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Observations of a pre-merger shock in colliding clusters of galaxies


Clusters of galaxies are the largest known gravitationally bound structures in the Universe. When clusters collide, they create merger shocks on cosmological scales, which transform most of the kinetic energy carried by the cluster gaseous halos into heat1,2,3. Observations of merger shocks provide key information on the merger dynamics, and enable insights into the formation and thermal history of the large-scale structures. Nearly all of the merger shocks are found in systems where the clusters have already collided4,5,6,7,8,9,10,11,12; knowledge of shocks in the pre-merger phase is a crucial missing ingredient13,14. Here, we report on the discovery of a unique shock in a cluster pair 1E 2216.0-0401 and 1E 2215.7-0404. The two clusters are observed at an early phase of major merger. Contrary to all the known merger shocks observed ubiquitously on merger axes, the new shock propagates outward along the equatorial plane of the merger. This discovery uncovers an important epoch in the formation of massive clusters, when the rapid approach of the cluster pair leads to strong compression of gas along the merger axis. Current theoretical models15,16 predict that the bulk of the shock energy might be dissipated outside the clusters, and eventually turn into heat of the pristine gas in the circum-cluster space.

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Data availability

The data 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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Competing interests

The authors declare no competing interests.

Additional information

Peer review information: Nature Astronomy thanks Jack Burns and Maxim Markevitch for their contribution to the peer review of this work.

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L.G. is supported by the RIKEN Special Postdoctoral Researcher Program. SRON is supported financially by NWO, the Netherlands Organization for Scientific Research. R.J.v.W acknowledges support from the VIDI research programme with project number 639.042.729, which is financed by the Netherlands Organisation for Scientific Research (NWO). F.M. is supported by the Lendület LP2016-11 grant awarded by the Hungarian Academy of Sciences. J.M. acknowledges support by STFC (UK) through the UK APAP network grant ST/R000743/1. The Leiden LOFAR team gratefully acknowledge support from the European Research Council under the European Unions Seventh Framework Programme (FP/2007-2013)/ERC Advanced Grant NEWCLUSTERS-321271.

This research has made use of data obtained from the Chandra Data Archive and the Chandra Source Catalog, and software provided by the Chandra X-ray Center (CXC) in the application packages CIAO, ChIPS and Sherpa. This research is partly based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. We thank the staff of the GMRT that made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. This paper is based in part on data obtained with the International LOFAR Telescope (ILT) under project code LC7_003 and LC9_014. LOFAR is the Low Frequency Array designed and constructed by ASTRON. It has observing, data processing and data storage facilities in several countries that are owned by various parties (each with their own funding sources), and that 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, UK; Ministry of Science and Higher Education, Poland. 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).

Author information

L.G. coordinated the research, led the XMM-Newton, LOFAR and GMRT proposals, reduced and analysed the Chandra and XMM-Newton data, and wrote the manuscript. H.A. led the Chandra proposal and analysed the Suzaku data. T.W.S., H.T.I., R.J.v.W. and F.d.G. performed the radio data reduction and worked on the radio index map. F.M., J.M., I.U. and J.d.P. assisted with the interpretation of the X-ray results, V.P. assisted with the GMRT proposal, H.J.A.R. and J.S.K. provided extensive suggestions on the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Liyi Gu.

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Fig. 1: Composite image of the pre-merging cluster 1E 2216.0-0401 and 1E 2215.7-0404.
Fig. 2: Original X-ray image and residual image from Chandra.
Fig. 3: ICM temperature map, temperature uncertainty map and pressure map of 1E 2216.0-0401 and 1E 2215.7-0404.