We present an analysis of the merging cluster MACS J1149.5+2223 using archival imaging from Subaru/Suprime-Cam and multi-object spectroscopy from Keck/DEIMOS and Gemini/GMOS. We employ two- and three-dimensional substructure tests and determine that MACS J1149.5+2223 is composed of two separate mergers among three subclusters occurring ∼1 Gyr apart. The primary merger gives rise to elongated X-ray morphology and a radio relic in the southeast. The brightest cluster galaxy is a member of the northern subcluster of the primary merger. This subcluster is very massive (). The southern subcluster is also very massive (), yet it lacks an associated X-ray surface brightness peak, and it has been unidentified previously despite the detailed study of this Frontier Field cluster. A secondary merger is occurring in the north along the line of sight (LOS) with a third, less massive subcluster (). We perform a Monte Carlo dynamical analysis on the main merger and estimate a collision speed at pericenter of km s-1. We show the merger to be returning from apocenter with core passage occurring Gyr before the observed state. We identify the LOS merging subcluster in a strong lensing analysis in the literature and show that it is likely bound to MACS J1149 despite having reached an extreme collision velocity of ∼4000 km s-1.

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.