The Case for Electron Re-Acceleration at Galaxy Cluster Shocks

TL;DR

This paper presents evidence that fossil relativistic electrons from radio galaxies are re-accelerated at cluster shocks, explaining the origin of radio relics in merging galaxy clusters.

Contribution

It provides the first direct observational link between a radio relic and a radio galaxy, highlighting the role of fossil electrons in shock acceleration.

Findings

Discovery of a connection between a radio relic and a radio galaxy in Abell 3411-3412.

Evidence that fossil electrons from active galactic nuclei are re-accelerated at cluster shocks.

Radio galaxies significantly influence the non-thermal intracluster medium in merging clusters.

Abstract

On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-size radio sources have been found, so-called radio relics. These relics are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach number collisionless shocks generated by cluster-cluster merger events. A long-standing problem is how low-Mach number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here we report on the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411-3412. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.