Production Efficiencies of Sound Waves in the Intracluster Medium Driven by AGN Jets

TL;DR

This study uses 3D hydrodynamic simulations to evaluate how effectively sound waves generated by AGN jets contribute to heating the intracluster medium, finding they are a minor heating source compared to shocks.

Contribution

It provides a detailed quantification of the energy stored in sound waves and shocks from AGN jets, highlighting the limited role of sound waves in cluster heating.

Findings

Approximately 9% of jet energy is stored in compressional waves after a single injection.

In self-regulated feedback, only about 3% of jet energy goes into sound waves.

Shocks dominate the energy contribution in the inner regions and are more significant at larger radii.

Abstract

Feedback from active galactic nuclei (AGN) is believed to be the most promising solution to the cooling flow problem in cool-core clusters, though how exactly the jet energy is transformed into heat is a subject of debate. Dissipation of sound waves is considered as one of the possible heating mechanisms; however, its relative contribution to heating remains unclear. To estimate the energy budget for heating, we perform 3D hydrodynamic simulations of AGN jet injections in a Perseus-like cluster and quantify the amount of energy stored in the forms of weak shocks and waves. We find that, for a single jet injection with typical parameters in cool-core clusters, $\sim9\%$ of the total jet energy is stored in compressional waves (including both shocks and waves). However, due to the destructive effects among randomly phased waves as well as the dissipation of shock energies, in our simulations including self-regulated AGN feedback, no more than $3\%$ of the total injected energy goes into compressional waves. We further separate the energy contribution from shocks and waves and find that, for a single outburst, the shocks can only contribute to $\sim20-30\%$ of the total compressional energy in the inner radii, and they dissipate very quickly as they travel outward. However, because of the repeated generation of shocks by multiple AGN outbursts, in the self-regulated case shocks completely dominate over sound waves in the inner region and can still provide $\sim40-50\%$ of the total compressional energy even at outer radii. Our results suggest that the production of sound waves is not as efficient as what was found in previous single-outburst simulations, and thus sound wave dissipation may be a subdominant source of heating in cool-core clusters.