The case for electron re-acceleration at galaxy cluster shocks

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 [1,2]. 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 [3]. A longstanding 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.

Chandra Observations of the Spectacular A3411–12 Merger Event

“…The gas mass fraction within r 500 is f g = 0.14 ± 0.01. The distribution of the hot X-ray emitting gas reveals a complex morphology, indicating an active merger history Giovannini et al 2013;van Weeren et al 2017). In particular, the gas distribution is not symmetric, but is elongated in the southeastnorthwest direction.…”

“…Here we measure at the location of the radio relic presented in van Weeren et al (2017) (Southeast Inner sector) M = 1.13 +0.14 −0.08 .…”

“…van Weeren et al (2017) also presented a clustering analysis applied to the three-dimensional galaxy distribution (right ascension, declination, and redshift) of their spectroscopic sample of cluster members (obtained with Keck). They considered mixtures of 1 to 7 multivariate Gaussian components.…”

New giant radio sources and underluminous radio halos in two galaxy clusters

“…However, the commonly used diffusive shock acceleration (DSA) model is challenged by the low Mach number of shocks associated with cluster mergers (M ∼ 1−3). Therefore the re-acceleration of a population of relativistic electrons might be required to explain the radio brightness of some relics (Markevitch et al 2005;Kang et al 2012;Vazza & Brüggen 2014;Bonafede et al 2014;Shimwell et al 2015;Eckert et al 2016;Botteon et al 2016;van Weeren et al 2016van Weeren et al , 2017.…”

Acceleration and Evolution of Cosmic Ray Electrons During Radio-Galaxy-Cluster-Shock Encounters

“…The encounters might also lead to enhanced synchrotron emissions due to compression and particle acceleration. In fact, such RG-shock encounters have been invoked to explain some of the giant, so-called "radio relics" seen in a number of merging clusters [4,5,6,7]. The basic physics of idealized shock-RG-cavity dynamical interactions has been treated before [4,3].…”