The Case for Electron Re-Acceleration at Galaxy Cluster Shocks
  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.
