The MeerKAT Fornax Survey VII. Characterisation of the Fornax cluster's magnetic field and new insights on magnetisation in large scale systems

TL;DR

This study characterizes the magnetic field in the Fornax galaxy cluster using high-resolution spectro-polarimetric data, revealing a steep power-law spectrum, high central magnetic strength, and correlations with cluster properties, advancing understanding of large-scale magnetisation.

Contribution

The paper provides the first detailed magnetic field model for the Fornax cluster based on a dense RM grid, including a power-law spectrum and insights into magnetic field evolution in clusters.

Findings

Magnetic field power spectrum slope of ~2.7 between 1 and 15 kpc.

Central magnetic field strength of ~5 μG, decreasing with plasma density.

Larger auto-correlation lengths in massive merging clusters.

Abstract

Large scale magnetic fields in galaxy clusters can influence their physics and the evolution of the cluster galaxies. These properties remain poorly constrained due to a historical lack of high-sensitivity and high-resolution spectro-polarimetric data. Thanks to the advent of the SKA pathfinders and precursors this is now dramatically changing. By exploiting the densest RM grid produced to date with broadband spectro-polarimetric data in the context of the MeerKAT Fornax Survey and presented in a previous paper (508 sources over 6.35 deg$^2$), we aim to study in detail the Fornax cluster's magnetic field. We compare the RM grid properties with numerical simulations to constrain the strength and the structure of the intra-cluster magnetic field. We model the magnetic field power spectrum with a power-law and we find a slope of $\sim$2.7 fluctuating between $\sim$1 and $\sim$15 kpc. It has a central strength of $\sim$5$\mu$G decreasing with the thermal plasma density according to a power-law exponent of $\sim$1.6, 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 larger auto-correlation lengths in the case of massive merging clusters and lower values for relaxed clusters and low-mass systems. We also observe a systematic increase of the central magnetic field strength as a function of central density, $B_0\propto n_0^{0.38}$. Finally, we argue that the steepening of the Fornax cluster's magnetic field profile and its relatively high central strength could be indicative of a recent re-amplification at the centre due to the extended central radio galaxy. The sample analysis supports the proposed scenario, although more detailed magnetic field studies conducted using consistent modelling on larger samples are needed to better understand magnetisation in clusters and groups.