Cool-core Clusters: The Role of BCG, Star Formation, and AGN-driven Turbulence

Recent observations of cool cluster cores that include the BCG gravity claim that the observed threshold in min(t cool/t ff) (cooling time to free-fall time ratio) lies at a somewhat higher value, close to 10–30, compared with the threshold seen in numerical simulations. There are only a few clusters in which this ratio falls much below 10. In this paper, we compare 3D hydrodynamic simulations of feedback active galactic nuclei (AGNs) jets interacting with the intracluster medium, with and without a BCG potential. We find that, for a fixed feedback efficiency, the presence of a BCG does not significantly affect the temperature, but increases (decreases) the core density (entropy) on average. Most importantly, min(t cool/t ff) is only affected slightly by the inclusion of the BCG gravity. Also notable is that the lowest value of min(t cool/t ff) in the NFW+BCG runs is about twice as large as in the NFW runs. We also look at the role of depletion of cold gas due to star formation, and show that it only affects the rotationally dominant component, while the radially dominant component remains largely unaffected. Stellar gas depletion also increases the repetition rate of AGN jets. The distribution of metals due to AGN jets in our simulations is predominantly along the jet direction, and the equatorial spread of metals is less compared with the observations. We also show that the turbulence in cool-core clusters is weak, which is consistent with recent Hitomi results on the Perseus cluster.