Cosmic-Ray Feedback Heating of the Intracluster Medium

Mateusz Ruszkowski,Christopher S Reynolds,H.-Y Karen Yang

doi:10.3847/1538-4357/aa79f8

Abstract

Abstract Active galactic nuclei (AGNs) play a central role in solving the decades-old cooling-flow problem. Although there is consensus that AGNs provide the energy to prevent catastrophically large star formation, one major problem remains: How is the AGN energy thermalized in the intracluster medium (ICM)? We perform a suite of three-dimensional magnetohydrodynamical adaptive mesh refinement simulations of AGN feedback in a cool core cluster including cosmic rays (CRs). CRs are supplied to the ICM via collimated AGN jets and subsequently disperse in the magnetized ICM via streaming, and interact with the ICM via hadronic, Coulomb, and streaming instability heating. We find that CR transport is an essential model ingredient at least within the context of the physical model considered here. When streaming is included, (i) CRs come into contact with the ambient ICM and efficiently heat it, (ii) streaming instability heating dominates over Coulomb and hadronic heating, (iii) the AGN is variable and the atmosphere goes through low-/high-velocity dispersion cycles, and, importantly, (iv) CR pressure support in the cool core is very low and does not demonstrably violate observational constraints. However, when streaming is ignored, CR energy is not efficiently spent on the ICM heating and CR pressure builds up to a significant level, creating tension with the observations. Overall, we demonstrate that CR heating is a viable channel for the AGN energy thermalization in clusters and likely also in ellipticals, and that CRs play an important role in determining AGN intermittency and the dynamical state of cool cores.

Highlights

In recent years hydrodynamic simulations made substantial progress in terms of understanding active galactic nuclei (AGN) accretion and feedback processes in clusters
While there is consensus that AGN provide most if not all of the energy needed to offset radiative losses in the intracluster medium (ICM) and prevent catastrophically large star formation rates, one major problem remains unsolved – how is the AGN energy thermalized in the ICM and what are the effective black hole feeding rates in realistic systems? We perform a suite of three-dimensional magneto-hydrodynamical (MHD) adaptive mesh refinement simulations of AGN feedback in a cool core cluster including cosmic ray (CR) physics
Our models demonstrate that CR heating is a viable channel for the thermalization of AGN energy in clusters, and likely in elliptical galaxies, and that CRs play an important role in determining AGN intermittency and the dynamical state of cool core atmospheres

Summary

Introduction

In recent years hydrodynamic simulations made substantial progress in terms of understanding AGN accretion and feedback processes in clusters. Ruszkowski, Yang, and Reynolds oretical and observational studies that focus on the role of thermal instability in the ICM in feeding the central supermassive black hole (e.g., McCourt et al (2012); Voit et al (2015)), simulations including cold-gas accretion and momentum-driven feedback have successfully reproduced the positive temperature gradients and properties of cold gas within the cool cores (Gaspari et al 2012a; Li et al 2015, 2016) These kinds of simulations provided valuable insights into the mysteries of how the AGN energy is transformed into heat and how the heat is distributed radially and isotropically throughout the cool core. Yang & Reynolds (2016a) showed that a gentle circulation flow on billionyear timescale is responsible for partially compensating cooling and transporting the heat provided by the AGN in an isotropic manner

Methods

Results

Conclusion