Rise and Fall of Radio Halos in Simulated Merging Galaxy Clusters

TL;DR

This study uses high-resolution MHD simulations to demonstrate how turbulence-driven reacceleration of cosmic-ray electrons can explain the transient radio halos observed in merging galaxy clusters, capturing their spectral diversity.

Contribution

First simulation to model cosmic-ray electron reacceleration by turbulence in merging clusters, reproducing key radio halo features and their transient nature.

Findings

Reacceleration can produce observable radio halos.

Clusters switch between radio loud and quiet states during mergers.

Simulated spectra match observed variety, including ultra-steep spectra.

Abstract

We present the first high resolution MHD simulation of cosmic-ray electron reacceleration by turbulence in cluster mergers. We use an idealised model for cluster mergers, combined with a numerical model for the injection, cooling and reacceleration of cosmic-ray electrons, to investigate the evolution of cluster scale radio emission in these objects. In line with theoretical expectations, we for the first time, show in a simulation that reacceleration of CRe has the potential to reproduce key observables of radio halos. In particular, we show that clusters evolve being radio loud or radio quiet, depending on their evolutionary stage during the merger. We thus recover the observed transient nature of radio halos. In the simulation the diffuse emission traces the complex interplay between spatial distribution of turbulence injected by the halo infall and the spatial distribution of the seed electrons to reaccelerate. During the formation and evolution of the halo the synchrotron emission spectra show the observed variety: from power-laws with spectral index of 1 to 1.3 to curved and ultra-steep spectra with index $> 1.5$.