Cool-Core Clusters : Role of BCG, Star Formation & AGN-Driven Turbulence

Abstract

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 10-30; there are only a few clusters in which this ratio falls much below 10. In this paper we compare numerical simulations of feedback AGN (Active Galactic Nuclei) 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 only affected slightly by the inclusion of the BCG gravity. Also notable is that the lowest value of min($t_{\rm cool}/t_{\rm ff}$) in the NFW+BCG runs are about twice larger than 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 (torus), while the radially dominant component (which regulates the feedback cycle) remains largely unaffected. 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. We also show that the turbulence in cool core clusters is weak, consistent with recent Hitomi results on Perseus cluster.