The detection of cluster magnetic fields via radio source depolarisation^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: osinga@strw.leidenuniv.nl
² Clay Center Observatory, Dexter Southfield, 20 Newton Street, Brookline, MA 02445, USA
³ Center for Astrophysics – Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St SE, Minneapolis, MN 55455, USA
⁵ DIFA – Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁶ INAF – IRA, via Gobetti 101, 40129 Bologna, Italy
⁷ IRA – INAF, via P. Gobetti 101, 40129 Bologna, Italy
⁸ Naval Research Laboratory, 4555 Overlook Avenue SW, Code 7213, Washington, DC 20375, USA
⁹ Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁰ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, Garching 85748, Germany
¹¹ Department of Liberal Arts and Sciences, Berklee College of Music, 7 Haviland Street, Boston, MA 02215, USA

Received: 11 March 2022
Accepted: 1 July 2022

Abstract

It has been well established that galaxy clusters have magnetic fields. The exact properties and origin of these magnetic fields are still uncertain even though these fields play a key role in many astrophysical processes. Various attempts have been made to derive the magnetic field strength and structure of nearby galaxy clusters using Faraday rotation of extended cluster radio sources. This approach needs to make various assumptions that could be circumvented when using background radio sources. However, because the number of polarised radio sources behind clusters is low, at the moment such a study can only be done statistically. In this paper, we investigate the depolarisation of radio sources inside and behind clusters in a sample of 124 massive clusters at z < 0.35 observed with the Karl G. Jansky Very Large Array. We detect a clear depolarisation trend with the cluster impact parameter, with sources at smaller projected distances to the cluster centre showing more depolarisation. By combining the radio observations with ancillary X-ray data from Chandra, we compare the observed depolarisation with expectations from cluster magnetic field models using individual cluster density profiles. The best-fitting models have a central magnetic field strength of 5−10 μG with power-law indices between n = 1 and n = 4. We find no strong difference in the depolarisation trend between sources embedded in clusters and background sources located at similar projected radii, although the central region of clusters is still poorly probed by background sources. We also examine the depolarisation trend as a function of cluster properties such as the dynamical state, mass, and redshift. We see a hint that dynamically disturbed clusters show more depolarisation than relaxed clusters in the r > 0.2R₅₀₀ region. In the core region, we did not observe enough sources to detect a significant difference between cool-core and non-cool-core clusters. Our findings show that the statistical depolarisation of radio sources is a good probe of cluster magnetic field parameters. Cluster members can be used for this purpose as well as background sources because the local interaction between the radio galaxies and the intracluster medium does not strongly affect the observed depolarisation trend.