Evidence of Halo Assembly Bias in Massive Clusters

TL;DR

This study provides strong observational evidence of halo assembly bias in galaxy clusters, showing that clusters with similar masses can have different large-scale clustering properties, impacting galaxy evolution models and cosmological measurements.

Contribution

First direct detection of halo assembly bias in galaxy clusters using weak lensing and auto-correlation functions, highlighting its significance for cosmology.

Findings

Halo assembly bias detected at 2.5σ and 4.4σ significance levels.

Clusters with similar masses show different large-scale biases.

Results impact galaxy evolution theories and precision cosmology.

Abstract

We present significant evidence of halo assembly bias for SDSS redMaPPer galaxy clusters in the redshift range $[0.1, 0.33]$. By dividing the 8,648 clusters into two subsamples based on the average member galaxy separation from the cluster center, we first show that the two subsamples have very similar halo mass of $M_{\rm 200m}\simeq 1.9\times 10^{14}~h^{-1}M_\odot$ based on the weak lensing signals at small radii $R<\sim 10~h^{-1}{\rm Mpc}$. However, their halo bias inferred from both the large-scale weak lensing and the projected auto-correlation functions differs by a factor of $\sim$1.5, which is a signature of assembly bias. The same bias hypothesis for the two subsamples is excluded at 2.5$\sigma$ in the weak lensing and 4.4$\sigma$ in the auto-correlation data, respectively. This result could bring a significant impact on both galaxy evolution and precision cosmology.