Radio Mini-Halo Emission from Cosmic Rays in Galaxy Clusters and Heating of the Cool Cores

TL;DR

This paper investigates the role of cosmic rays in heating galaxy cluster cool cores and their associated radio mini-halos, using simulations and observations to support the hypothesis that CRs contribute to core stability and produce observable synchrotron emission.

Contribution

The study combines numerical simulations with X-ray observations to model CR pressure profiles and predict radio mini-halo emissions, providing new evidence for CR heating in galaxy cluster cores.

Findings

CR heating can stabilize cool cores more effectively with sound velocity streaming.

Predicted synchrotron profiles match observed mini-halos if CR spectra are steep.

Non-thermal fluxes in X-ray and gamma-ray bands are predicted to be small.

Abstract

It has been proposed that the cool cores of galaxy clusters are stably heated by cosmic rays (CRs). If this is the case, radio mini-halos, which are often found in the central regions of cool core clusters, may be attributed to the synchrotron emission from the CRs. Based on this idea, we investigate the radial profiles of the mini-halos. First, using numerical simulations, we confirm that it is appropriate to assume that radiative cooling of the intracluster medium (ICM) is balanced with the heating by CR streaming. In these simulations, we assume that the streaming velocity of the CRs is the sound velocity of the ICM, and indicate that the heating is even more stable than the case where the streaming velocity is the Alfven velocity. Then, actually assuming the balance between cooling and heating, we estimate the radial profiles of CR pressure in six clusters only from X-ray observations. Since the CR protons interact with the ICM protons, we can predict the radial profiles of the resultant synchrotron radiation. We compare the predictions with the observed radial profiles of the mini-halos in the six clusters and find that they are consistent if the momentum spectra of the CRs are steep. These results may indicate that the cores are actually being heated by the CRs. We also predict broad-band spectra of the six clusters, and show that the non-thermal fluxes from the clusters are small in hard X-ray and gamma-ray bands.