the standard flat LCDM model (\Omega_m ~= 0.3, h ~= 0.7), and is inconsistent with the considerably weaker correlations predicted by \Omega_m = 1 models. An analytic relation for the correlation scale versus cluster mean separation, r_0 - d, that best describes the observations and the LCDM prediction is r_0 ~= 2.6 \sqrt{d} (for d ~= 20 - 90 h^{-1} Mpc). Data from the complete Sloan Digital Sky Survey, when available, will greatly enhance the accuracy of the results and allow a more precise determination of cosmological parameters.