Feedback Heating by Cosmic Rays in Clusters of Galaxies

TL;DR

This paper proposes a new model where cosmic rays efficiently heat the intracluster medium in galaxy clusters, suppressing cooling flows without fine-tuning, and aligns well with observations and potential gamma-ray detections.

Contribution

The study introduces a successful cosmic-ray heating model for galaxy clusters that reduces cooling flows and matches observed profiles without requiring fine-tuning.

Findings

Cosmic-ray heating suppresses cooling catastrophe in simulations.

Cluster profiles match observations with reduced accretion rates.

No fine-tuning of conduction suppression needed in the model.

Abstract

Recent observations show that the cooling flows in the central regions of galaxy clusters are highly suppressed. Observed AGN-induced cavities/bubbles are a leading candidate for suppressing cooling, usually via some form of mechanical heating. At the same time, observed X-ray cavities and synchrotron emission point toward a significant non-thermal particle population. Previous studies have focused on the dynamical effects of cosmic-ray pressure support, but none have built successful models in which cosmic-ray heating is significant. Here we investigate a new model of AGN heating, in which the intracluster medium is efficiently heated by cosmic-rays, which are injected into the ICM through diffusion or the shredding of the bubbles by Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal conduction as well. Using numerical simulations, we show that the cooling catastrophe is efficiently suppressed. The cluster quickly relaxes to a quasi-equilibrium state with a highly reduced accretion rate and temperature and density profiles which match observations. Unlike the conduction-only case, no fine-tuning of the Spitzer conduction suppression factor f is needed. The cosmic ray pressure, P_c/P_g <~ 0.1 and dP_c/dr <~ 0.1 \rho g, is well within observational bounds. Cosmic ray heating is a very attractive alternative to mechanical heating, and may become particularly compelling if GLAST detects the gamma-ray signature of cosmic-rays in clusters.