Cross-correlations of X-ray and optically selected clusters with near-infrared and optical galaxies

We compute the real-space cluster-galaxy cross-correlation. xi(cg)(r) using the ROSAT-ESO Flux Limited X-ray (REFLEX) cluster survey, a group catalogue (2dFGGC) constructed from the final version of the Two-degree Field Galaxy Redshift Survey (2dFGRS), and galaxies extracted from the Two Micron All Sky Survey (2MASS) and Automated Plate Measurement (APM) surveys. This first detailed calculation of the cross-correlation for X-ray clusters and groups is consistent with previous works and shows that. xi(cg)(r) cannot be described by a single power law. We analyse the clustering dependence on the cluster X-ray luminosity L-X and virial mass M-vir thresholds as well as on the galaxy limiting magnitude. We also make a comparison of our results with those obtained for the halo-mass cross-correlation function in a Lambda CDM N-body simulation to infer the scale dependence of galaxy bias around clusters. Our results indicate that the distribution of galaxies shows a significant anti-bias at highly non-linear small cluster-centric distances, b(cg)(r) similar or equal to 0.7, irrespective of the group/cluster virial mass or X-ray luminosity and galaxy characteristics. This shows that a generic process controls the efficiency of galaxy formation and evolution in high-density regions. On larger scales, bcg(r) rises to a nearly constant value of the order of unity, the transition occurring at approximately 2 h(-1) Mpc for 2dF groups and 5 h(-1) Mpc for REFLEX clusters.