How AGN Jets Heat the Intracluster Medium -- Insights from Hydrodynamic Simulations

TL;DR

This study uses 3D hydrodynamic simulations to explore how AGN jets heat the intracluster medium, revealing that shock heating and mixing within jet cones are primary mechanisms, leading to a self-regulating, gentle circulation process.

Contribution

It provides new insights into the dominant heating mechanisms of the ICM by combining cold gas accretion and momentum-driven jets in detailed simulations, highlighting the self-regulating nature of AGN feedback.

Findings

Net heating occurs mainly within jet cones via shock heating and mixing.

Outside jet cones, weak shocks provide insufficient heating to counter cooling.

Turbulent heating is negligible compared to thermal energy.

Abstract

Feedback from active galactic nuclei (AGN) is believed to prevent catastrophic cooling in galaxy clusters. However, how the feedback energy is transformed into heat, and how the AGN jets heat the intracluster medium (ICM) isotropically, still remain elusive. In this work, we gain insights into the relative importance of different heating mechanisms using three-dimensional hydrodynamic simulations including cold gas accretion and momentum-driven jet feedback, which are the most successful models to date in terms reproducing the properties of cool cores. We find that there is net heating within two `jet cones' (within ~30 degrees from the axis of jet precession) where the ICM gains entropy by shock heating and mixing with the hot thermal gas within bubbles. Outside the jet cones, the ambient gas is heated by weak shocks, but not enough to overcome radiative cooling, therefore forming a `reduced' cooling flow. Consequently, the cluster core is in a process of `gentle circulation' over billions of years. Within the jet cones, there is significant adiabatic cooling as the gas is uplifted by buoyantly rising bubbles; outside the cones, energy is supplied by inflow of already-heated gas from the jet cones as well as adiabatic compression as the gas moves toward the center. In other words, the fluid dynamics self-adjusts such that it compensates and transports the heat provided by the AGN, and hence no fine-tuning of the heating profile of any process is necessary. Throughout the cluster evolution, turbulent energy is only at the percent level compared to gas thermal energy, and thus turbulent heating is not the main source of heating in our simulation.