The "toothbrush-relic": Evidence for a coherent linear 2-Mpc scale shock wave in a massive merging galaxy cluster?

R. J. Van Weeren, H. J.A. Röttgering, H. T. Intema, L. Rudnick, M. Brüggen, M. Hoeft, J. B.R. Oonk

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Some merging galaxy clusters host diffuse extended radio emission, so-called radio halos and relics, unrelated to individual galaxies. The origin of these halos and relics is still debated, although there is compelling evidence now that they are related to cluster merger events. Here we present detailed Westerbork Synthesis Radio Telescope (WSRT) and Giant Metrewave Radio Telescope (GMRT) radio observations between 147 MHz and 4.9 GHz of a new radio-selected galaxy cluster 1RXS J0603.3+4214, for which we find a redshift of 0.225. The cluster is detected as an extended X-ray source in the ROSAT All Sky Survey with an X-ray luminosity of LX, 0.1-2.4 keV ∼ 1 × 1045 erg s-1. The cluster hosts a large bright 1.9 Mpc radio relic, an elongated ∼2 Mpc radio halo, and two fainter smaller radio relics. The large radio relic has a peculiar linear morphology. For this relic we observe a clear spectral index gradient from the front of the relic towards the back, in the direction towards the cluster center. Parts of this relic are highly polarized with a polarization fraction of up to 60%. We performed rotation measure (RM) synthesis between 1.2 and 1.7 GHz. The results suggest that for the west part of the large relic some of the Faraday rotation is caused by the intracluster medium and not only due to galactic foregrounds. We also carried out a detailed spectral analysis of this radio relic and created radio color-color diagrams. We find (i) an injection spectral index of-0.6 to-0.7; (ii) steepening spectral index and increasing spectral curvature in the post-shock region; and (iii) an overall power-law spectrum between 74 MHz and 4.9 GHz with α =-1.10 ± 0.02. Mixing of emission in the beam from regions with different spectral ages is probably the dominant factor that determines the shape of the radio spectra. Changes in the magnetic field, total electron content, or adiabatic gains/losses do not play a major role. A model in which particles are (re)accelerated in a first order Fermi process at the front of the relic provides the best match to the observed spectra. We speculate that in the post-shock region particles are re-accelerated by merger induced turbulence to form the radio halo as the relic and halo are connected. The orientation of the bright relic and halo indicate a north-south merger event, but the peculiar linear shape and the presence of another relic, perpendicular to the bright relic, suggest a more complex merger event. Deep X-ray observations will be needed to determine the merger scenario.

Original languageEnglish (US)
Article numberA124
JournalAstronomy and Astrophysics
StatePublished - 2012

Bibliographical note

Funding Information:
We would like to thank the anonymous referee for useful comments. We thank the staff of the 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 Westerbork Synthesis Radio Telescope is operated by ASTRON (Netherlands Institute for Radio Astronomy) with support from the Netherlands Foundation for Scientific Research (NWO). The William Herschel Telescope are 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. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Support for this work was provided by NASA through Einstein Postdoctoral grant number PF2-130104 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. R.J.v.W. acknowledges funding from the Royal Netherlands Academy of Arts and Sciences. M.B. and M.H. acknowledges support by the research group FOR 1254 funded by the Deutsche Forschungsgemeinschaft. LR acknowledges support from the US National Science Foundation, under grant AST0908668 to the University of Minnesota. H.T.I. is Jansky Fellow of the National Radio Astronomy Observatory. R.J.v.W. would like to thank C. Pfrommer for discussions and G. Heald for his explanation of the RM-CLEAN algorithm.


  • Galaxies: active
  • Large-scale structure of Universe
  • Radio continuum: galaxies

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