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 found1,2. 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 events3. 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.
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Support for this work was provided by the National Aeronautics and Space Administration through Chandra Award Numbers GO3-14131X and GO5-16133X issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under Contract NAS8-03060. We thank the staff of the Giant Metrewave Radio Telescope (GMRT) who have made these observations possible. The GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the US National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Part of this work was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership between the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The Isaac Newton Telescope is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. R.J.W. was supported by a Clay Fellowship awarded by the Harvard-Smithsonian Center for Astrophysics. V.M.P. acknowledges support for this work from grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation. D.R. was supported by the National Research Foundation of Korea through Grant 2016R1A5A1013277. H.K. was supported by the National Research Foundation of Korea through Grant 2014R1A1A2057940. R.M.S. acknowledges CAPES (PROEX), CNPq, PRPG/USP, FAPESP and INCT-A funding. M.J.J. acknowledges support from KASI and NRF of Korea to CGER. D.S. acknowledges financial support from the Netherlands Organisation for Scientific research (NWO) through a Veni fellowship. G.A.O. is supported by NASA through Hubble Fellowship Grant HST-HF2-51345.001-A, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under Contract NAS5-26555.
The authors declare no competing financial interests.
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van Weeren, R., Andrade-Santos, F., Dawson, W. et al. The case for electron re-acceleration at galaxy cluster shocks. Nat Astron 1, 0005 (2017). https://doi.org/10.1038/s41550-016-0005
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