Identifying heating processes in simulations with an entropy-based scheme: A single jet episode in a galaxy cluster

TL;DR

This paper introduces an entropy-based method to analyze heating mechanisms in galaxy cluster simulations, distinguishing contributions from shocks, turbulence, and cosmic rays during AGN jet activity.

Contribution

The study develops and validates a passive entropy scalar technique to systematically identify dominant heating processes in MHD simulations of galaxy clusters.

Findings

Light jets produce wider bubbles and heat primarily through turbulence.

Denser jets sustain strong shocks and dissipate energy via shocks.

Shock heating dominates early, turbulence dominates later in jet evolution.

Abstract

Understanding heating processes in galaxy clusters is essential for predicting the regulation of radiative cooling and star formation, and for clarifying the mechanisms underlying active galactic nucleus (AGN) feedback in cool-core clusters. We investigate the processes through which AGN jets deposit heat into the intracluster medium (ICM) by tracking passive entropy scalars in magneto-hydrodynamic (MHD) simulations. This enables us to systematically disentangle the contributions from different heating channels. We successfully validate this method with several idealized tests, including turbulent heating, heating by anisotropic Braginskii viscosity, dissipative and adiabatic heating by shocks using in-situ shock-detection methods, and cosmic ray (CR) heating through the excitations and damping of Alfv\'en waves. Using this methodology, we simulate single-epoch outbursts of high-power jets with varying densities in a cluster environment. Light jets produce wider bubbles, displacing a larger fraction of the gas in the cluster's core, whereas comparatively denser jets propagate more efficiently to larger distances without significantly disturbing the central region. During early evolution, shock heating dominates for the jets irrespective of their densities. At later times, light jets primarily heat the ICM through turbulent dissipation, while the denser jets continue to dissipate most of their energy via shocks. Turbulent and/or mixing-driven heating prevails inside the cocoon, whereas shock and acoustic compressions dominate outside. In light jets, the forward shock weakens rapidly, whereas dense jets can sustain strong bow shocks to large distances. This heating estimator allows us to identify the dominant heating mechanism responsible for resolving the cooling flow problem in future self-regulated AGN jet simulations.