The radio relics and halo of EL Gordo, A massive Z = 0.870 cluster merger

Robert R. Lindner, Andrew J. Baker, John P. Hughes, Nick Battaglia, Neeraj Gupta, Kenda Knowles, Tobias A. Marriage, Felipe Menanteau, Kavilan Moodley, Erik D. Reese, Raghunathan Srianand

Research output: Contribution to journalArticlepeer-review


We present 610 MHz and 2.1 GHz imaging of the massive Sunyaev-Zel'dovich Effect selected z = 0.870 cluster merger ACT-CL J0102-4915 ("El Gordo"), obtained with the Giant Metrewave Radio Telescope and the Australia Telescope Compact Array (ATCA), respectively. We detect two complexes of radio relics separated by 3.′4 (1.6 Mpc) along the system's northwest-to-southeast collision axis that have high integrated polarization fractions (33%) and steep spectral indices (α between 1 and 2; S ∝ν), consistent with creation via Fermi acceleration by shocks in the intracluster medium triggered by the cluster collision. From the spectral index of the relics, we compute a Mach number and shock speed of . With our wide-bandwidth, full-polarization ATCA data, we compute the Faraday depth φ across the northwest relic and find a range of values spanning Δφ = 30 rad m-2, with a mean value of 〈φ〉 = 11 rad m-2 and standard deviation σφ = 6 rad m-2. With the integrated line-of-sight gas density derived from new Chandra X-ray observations, our Faraday depth measurement implies B ∼ 0.01 μG in the cluster outskirts. The extremely narrow shock widths in the relics (d shock ≤ 23 kpc), caused by the short synchrotron cooling timescale of relativistic electrons at z = 0.870, prevent us from placing a meaningful constraint on the magnetic field strength B using cooling time arguments. In addition to the relics, we detect a large (r H ≃ 1.1 Mpc radius), powerful (log (L 1.4/W Hz-1) = 25.66 ± 0.12) radio halo with a shape similar to El Gordo's "bullet"-like X-ray morphology. The spatially resolved spectral-index map of the halo shows the synchrotron spectrum is flattest near the relics, along the system's collision axis, and in regions of high T gas, all locations associated with recent energy injection. The spatial and spectral correlation between the halo emission and cluster X-ray properties supports primary-electron processes like turbulent reacceleration as the halo production mechanism. The halo's integrated 610 MHz to 2.1 GHz spectral index is a relatively flat α = 1.2 ± 0.1, consistent with the cluster's high T gas in view of previously established global scaling relations. El Gordo is the highest-redshift cluster known to host a radio halo and/or radio relics, and provides new constraints on the non-thermal physics in clusters at z > 0.6.

Original languageEnglish (US)
Article number49
JournalAstrophysical Journal
Issue number1
StatePublished - May 1 2014


  • galaxies: clusters: individual (ACT-CL J0102?4915)
  • galaxies: clusters: intracluster medium
  • large-scale structure of universe
  • radiation mechanisms: non-thermal
  • radio continuum: general
  • relativistic processes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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