Cosmic rays and thermal instability in self-regulating cooling flows of massive galaxy clusters

TL;DR

This study uses magneto-hydrodynamical simulations to show that cosmic rays from AGN jets can prevent cooling flows in galaxy clusters for billions of years, but do not stop thermal instability, with implications for observed gamma-ray flux.

Contribution

It demonstrates that a small fraction of AGN energy converted into cosmic rays can effectively prevent cooling flows over cosmic timescales, highlighting the role of CRs in cluster thermodynamics.

Findings

10% CR energy injection prevents cooling flows for billions of years.

CR-dominated jets form extended warm central nebulae supported by CR pressure.

CR redistribution is mainly via advection, with streaming heating maintaining gas phases.

Abstract

One of the key physical processes that helps prevent strong cooling flows in galaxy clusters is the continued energy input from the central active galactic nucleus (AGN) of the cluster. However, it remains unclear how this energy is thermalised so that it can effectively prevent global thermal instability. One possible option is that a fraction of the AGN energy is converted into cosmic rays (CRs), which provide non-thermal pressure support, and can retain energy even as thermal energy is radiated away. By means of magneto-hydrodynamical simulations, we investigate how CR injected by the AGN jet influence cooling flows of a massive galaxy cluster. We conclude that converting a fraction of the AGN luminosity as low as 10\% into CR energy prevents cooling flows on timescales of billion years, without significant changes in the structure of the multi-phase intra-cluster medium. CR-dominated jets, by contrast, lead to the formation of an extended, warm central nebula that is supported by CR pressure. We report that the presence of CRs is not able to suppress the onset of thermal instability in massive galaxy clusters, but CR-dominated jets do significantly change the continued evolution of gas as it continues to cool from isobaric to isochoric. The CR redistribution in the cluster is dominated by advection rather than diffusion or streaming, but the heating by CR streaming helps maintain gas in the hot and warm phase. Observationally, self-regulating, CR-dominated jets produce a \gammaray~ flux in excess of current observational limits, but low CR fractions in the jet are not ruled out.