DETECTION of the SPLASHBACK RADIUS and HALO ASSEMBLY BIAS of MASSIVE GALAXY CLUSTERS

We show that the projected number density profiles of Sloan Digital Sky Survey photometric galaxies around galaxy clusters display strong evidence for the splashback radius, a sharp halo edge corresponding to the location of the first orbital apocenter of satellite galaxies after their infall. We split the clusters into two subsamples with different mean projected radial distances of their members, 〈R_mem〉, at fixed richness and redshift. The sample with smaller 〈R_mem〉 has a smaller ratio of the splashback radius to the traditional halo boundary R_200m than the subsample with larger 〈R_mem〉, indicative of different mass accretion rates for these subsamples. The same subsamples were recently used by Miyatake et al. to show that their large-scale clustering differs despite their similar weak lensing masses, demonstrating strong evidence for halo assembly bias. We expand on this result by presenting a 6.6σ difference in the clustering amplitudes of these samples using cluster-photometric galaxy crosscorrelations. This measurement is a clear indication that halo clustering depends on parameters other than halo mass. If 〈R_mem〉 is related to the mass assembly history of halos, the measurement is a manifestation of the halo assembly bias. However, our measured splashback radii are smaller, while the strength of the assembly bias signal is stronger, than the predictions of collisionless λ cold dark matter simulations. We show that dynamical friction, cluster mis-centering, or projection effects are not likely to be the sole source of these discrepancies. However, further investigations regarding unknown catastrophic weak lensing or cluster identification systematics are warranted.