AGNs do not Regulate ICM Cooling at Cluster Cores

Our most cherished example of AGN feedback is found at the cores of galaxy clusters: here, relativistic jets from an AGN in the Brightest Cluster Galaxy (BCG) are invoked to re-heat the intracluster medium (ICM), which radiates prodigiously in X-rays. The mechanical energy deposited by BCG-AGN jets into the ICM is inferred to closely balance –regulate –cooling of the ICM, although large amounts of cool gas (tracing an X-ray cooling flow) and vigorous star formation are often found in clusters having particularly luminous X-ray cores. Why, however, does AGN feedback so closely balance ICM cooling? Here, we show from exquisitely detailed observations of massive clusters that present understanding of what dictates ICM cooling misses the big picture: the role of cluster mergers in halting and delaying X-ray cooling flows. We demonstrate that AGN re-heating is not required to explain the observed phenomenology at cluster cores; rather, an X-ray cooling flow turns on long after a merger as clusters become more dynamically relaxed.