We present deep Chandra observations of A3411-12, a remarkable merging cluster that hosts the most compelling evidence for electron reacceleration at cluster shocks to date. Using the YX-M scaling relation, we find r 500 ∼ 1.3 Mpc, M500 =(7.1 ± 0.7) × 1014 M⊙, kT =6.5 ± 0.1 keV, and a gas mass of Mg,500 =(9.7 ± 0.1) × 1013 M⊙. The gas mass fraction within r 500 is fg = 0.14 ± 0.01. We compute the shock strength using density jumps to conclude that the Mach number of the merging subcluster is small (M ≤ 1.15-0.09+0.14). We also present density, temperature, pseudo-pressure, and pseudo-entropy maps. Based on the pseudo-entropy map, we conclude that the cluster is undergoing a mild merger, consistent with the small Mach number. On the other hand, radio relics extend over Mpc scale in the A3411-12 system, which strongly suggests that a population of energetic electrons already existed over extended regions of the cluster.

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-sized radio sources have been found 1,2. Also known as radio relics, these regions of diffuse radio emission are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach-number shocks generated by cluster-cluster merger events 3. A long-standing problem is how low-Mach-number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here, we report the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411-3412 by combining radio, X-ray and optical observations. 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.