Luminous red galaxies in clusters: central occupation, spatial distributions and miscentring

Abstract

Luminous Red Galaxies (LRG) from the Sloan Digital Sky Survey are considered\namong the best understood samples of galaxies, and they are employed in a broad\nrange of cosmological studies. Because they form a relatively homogeneous\npopulation, with high stellar masses and red colors, they are expected to\noccupy halos in a relatively simple way. In this paper, we study how LRGs\noccupy massive halos via direct counts in clusters and we reveal several\nunexpected trends suggesting that the connection between LRGs and dark matter\nhalos may not be straightforward. Using the redMaPPer cluster catalog, we\nderive the central occupation of LRGs as a function richness, Ncen({\\lambda}).\nAssuming no correlation between cluster mass and central galaxy luminosity at\nfixed richness, we show that clusters contain a significantly lower fraction of\ncentral LRGs than predicted from the two-point correlation function. At halo\nmasses of 10^14.5 Msun, we find Ncen=0.73, compared to Ncen of 0.89 from\ncorrelation studies. Our central occupation function for LRGs converges to 0.95\nat large halo masses. A strong anti-correlation between central luminosity and\ncluster mass at fixed richness is required to reconcile our results with those\nbased on clustering studies. We also derive P_BNC, the probability that the\nbrightest cluster member is not the central galaxy. We find P_BNC ~ 20-30%\nwhich is a factor of ~2 lower than the value found by Skibba et al. 2011.\nFinally, we study the radial offsets of bright non-central LRGs from cluster\ncenters and show that bright non-central LRGs follow a different radial\ndistribution compared to red cluster members, which follow a\nNavarro-Frank-White profile. This work demonstrates that even the most massive\nclusters do not always have an LRG at the center, and that the brightest galaxy\nin a cluster is not always the central galaxy.\n