Cosmic ray transport in galaxy clusters: implications for radio halos and gamma-rays

TL;DR

This paper explores how cosmic ray transport and turbulence in galaxy clusters influence the distribution of cosmic rays, explaining the observed bimodality in radio halos and gamma-ray emissions.

Contribution

It introduces a model linking CR propagation and turbulence to the bimodal distribution of radio and gamma-ray emissions in galaxy clusters, independent of specific halo formation mechanisms.

Findings

Bimodal CR distribution correlates with cluster dynamical state.

Turbulence influences the spatial concentration of CRs.

Explains radio halo bimodality without relying on specific formation models.

Abstract

Observations of giant radio halos provide unambiguous evidence for the existence of cosmic ray (CR) electrons and magnetic fields in galaxy clusters. The physical mechanism generating radio halos is still heavily debated. We critically discuss the proposed models for the radio halo emission and highlight the weaknesses underlying each explanation. We present an idea how the interplay of CR propagation and turbulent advection selects a bimodal spatial CR distribution that is characteristic for the dynamical state of a cluster. As a result, strongly turbulent, merging clusters should have a more centrally concentrated CR energy density profile with respect to relaxed ones with very subsonic turbulence. This translates into a bimodality of the expected diffuse radio and gamma ray emission of clusters. Thus, the observed bimodality of cluster radio halos appears to be a natural consequence of the interplay of CR transport processes, independent of the model of radio halo formation, be it hadronic interactions of CR protons or re-acceleration of low-energy CR electrons.