Fast magnetic field amplification in distant galaxy clusters


In the present-day Universe, magnetic fields pervade galaxy clusters1 and have strengths of a few microgauss, as measured from Faraday rotation2. Evidence for cluster magnetic fields is also provided by the observation of megaparsec-scale radio emission, namely radio halos and relics3. These are commonly found in merging systems4 and are characterized by a steep radio spectrum Sν (α < −1, where Sν να and is ν the observing frequency). It is widely believed that magneto-hydrodynamical turbulence and shock waves (re-)accelerate cosmic rays5 and produce radio halos and relics. The origin and amplification of magnetic fields in clusters is not well understood. It has been proposed that turbulence drives a small-scale dynamo6,7,8,9,10,11 that amplifies seed magnetic fields (which are primordial and/or injected by galactic outflows, such as active galactic nuclei, starbursts or winds12). At high redshift, radio halos are expected to be faint, owing to losses from inverse Compton scattering and the dimming effect with distance. Moreover, Faraday rotation measurements are difficult to obtain. If detected, distant radio halos provide an alternative tool to investigate magnetic field amplification. Here, we report Low Frequency Radio Array observations that reveal diffuse radio emission in massive clusters when the Universe was only half of its present age, with a sample occurrence fraction of about 50%. The high radio luminosities indicate that these clusters have similar magnetic field strengths to those in nearby clusters, and suggest that magnetic field amplification is fast during the first phases of cluster formation.

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Fig. 1: Examples of observed radio emission in our high-z galaxy cluster sample.
Fig. 2: X-ray images of a subsample of galaxy clusters from Fig. 1.
Fig. 3: Cluster magnetic field estimation and theoretical magnetic field evolution.

Data availability

The radio observations are available in the LOFAR Long Term Archive (LTA; and in the VLA archive (, project code 15A_270). The X-ray observations are available in the XMM-Newton and Chandra data archives ( and The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.

Code availability

The codes that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


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We thank C. Giocoli and his team for the discussion of the cosmological derivations in the manuscript. This manuscript is based on data obtained with the International LOFAR Telescope (ILT). LOFAR is the Low Frequency Radio Array designed and constructed by ASTRON. It has observing, data processing and data storage facilities in several countries, which are owned by various parties (each with their own funding sources), and which are collectively operated by the ILT foundation under a joint scientific policy. The ILT resources have benefited from the following recent major funding sources: CNRS-INSU, Observatoire de Paris and Université d’Orléans, France; BMBF, MIWF-NRW, MPG, Germany; Science Foundation Ireland (SFI), Department of Business, Enterprise and Innovation (DBEI), Ireland; NWO, the Netherlands; the Science and Technology Facilities Council (STFC), United Kingdom; Ministry of Science and Higher Education, Poland; the Istituto Nazionale di Astrofisica (INAF), Italy. This research made use of the Dutch national e-infrastructure with support of the SURF Cooperative (e-infra 180169) and the LOFAR e-infra group. The Jülich LOFAR Long Term Archive and the German LOFAR network are both coordinated and operated by the Jülich Supercomputing Centre (JSC), and computing resources on the supercomputer JUWELS at JSC were provided by the Gauss Centre for Supercomputing e.V. (grant CHTB00) through the John von Neumann Institute for Computing (NIC). This research made use of the University of Hertfordshire high-performance computing facility and the LOFAR-UK computing facility located at the University of Hertfordshire and supported by STFC (ST/P000096/1), and of the Italian LOFAR IT computing infrastructure supported and operated by INAF, and by the Physics Department of Turin University (under an agreement with Consorzio Interuniversitario per la Fisica Spaziale) at the C3S Supercomputing Centre, Italy. The National Radio Astronomy Observatory is a facility of the US National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. The scientific results reported in this manuscript are based in part on data obtained from the Chandra Data Archive. The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, the Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, NASA (grant no. NNX08AR22G) issued through the Planetary Science Division of the NASA Science Mission Directorate, the US National Science Foundation (grant no. AST-1238877), the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory and the Gordon and Betty Moore Foundation. G.D.G. and R.J.v.W. acknowledge support from the ERC starting grant ‘ClusterWeb’ (no. 804208). G.B., R.C., F.G. and M.R. acknowledge support from INAF through the mainstream project ‘Galaxy clusters science with LOFAR’. A. Botteon and R.J.v.W. acknowledge support from the VIDI research programme (no. 639.042.729), which is financed by the Netherlands Organisation for Scientific Research. H.J.A.R. acknowledges support from the ERC Advanced Investigator programme ‘NewClusters’ (no. 32127). A. Bonafede acknowledges support from the ERC starting grant ‘DRANOEL’ (no. 714245) and from the MIUR grant FARE ‘SMS’. P.D.-F. acknowledges financial support from the ERC Starting ‘MAGCOW’ (no. 714196).

Author information




G.D.G. coordinated the research, performed the radio imaging, reduced the VLA and Chandra data and wrote the manuscript. R.J.v.W., A. Botteon and F.d.G. performed the additional calibration on the LOFAR data and wrote the data reduction software. G.B. and R.C. performed the magnetic field evolution modelling. R.J.v.W., G.B. and R.C. helped with the writing of the manuscript. M.B. and M.H. helped with the interpretation of the radio and modelling results and provided extensive feedback on the manuscript. H.J.A.R. and T.W.S. led the LoTSS survey and coordinated the LOFAR data reduction. A.S. carried out the XMM-Newton data reduction. F.G. and M.R. helped with the interpretation of the X-ray data and sample selection. A. Bonafede helped design the experiment and prepare the observing proposal. V.C., D.D., P.D.-F., T.A.E. and S.M. helped with the interpretation of the radio and modelling results and gave feedback on the manuscript. All of the authors of this manuscript are members of the LOFAR Surveys Key Science Project.

Corresponding author

Correspondence to Gabriella Di Gennaro.

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Peer review information thanks Kaustuv Basu, Surajit Paul and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Observed radio emission in our high-z galaxy cluster sample.

In colorscale we show the full-resolution LOFAR images. Low- resolution source-subtracted radio contours, displayed at the [-2,2,3,4,5,8,16]xσrms level, are shown only for clusters that host diffuse radio emission (with σrms the individual map noise; the negative contour levels are indicated with a short- dashed line style). The full- and low-resolution LOFAR beams are displayed in the bottom left corner (in pink and grey colors, respectively). In the header of each image, the galaxy cluster name, mass and redshift are reported. The dashed black circle in each map shows the R = 0.5RSZ,500 region, obtained from MSZ,500. Source data

Extended Data Fig. 2 X-ray images of all the galaxy clusters in our.

In colorscale we show the Chandra/XMM-Newton images. LOFAR radio contours are drawn as Fig. 1, with the LOFAR.

Supplementary information

Supplementary Information

Supplementary Figs. 1–2, Discussion and Tables 1–4.

Source data

Source Data Fig. 1

R.m.s. map noise levels for each cluster (σrms). The same noise levels are used for Fig. 2.

Source Data Fig. 3

Data used in the plot; the file includes also the data to reproduce the black line in the plot.

Source Data Extended Data Fig. 1

R.m.s. map noise levels for each cluster of our sample (σrms). The same noise levels are used for Extended Data Fig. 2.

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Di Gennaro, G., van Weeren, R.J., Brunetti, G. et al. Fast magnetic field amplification in distant galaxy clusters. Nat Astron (2020).

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