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

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 γ-ray flux in excess of current observational limits, but low CR fractions in the jet are not ruled out.

Cosmic ray feedback in galaxies and galaxy clusters

“…In agreement with the linear analysis of Kempski and Quataert ( 2020 ), non-linear simulations of thermal instability development in the ICM in the presence of AGN feedback heating and CRs demonstrate that CRs are initially unable to stabilize most of the hot and dilute plasma against TI (i.e., the gas is mostly characterized by and ; Beckmann et al. 2022a ). However, once the gas has cooled out of the hot phase, it either settles down to a thermally stable state with , or to a state where but and further collapse can be prevented by significant CR pressure (see Fig.…”

“…15 in Beckmann et al. 2022a ). Such significant pressure support ( ) in the cold phase is expected even if the initial in the hot phase because of adiabatic compression of CRs combined with dramatic loss of thermal pressure due to radiative cooling.…”

“…As suggested by Beckmann et al. ( 2022a ) in the context of the ICM, secondary CR electrons may dissociate CO molecules, which may help to explain strong C ii lines (Mittal et al. 2012 ) and high C ii /CO abundance (Mashian et al.…”

Multiphase Gas in Elliptical Galaxies: The Role of Type Ia Supernovae

“…We have ignored the effect of important physics such as magnetic fields (Hopkins et al 2020;Wang et al 2021;Buie et al 2022), cosmic rays (Butsky et al 2020;Kempski & Quataert 2020;Beckmann et al 2022), and conduction (Parrish et al 2009;El-Badry et al 2019), see Faucher-Giguère & Oh (2023) for a review. Magnetic fields and cosmic rays are expected to be energetically important and can affect the properties of the thermal energy-driven outflow.…”

Evolution and feedback of AGN Jets of different Cosmic-ray Composition

“…More recently, the properties of CRp dominated jets have also seen growing attention in the literature (e.g. Mathews & Brighenti 2008;Guo & Mathews 2011;Ruszkowski et al 2017;Ehlert et al 2018;Yang et al 2019;Su et al 2021;Beckmann et al 2022). These works have demonstrated that differences in the energy content of AGN bubbles could lead to very different heating impacts and dynamical evolution of the ICM.…”