Fountains and storms: The effects of AGN feedback and mergers on the evolution of the intracluster medium in the romulusc simulation

The intracluster medium (ICM) is a multiphase environment, dynamically regulated by active galactic nuclei (AGN), the motions of cluster galaxies, and mergers with other clusters or groups. AGN provide a central heating source capable of preventing runaway cooling flows and quenching star formation, but how they achieve this is still poorly understood. We investigate the effects of AGN feedback and mergers on the ICM using the high-resolution romulusc cosmological simulation of a 1014 M galaxy cluster. We demonstrate that AGN feedback regulates and quenches star formation in the brightest cluster galaxy gently, without any explosive episodes, and co-exists with a low entropy core with sub-Gyr cooling times. In contrast, the merger disrupts the ICM structure, heating the core and cutting off the supply of low-entropy, infalling gas that until then fuelled the AGN. We find that this removal of the low-entropy phase correlates with the ratio tcool / tff increasing above 30 in the core, matching observations that cooling gas is only found in clusters where this ratio is 5-30. Importantly, we find that evolution in the inner entropy profile and the ratio of cooling to free-fall time-scale are directly connected to the quenching of star formation in the BCG. This is in line with previous results from idealized simulations and confirmed here within a fully cosmological simulation for the first time.