Radio Halos and the importance of future observations at low frequency

TL;DR

This paper discusses the significance of low-frequency radio observations for understanding radio halos in galaxy clusters, highlighting how future LOFAR surveys can test models of turbulent reacceleration of electrons during cluster mergers.

Contribution

It emphasizes the importance of low-frequency observations and proposes that upcoming LOFAR surveys will critically test the turbulent reacceleration model for radio halo formation.

Findings

Radio halos are linked to turbulent acceleration during cluster mergers.

A bimodal distribution exists in radio properties of galaxy clusters.

Many radio halos have steep spectra detectable only at low frequencies.

Abstract

Radio observations discovered large scale non thermal sources in the central Mpc regions of dynamically disturbed galaxy clusters (radio halos). The morphological and spectral properties of these sources suggest that the emitting electrons are accelerated by spatially distributed and gentle mechanisms, providing some indirect evidence for turbulent acceleration in the inter-galactic-medium (IGM). Radio and X-ray surveys allow to investigate the statistics of radio halos and unveil a bimodal behaviour of the radio properties of galaxy clusters: merging clusters host radio halos and trace the well known radio--X correlation, while more relaxed clusters do not host radio halos and populate a region well separated from that spanned by the above correlation. This appears consistent with the hypothesis that relativistic electrons can be reaccelerated by MHD turbulence generated during cluster mergers. In the context of this model the population of radio halos consists of a mixture of halos with different spectral properties, most of them with very steep spectrum and visible only at low radio frequencies. For this reason the future LOFAR surveys may provide a robust test to this theoretical hypothesis.