Strong Conformity and Assembly Bias: Towards a Physical Understanding of the Galaxy-Halo Connection in SDSS Clusters

Ying Zu; Yunjia Song; Zhiwei Shao; Xiaokai Chen; Yun Zheng; Hongyu Gao; Yu Yu; Huanyuan Shan; Yipeng Jing

doi:10.1093/mnras/stac125

paper

Strong Conformity and Assembly Bias: Towards a Physical Understanding of the Galaxy-Halo Connection in SDSS Clusters

arXiv:2108.06790 · doi:10.1093/mnras/stac125

Abstract

Understanding the physical connection between cluster galaxies and massive haloes is key to mitigating systematic uncertainties in next-generation cluster cosmology. We develop a novel method to infer the level of conformity between the stellar mass of the brightest central galaxies~(BCGs) $M_{*}^{B C G}$ and the satellite richness $λ$ , defined as their correlation coefficient $ρ_{cc}$ at fixed halo mass, using the abundance and weak lensing of SDSS clusters as functions of $M_{*}^{B C G}$ and $λ$ . We detect a halo mass-dependent conformity as $ρ_{cc} = 0.60 + 0.08 ln (M_{h} /3 \times 1 0^{14} M_{⊙} / h)$ . The strong conformity successfully resolves the "halo mass equality" conundrum discovered in Zu et al. 2021 -- when split by $M_{*}^{B C G}$ at fixed $λ$ , the low and high- $M_{*}^{B C G}$ clusters have the same average halo mass despite having a $0.34$ dex discrepancy in average $M_{*}^{B C G}$ . On top of the best-fitting conformity model, we develop a cluster assembly bias~(AB) prescription calibrated against the CosmicGrowth simulation, and build a conformity+AB model for the cluster weak lensing measurements. Our model predicts that with a $\sim 20%$ lower halo concentration $c$ , the low- $M_{*}^{B C G}$ clusters are $\sim 10%$ more biased than the high- $M_{*}^{B C G}$ systems, in excellent agreement with the observations. We also show that the observed conformity and assembly bias are unlikely due to projection effects. Finally, we build a toy model to argue that while the early-time BCG-halo co-evolution drives the $M_{*}^{B C G}$ - $c$ correlation, the late-time dry merger-induced BCG growth naturally produces the $M_{*}^{B C G}$ - $λ$ conformity despite the well-known anti-correlation between $λ$ and $c$ . Our method paves the path towards simultaneously constraining cosmology and cluster formation with future cluster surveys.

arXiv commentFinal MN accepted version (w/ Fig 6 replaced)

Strong Conformity and Assembly Bias: Towards a Physical Understanding of the Galaxy-Halo Connection in SDSS Clusters

Abstract

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