Day 852

Thursday.

1503.05200
Luminous red galaxies in clusters: central occupation, spatial distributions, and mis-cenetering
Hoshino, Leauthaud, Lackner, et al

LRG from SDSS are considered among the best understood samples of galaxies, and they are employed in a broad range of cosmo studies.  Because they form a relatively homogeneous population, with high stellar masses and red colors, they are expected to occupy haloes in a relatively simple way.  In this paper, study how LRGs occupy massive haloes via direct counts in clusters and reveal several unexpected trends suggesting that the connection between LRGs and DM haloes may not be straightforward.  Using the redMaPPer cluster catalog, derive the central occupation of LRGs as a function of richness, N_cen(lambda).  Assuming no correlation between cluster mass and central galaxy luminosity at fixed richness, show that clusters contain a significantly lower fraction of central LRGs than predicted from the 2pt correlation function.  At halo masses of 1e14.5 Msun, find N_cen=0.73, compared to N_cen of 0.89 from correlation studies.  The central occupation function for LRGs converges to 0.95 at large halo masses.  A strong anti-correlation between central luminosity and cluster mass at fixed richness is required to reconcile the results with these based on clustering studies.  Also derive P_BNC, the probability that the brightest cluster member is not the central galaxy.  Find P_BNC~20-30% which is a factor of ~2 lower than the value found by Skibba+ 2011.  Finally, study the radial offsets of bright non-central LRGs from cluster centers and show that bright non-central LRGs follow a different radial distribution compared to red cluster members, which follow a NFW profile.  This work demonstrate that even the most massive clusters do not always have an LRG at the center, and that the brightest galaxy in a cluster is not always the central galaxy.