The Planck clusters in the LOFAR sky

VI. LoTSS-DR2: Properties of radio relics^⋆

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: ajones@hs.uni-hamburg.de
² INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
³ DIFA – Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁴ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁵ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁶ INAF, IASF-Milano, via A. Corti 12, 20133 Milano, Italy
⁷ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁸ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
⁹ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
¹⁰ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹¹ GEPI & USN, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
¹² Department of Physics & Electronics, Rhodes University, PO Box 94 Grahamstown 6140, South Africa

Received: 29 September 2022
Accepted: 5 December 2022

Abstract

Context. It is well established that shock waves in the intracluster medium launched by galaxy cluster mergers can produce synchrotron emission, which is visible to us at radio frequencies as radio relics. However, the particle acceleration mechanism producing these relics is still not fully understood. It is also unclear how relics relate to radio halos, which trace merger-induced turbulence in the intracluster medium.

Aims. We aim to perform the first statistical analysis of radio relics in a mass-selected sample of galaxy clusters, using homogeneous observations.

Methods. We analysed all relics observed by the Low Frequency Array Two Metre Sky Survey Data Release 2 (LoTSS DR2) at 144 MHz, hosted by galaxy clusters in the second Planck catalogue of SZ sources (PSZ2). We measured and compared the relic properties in a uniform, unbiased way. In particular, we developed a method to describe the characteristic downstream width in a statistical manner. Additionally, we searched for differences between radio relic-hosting clusters with and without radio halos.

Results. We find that, in our sample, ∼10% of galaxy clusters host at least one radio relic. We confirm previous findings, at higher frequencies, of a correlation between the relic-cluster centre distance and the longest linear size, as well as the radio relic power and cluster mass. However, our findings suggest that we are still missing a population of low-power relics. We also find that relics are wider than theoretically expected, even with optimistic downstream conditions. Finally, we do not find evidence of a single property that separates relic-hosting clusters with and without radio halos.