Strong Conformity and Assembly Bias: Towards a Physical Understanding of the Galaxy-Halo Connection in SDSS Clusters. (arXiv:2108.06790v2 [astro-ph.CO] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Zu_Y/0/1/0/all/0/1">Ying Zu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Song_Y/0/1/0/all/0/1">Yunjia Song</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shao_Z/0/1/0/all/0/1">Zhiwei Shao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chen_X/0/1/0/all/0/1">Xiaokai Chen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zheng_Y/0/1/0/all/0/1">Yun Zheng</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gao_H/0/1/0/all/0/1">Hongyu Gao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yu_Y/0/1/0/all/0/1">Yu Yu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shan_H/0/1/0/all/0/1">Huanyuan Shan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jing_Y/0/1/0/all/0/1">Yipeng Jing</a>
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) $M_*^{BCG}$ and the
satellite richness $lambda$, defined as their correlation coefficient
$rho_{cc}$ at fixed halo mass, using the abundance and weak lensing of SDSS
clusters as functions of $M_*^{BCG}$ and $lambda$. We detect a halo
mass-dependent conformity as
$rho_{cc}{=}0.60{+}0.08ln(M_h/3{times}10^{14}M_{odot}/h)$. The strong
conformity successfully resolves the “halo mass equality” conundrum discovered
in Zu et al. 2021 — when split by $M_*^{BCG}$ at fixed $lambda$, the low and
high-$M_*^{BCG}$ clusters have the same average halo mass despite having a
$0.34$ dex discrepancy in average $M_*^{BCG}$. 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-$M_*^{BCG}$ clusters are
${sim}10%$ more biased than the high-$M_*^{BCG}$ 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
$M_*^{BCG}$-$c$ correlation, the late-time dry merger-induced BCG growth
naturally produces the $M_*^{BCG}$-$lambda$ conformity despite the well-known
anti-correlation between $lambda$ and $c$. Our method paves the path towards
simultaneously constraining cosmology and cluster formation with future cluster
surveys.
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) $M_*^{BCG}$ and the
satellite richness $lambda$, defined as their correlation coefficient
$rho_{cc}$ at fixed halo mass, using the abundance and weak lensing of SDSS
clusters as functions of $M_*^{BCG}$ and $lambda$. We detect a halo
mass-dependent conformity as
$rho_{cc}{=}0.60{+}0.08ln(M_h/3{times}10^{14}M_{odot}/h)$. The strong
conformity successfully resolves the “halo mass equality” conundrum discovered
in Zu et al. 2021 — when split by $M_*^{BCG}$ at fixed $lambda$, the low and
high-$M_*^{BCG}$ clusters have the same average halo mass despite having a
$0.34$ dex discrepancy in average $M_*^{BCG}$. 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-$M_*^{BCG}$ clusters are
${sim}10%$ more biased than the high-$M_*^{BCG}$ 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
$M_*^{BCG}$-$c$ correlation, the late-time dry merger-induced BCG growth
naturally produces the $M_*^{BCG}$-$lambda$ conformity despite the well-known
anti-correlation between $lambda$ and $c$. Our method paves the path towards
simultaneously constraining cosmology and cluster formation with future cluster
surveys.
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