The MeerKAT Fornax Survey

VII. Characterisation of the Fornax cluster’s magnetic field and new insights on magnetisation in large-scale systems

¹ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, Selargius, Italy
² Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
³ Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, 2000 Johannesburg, South Africa
⁴ Netherlands Institute for Radio Astronomy (ASTRON), Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 20 March 2026
Accepted: 9 April 2026

Abstract

Large-scale magnetic fields in galaxy clusters can influence their physics and the evolution of cluster-embedded galaxies. These properties remain poorly constrained due to a historical lack of high-sensitivity and high-resolution spectropolarimetric data. Thanks to the advent of the Square Kilometre Array pathfinders and precursors, this situation is now dramatically changing. Using the densest rotation measure (RM) grid produced to date from broadband spectropolarimetric data within the MeerKAT Fornax Survey (508 sources over 6.35 deg²; presented in a previous paper), we aim to study the Fornax cluster’s magnetic field in detail. We compared the RM grid properties with numerical simulations to constrain the strength and the structure of the intracluster magnetic field. We modelled the magnetic field power spectrum with a power law, and find a slope of 2.7^+0.2_−0.4, fluctuating between minimum and a maximum scales of 1.01^+0.01_−0.02 and 15⁺⁹₋₂ kpc, respectively. It has a central strength of 5.0^+0.3_−0.4 μG, decreasing with the thermal plasma density according to a power–law exponent η = 1.6^+0.3_−0.5, the highest value to date in large-scale systems. By analysing a sample of 17 galaxy clusters and groups with magnetic field estimates from the literature, we observe longer autocorrelation lengths in the case of massive merging clusters and lower values for relaxed clusters and low-mass clusters or galaxy groups. We also observe a systematic increase in the central magnetic field strength as a function of central density, B₀ ∝ n₀^{(0.38 ± 0.14)}. We argue that the steepening of the Fornax cluster’s magnetic field profile and its relatively high central strength could indicate a recent re-amplification at the centre due to the extended central radio galaxy. The sample analysis supports the proposed scenario; however, more detailed magnetic field studies conducted using consistent modelling on larger samples are needed to better understand magnetisation in clusters and groups.