Title：Cluster Dynamical State not AGN Feedback dictates X-ray Cooling Flows

Speaker：Prof. Jeremy Lim ( Department of Physics, the University of Hong Kong)

Abstract：The intracluster medium (ICM), which emits in X-rays, encodes the mass assembly of galaxy clusters — except, supposedly, at cluster cores, where the prevailing paradigm demands ICM cooling owing to radiative loss be closely balanced by reheating owing to relativistic jets from a supermassive black hole (which accretes the cooled gas) in the brightest cluster galaxy (BCG). Such feedback from an active galactic nucleus (AGN) is widely invoked to help regulate star formation in massive galaxies, constituting an essential ingredient in cosmological simulations to moderate BCG stellar growth. AGN feedback presumably helps drive a range of cluster X-ray surface brightness profiles, from moderately centrally peaked (a sign of weak cool cores; WCCs) to highly centrally peaked (strong cool cores; SCCs). Here, we show that the ICM thermodynamics of massive clusters — even at their cores — along with the occurrence of cool gas and star formation in their BCGs are dictated, instead, by the cluster global dynamical state: (i) whereas merging clusters do not possess cool cores (non cool cores; NCCs), progressively more dynamically-relaxed clusters possess cooler and denser cores transitioning from WCCs to SCCs; and (ii) only the most dynamically-relaxed clusters — specifically, all those possessing SCCs — host BCGs displaying cool gas and star formation, despite also exhibiting the most powerful AGN jets. These findings imply that mergers destroy cool cores, after which clusters evolve to WCCs and then SCCs as they dynamically relax. Only when a SCC is reestablished does vigorous ICM cooling (a “hidden” cooling flow) resume, implying limited AGN reheating, to deposit copious cool gas and fuel star formation in BCGs. As the SCC phase occupies only a small segment of the interval between cluster mergers, BCGs experience only brief episodes of star formation fueled by ICM cooling over cosmic history — removing the need for closely-balanced reheating over a Hubble time.

Related papers：

https://iopscience.iop.org/article/10.3847/1538-4357/ad5293/pdf

https://www.nature.com/articles/s41550-019-0909-6

报告地点：紫台仙林园区5-516会议室

报告时间：2026年10月15日周四10:00

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