Probing the magnetic field at the cluster virial radius with volume-filling radio emission

TL;DR

This study uses ultra-deep low-frequency radio observations combined with simulations to explore magnetic fields and relativistic electrons at the outskirts of galaxy clusters, reaching the virial radius and revealing efficient energy transfer processes.

Contribution

It provides the deepest low-frequency radio imaging of a galaxy cluster's outskirts and combines this with simulations to estimate magnetic fields and energy budgets at large radii.

Findings

Radio emission extends up to the virial radius of 5 Mpc.

Evidence of efficient kinetic energy transfer to nonthermal components.

Pervasive radio emission indicates widespread relativistic electrons and magnetic fields.

Abstract

Diffuse synchrotron emission in the form of radio halos and radio relics probe the existence of relativistic electrons and magnetic fields in galaxy clusters. These nonthermal components are generated from the dissipation of kinetic energy released by turbulence and shocks injected in the intracluster medium (ICM) during the large-scale structure formation process. By using the deepest images ever obtained on a galaxy cluster at low-frequency (72 h LOFAR-HBA + 72 h LOFAR-LBA), in arXiv:2211.01493 we provided an unprecedented view of the distribution of relativistic electrons and magnetic fields in the far outskirts of Abell 2255. In particular, we observed pervasive radio emission that fills the entire cluster volume and extends up to the cluster virial radius, reaching a maximum projected linear size of 5 Mpc. By combining radio and X-ray observations with advanced numerical simulations, we estimated the magnetic field and energy budget associated to turbulent motions at such large distances from the cluster center. Our results suggest an efficient transfer of kinetic energy into nonthermal components in the extremely diluted cluster outskirts. In the past two years, the total LOFAR-HBA observation time on Abell 2255 has increased to 336 hours. The analysis of this ultra-deep dataset aims to further advance our understanding of relativistic electrons and magnetic fields in cluster peripheries.