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

Recent analysis shows that it is important to explicitly include the gravitational potential of the central brightest central galaxy (BCG) to infer the acceleration due to gravity ($g$) and the free-fall time ($t_{\rm ff} \equiv [2r/g]^{1/2}$) in cool cluster cores. Accurately measuring $t_{\rm ff}$ is crucial because according to numerical simulations cold gas condensation and strong feedback occur in cluster cores with min($t_{\rm cool}/t_{\rm ff}$) below a threshold value close to 10. Recent observations which include the BCG gravity show that the observed threshold in min($t_{\rm cool}/t_{\rm ff}$) lies at a somewhat higher value, close to 20-30, there are only a few clusters in which this ratio falls below 10. In this paper we compare numerical simulations of feedback AGN (Active Galactic Nucleus) jets interacting with the intracluster medium (ICM) 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($t_{\rm cool}/t_{\rm ff}$) is not affected much by the inclusion of the BCG gravity. We also look at the role of depletion of cold gas due to star formation in cool core clusters using a simplified model for the removal of cold gas. We show that the depletion of cold gas affects only the rotationally dominant component (torus) while the radially dominant component (which regulates the feedback cycle) remains largely unaffected. Stellar gas depletion also increases the duty cycle of AGN jets. The distribution of metals due to AGN jets in our simulations is predominantly along the jet direction and the radial spread of metals is less compared to the observations. We also show that the turbulence in cool core clusters is weak, consistent with recent Hitomi results on Perseus cluster.