• An Erratum to this article was published on 13 January 2017


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 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.

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  1. 1.

    , , & Clusters of galaxies: observational properties of the diffuse radio emission. Astron. Astrophys. Rev. 20, 54 (2012).

  2. 2.

    & Cosmic rays in galaxy clusters and their nonthermal emission. Int. J. Mod. Phys. D 23, 1430007–98 (2014).

  3. 3.

    , , & Cluster radio relics as a tracer of shock waves of the large-scale structure formation. Astron. Astrophys. 332, 395–409 (1998).

  4. 4.

    , , & Cosmological shock waves and their role in the large-scale structure of the Universe. Astrophys. J. 593, 599–610 (2003).

  5. 5.

    & Cosmic ray spectrum from diffusive shock acceleration. Astrophys. Space Sci. 336, 263–268 (2011).

  6. 6.

    et al. A shock front in the merging galaxy cluster A754: X-ray and radio observations. Astrophys. J. 728, 82 (2011).

  7. 7.

    et al. Another shock for the Bullet cluster, and the source of seed electrons for radio relics. Mon. Not. R. Astron. Soc. 449, 1486–1494 (2015).

  8. 8.

    & Diffusive shock acceleration simulations of radio relics. Astrophys. J. 756, 97 (2012).

  9. 9.

    & Giant radio relics in galaxy clusters: reacceleration of fossil relativistic electrons? Mon. Not. R. Astron. Soc. 435, 1061–1082 (2013).

  10. 10.

    & Do radio relics challenge diffusive shock acceleration? Mon. Not. R. Astron. Soc. 437, 2291–2296 (2014).

  11. 11.

    et al. Suzaku observations of the galaxy cluster 1RXS J0603.3+4214: implications of particle acceleration processes in the “Toothbrush” radio relic. Publ. Astron. Soc. Jpn. 67, 113 (2015).

  12. 12.

    LOFAR, VLA, and Chandra observations of the Toothbrush galaxy cluster. Astrophys. J. 818, 204 (2016).

  13. 13.

    & Particle acceleration at astrophysical shocks: a theory of cosmic ray origin. Phys. Rep. 154, 1–75 (1987).

  14. 14.

    et al. A merger mystery: no extended radio emission in the merging cluster Abell 2146. Mon. Not. R. Astron. Soc. 417, 1–5 (2011).

  15. 15.

    & Non-thermal Electron acceleration in low mach number collisionless shocks. II. Firehose-mediated Fermi acceleration and its dependence on pre-shock conditions. Astrophys. J. 797, 47 (2014).

  16. 16.

    , , & A shock at the radio relic position in Abell 115. Mon. Not. R. Astron. Soc. 460, 84–88 (2016).

  17. 17.

    et al. A shock front at the radio relic of Abell 2744. Mon. Not. R. Astron. Soc. 461, 1302–1307 (2016).

  18. 18.

    , , & Bow shock and radio halo in the merging cluster A520. Astrophys. J. 627, 733–738 (2005).

  19. 19.

    & Curved radio spectra of weak cluster shocks. Astrophys. J. 809, 186 (2015).

  20. 20.

    et al. Evidence for particle re-acceleration in the radio relic in the galaxy cluster PLCKG287.0+32.9. Astrophys. J. 785, 1 (2014).

  21. 21.

    , & The Coma cluster radio source 1253+275, revisited. Astron. Astrophys. 252, 528–537 (1991).

  22. 22.

    et al. Discovery of the first giant double radio relic in a galaxy cluster found in the Planck Sunyaev-Zel’dovich Cluster Survey: PLCK G287.0+32.9. Astrophys. J. 736, 8 (2011).

  23. 23.

    & , Reviving fossil radio plasma in clusters of galaxies by adiabatic compression in environmental shock waves. Astron. Astrophys. 366, 26–34 (2001).

  24. 24.

    & On the formation of cluster radio relics. Mon. Not. R. Astron. Soc.. 331, 1011–1019 (2002).

  25. 25.

    et al. Complex diffuse radio emission in the merging Planck ESZ Cluster A3411. Astrophys. J. 769, 101 (2013).

  26. 26.

    et al. The nature of the giant diffuse non-thermal source in the A3411-A3412 complex. Mon. Not. R. Astron. Soc. 435, 518–523 (2013).

  27. 27.

    , , & Particle acceleration on megaparsec scales in a merging galaxy cluster. Science 330, 347 (2010).

  28. 28.

    , , , & Turbulent cosmic-ray reacceleration at radio relics and halos in clusters of galaxies. Astrophys. J. 815, 116 (2015).

  29. 29.

    , , & Electron and proton acceleration efficiency by merger shocks in galaxy clusters. Astrophys. J. 451, 2198–2211 (2015).

  30. 30.

    , , & Exploring the nature of collisionless shocks under laboratory conditions. Sci. Rep. 4, 3934 (2014).

  31. 31.

    , , , & in Astronomical Data Analysis Software and Systems XVI (eds Shaw, R. A., Hill, F. & Bel, D. J.) 127 (Astron. Soc. Pacif. Conf. Ser. Vol. 376, 2007).

  32. 32.

    , & The noncoplanar baselines effect in radio interferometry: the W-projection algorithm. IEEE J. Sel. Top. Signal Process. 2, 647–657 (2008).

  33. 33.

    , & in Astronomical Data Analysis Software and Systems XIV (eds Shopbel, P. L, Britton, M. & Ebert, R. ) 86 (Astron. Soc. Pacif. Conf. Ser. Vol. 376, 2005).

  34. 34.

    et al. The discovery of lensed radio and X-Ray sources behind the frontier fields cluster MACS J0717.5+3745 with the JVLA and Chandra. Astrophys. J. 817, 98 (2016).

  35. 35.

    et al. Post-correlation radio frequency interference classification methods Mon. Not. R. Astron. Soc. 405, 155–167 (2010).

  36. 36.

    , & A Rotation measure image of the sky. Astrophys. J. 702, 1230–1236 (2009).

  37. 37.

    & The morphology of extragalactic radio sources of high and low luminosity. Mon. Not. R. Astron. Soc. 167, 31–36 (1974).

  38. 38.

    et al. Chandra temperature profiles for a sample of nearby relaxed galaxy clusters. Astrophys. J. 628, 655–672 (2005).

  39. 39.

    , & The cool-core bias in X-ray galaxy cluster samples. I: Method and application to HIFLUGCS. Astron. Astrophys. 526, 79 (2011).

  40. 40.

    et al. Challenges to our understanding of radio relics: X-ray observations of the Toothbrush cluster. Mon. Not. R. Astron. Soc. 433, 812–824 (2013).

  41. 41.

    et al. Subaru Prime Focus Camera — Suprime-Cam. Publ. Astron. Soc. Jpn. 54, 833–853 (2002).

  42. 42.

    et al. MC2: Constraining the dark matter distribution of the violent merging galaxy cluster CIZA J2242.8+5301 by piercing through the milky Way. Astrophys. J. 802, 46 (2015).

  43. 43.

    et al. The role of cluster mergers and travelling shocks in shaping the Hα luminosity function at z0.2: ‘sausage’ and ‘toothbrush’ clusters. Mon. Not. R. Astron. Soc. 438, 1377–1390 (2014).

  44. 44.

    et al. MC2: boosted AGN and star formation activity in CIZA J2242.8+5301, a massive post-merger cluster at z=0.19. Mon. Not. R. Astron. Soc. 450, 630–645 (2015).

  45. 45.

    in Astronomical Data Analysis Software and Systems (eds Gabriel, C., Arviset, C., Ponz, D. & Enrique, S.) 112 (Astron. Soc. Pacif. Conf. Ser. Vol. 351, 2006).

  46. 46.

    et al. The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4) Astron. J. 145, 44 (2013).

  47. 47.

    et al. in Astronomical Data Analysis Software and Systems XI (eds Bohlender, D. A., Durand, D. & Handley, T. H.) 228 (Astron. Soc. Pacif. Conf. Ser. Vol. 281, 2002).

  48. 48.

    et al. in Proc. SPIE - International Society for Optical Engineering (eds Iye, M. & Moorwood, A. F. M) Vol. 4841, 1657–1669 (International Society for Optics and Photonics, 2003).

  49. 49.

    et al. MC2: galaxy imaging and redshift analysis of the merging cluster CIZA J2242.8+5301. Astrophys. J. 805, 143 (2015).

  50. 50.

    et al. The DEEP2 Galaxy Redshift Survey: design, observations, data reduction, and redshifts. Astrophys. J. Suppl. Ser. 208, 5 (2013).

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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.

Author information


  1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

    • Reinout J. van Weeren
    • , Felipe Andrade-Santos
    • , William R. Forman
    • , Christine Jones
    •  & Ralph P. Kraft
  2. Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA

    • William A. Dawson
  3. University of California, 1 Shields Avenue, Davis, California 95616, USA

    • Nathan Golovich
    •  & David Wittman
  4. National Centre for Radio Astrophysics, TIFR, Pune University Campus, Post Bag 3, Pune 411007, India

    • Dharam V. Lal
  5. Department of Earth Sciences, Pusan National University, Busan 46241, Korea

    • Hyesung Kang
  6. Department of Physics, UNIST, Ulsan 44919, Korea

    • Dongsu Ryu
  7. Korea Astronomy and Space Science Institute, Daejeon 34055, Korea

    • Dongsu Ryu
  8. Hamburger Sternwarte, Hamburg University, Gojenbergsweg 112, 21029 Hamburg, Germany

    • Marcus Brüggen
  9. Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, California 94305-4085, USA

    • Georgiana A. Ogrean
  10. Department of Physics and JINA Center for the Evolution of the Elements, University of Notre Dame, Notre Dame, Illinois 46556, USA

    • Vinicius M. Placco
  11. Departamento de Astronomia - Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, SP 05508-900, Brazil

    • Rafael M. Santucci
  12. Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, Lisbon 1749-016, Portugal

    • David Wittman
  13. Department of Astronomy and Center for Galaxy Evolution Research, Yonsei University, 50 Yonsei-ro, Seoul 03722, Korea

    • M. James Jee
  14. Department of Physics, Lancaster University, Lancaster LA1 4YB, UK

    • David Sobral
  15. Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands

    • David Sobral
  16. European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching bei Mìchen, Germany

    • Andra Stroe
  17. Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218-2686, USA

    • Kevin Fogarty


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R.J.W. coordinated the research, wrote the manuscript, reduced the VLA data, and led the Chandra observing proposal. F.A.S., K.F. and G.A.O. performed the Chandra data reduction and worked on the X-ray surface brightness profile fitting. H.K. and D.R. carried out the re-acceleration modeling. M.B., W.R.F. and C.J. helped with the interpretation of the radio and X-ray results and provided extensive feedback on the manuscript. C.J. led the GMRT observing proposal. D.V.L. obtained the GMRT observations and carried out the GMRT data reduction. V.M.P. and R.M.A. obtained the SOAR observations and performed the corresponding data reduction. D.S. and A.S. obtained the INT observations and reduced the data. W.A.D. carried out the dynamical modeling of the merger event. W.A.D, N.G. and M.J.J. obtained the Keck and Subaru observations and reduced the data. D.W. helped with the interpretation of the dynamical modeling and led the Keck and Subaru observing proposals. R.P.K. assisted with the writing of the Chandra observing proposal.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Reinout J. van Weeren.

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