MeerKAT view of the diffuse radio sources in Abell 3667 and their interactions with the thermal plasma

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: fdg@hs.uni-hamburg.de
² INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
³ Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455, USA
⁴ Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2A, Helsinki 00014, Finland
⁵ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 92/3, 40129 Bologna, Italy
⁶ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁷ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
⁸ Department of Physics and Astronomy, Faculty of Physical Sciences, University of Nigeria, Carver Building, 1 University Road, Nsukka, Nigeria
⁹ US Naval Research Laboratory, Code 7213, 4555 Overlook Ave SE, Washington, DC 20375, USA
¹⁰ NRAO – National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA
¹¹ SARAO – South African Radio Astronomy Observatory, 2 Fir St., Observatory, South Africa
¹² Department of Physics, School of Natural Sciences UNIST, Ulsan 44919, Republic of Korea
¹³ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa
¹⁴ School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin D09 W6Y4, Ireland

Received: 12 November 2021
Accepted: 31 January 2022

Abstract

Context. During their lifetimes, galaxy clusters grow through the accretion of matter from the filaments of the large-scale structure and from mergers with other clusters. These mergers release a large amount of energy into the intracluster medium (ICM) through merger shocks and turbulence. These phenomena are associated with the formation of radio sources known as radio relics and radio halos, respectively. Radio relics and halos are unique proxies for studying the complex properties of these dynamically active regions of clusters and the microphysics of the ICM more generally.

Aims. Abell 3667 is a spectacular example of a merging system that hosts a large pair of radio relics. Due to its proximity (z = 0.0553) and large mass, the system enables the study of these sources to a uniquely high level of detail. However, being located at Dec = −56.8°, the cluster could only be observed with a limited number of radio facilities.

Methods. We observed Abell 3667 with MeerKAT as part of the MeerKAT Galaxy Cluster Legacy Survey. We used these data to study the large-scale emission of the cluster, including its polarisation and spectral properties. The results were then compared with simulations.

Results. We present the most detailed view of the radio relic system in Abell 3667 to date, with a resolution reaching 3 kpc. The relics are filled with a network of filaments with different spectral and polarisation properties that are likely associated with multiple regions of particle acceleration and local enhancements of the magnetic field. Conversely, the magnetic field in the space between filaments has strengths close to what would be expected in unperturbed regions at the same cluster-centric distance. Comparisons with magnetohydrodynamic cosmological and Lagrangian simulations support the idea of filaments as multiple acceleration sites. Our observations also confirm the presence of an elongated radio halo, developed in the wake of the bullet-like sub-cluster that merged from the south-east. Finally, we associate the process of magnetic draping with a thin polarised radio source surrounding the remnant of the bullet’s cool core.

Conclusions. Our observations have unveiled the complexity of the interplay between the thermal and non-thermal components in the most active regions of a merging cluster. Both the intricate internal structure of radio relics and the direct detection of magnetic draping around the merging bullet are powerful examples of the non-trivial magnetic properties of the ICM. Thanks to its sensitivity to polarised radiation, MeerKAT will be transformational in the study of these complex phenomena.