The MeerKAT Fornax Survey IV. A close look at the cluster physics through the densest rotation measure grid

Abstract: Using the Square Kilometre Array (SKA) mid precursor MeerKAT, we acquired broadband spectro-polarimetric data in the context of the MeerKAT Fornax Survey to study the Fornax cluster's magnetic fields in detail by building the densest rotation measure (RM) grid to date. Here, we present the survey, the analysis, and a discussion of the RM grid properties. We analyzed a circular region centered on the Fornax cluster center with a radius of $\sim1.4^\circ$; that is, $\rm\sim 0.73 R_{vir}$. The mosaics have a resolution of 13arcsec and cover the frequencies between 900\,MHz and 1.4\,GHz, reaching an average noise of 16$\mu$Jy beam$^{-1}$ in total intensity and 3$\mu$Jy beam$^{-1}$ in the Q and U Stokes images. With these data, we detected 508 polarized sources over an area of $\sim$6.35 deg$^2$ corresponding to a density of $\sim$80 polarized sources/deg$^2$. This is the densest RM grid ever built. Of the polarized sources, five are cluster sources. Excluding the cluster sources, we built the Euclidean-normalized differential source counts in polarization and we went a factor of ten deeper than previous surveys. We tentatively detect for the first time an increment in the differential source counts at low polarized flux densities; that is, $\sim$9\,$\mu$Jy at 1.4\,GHz. The average degree of polarization of about 3--4\% suggests that the sub$-\mu$Jansky population is not dominated by star-forming galaxies, typically showing a degree of polarization lower than 1\%. The majority of the polarized sources are Faraday simple; in other words, their polarization plane rotates linearly with the wavelength squared. The RM shows the typical decrement going from the center to the outskirts of the Fornax cluster. However, interesting features are observed both in the RM grid and in the RM radial profiles across different directions. A combination of the ...

• The paper constructs the densest rotation measure grid to date using MeerKAT, mapping 508 polarized sources over 6.35 square degrees.
• The methodology utilizes broadband spectro-polarimetric data from 900 MHz to 1.4 GHz with a 13 arcsecond resolution, ensuring precise magnetic field measurements.
• The study reveals a radial RM profile with an enhancement near 300 kpc and a rise in differential source counts at low polarized flux, setting new benchmarks for cluster physics.

Insights into the Dense Rotation Measure Grid of the Fornax Cluster

The paper "The MeerKAT Fornax Survey IV: A close look at the cluster physics through the densest rotation measure grid" explores the intricate details of the magnetic fields within the Fornax cluster using the MeerKAT radio telescope. This work capitalizes on the advanced capabilities of MeerKAT to achieve the densest rotation measure (RM) grid to date, providing a novel approach to investigating cluster physics. The focus is to discern the RM grid properties and their implications on our understanding of magnetic fields in galaxy clusters.

The methodology centers on collecting broadband spectro-polarimetric data across a frequency range of 900 MHz to 1.4 GHz, achieving an impressive resolution of 13 arcseconds. The result is a densely populated RM grid comprising 508 polarized sources over approximately 6.35 square degrees, equating to a density of about 80 polarized sources per square degree. This density is unprecedented and allows for detailed mapping of the cluster's magnetic field.

One of the significant achievements of this study is the construction of the Euclidean-normalized differential source counts in polarization, reaching a depth an order of magnitude greater than previous surveys. The results tentatively suggest, for the first time, a rise in the differential source counts at low polarized flux densities, around 9 μJy at 1.4 GHz. The polarization measurements imply that at sub-μJy levels, the population is unlikely dominated by star-forming galaxies, which usually exhibit lower polarization degrees.

The paper also provides a thorough examination of Faraday simplicity among the polarized sources, demonstrating that the majority are Faraday simple, indicated by the linear rotation of their polarization plane as a function of wavelength squared, with a few exceptions. This confirms that the RM values are largely representative of the Faraday screens between the observer and the source, largely discounting contributions from the intrinsic rotation within the sources themselves.

Notably, the RM data suggest a decrement from the center to the outskirts of the Fornax cluster, a typical characteristic attributed to the dilution of the magnetic field and thermal plasma density. However, the study reveals a peculiar radial profile: the RM values exhibit an enhancement at around 300 kpc from the cluster center. This feature is accompanied by a standard deviation plateau at large radii, which might reflect not only the intrinsic properties of the Fornax intra-cluster medium but also interactions with large-scale structure filaments surrounding the cluster.

The implications of these findings are manifold. Practically, this dense RM grid sets a new benchmark for future radio astronomical studies, particularly in understanding large-scale magnetic fields in low-mass galaxy clusters. Theoretically, it provides a robust framework for examining magneto-genesis and magnetic field evolution in the cosmic web. The data suggest that LSS and cluster dynamics, like sloshing events and mergers, could significantly impact the observed RM characteristics, offering pathways for future detailed simulations and observations.

Future developments in radio astronomy, especially with the advent of the Square Kilometre Array (SKA), have the potential to further unravel these complex magnetic structures. This paper paves the way for such research by demonstrating the intricate relationship between observed RMs and the dynamic processes within and surrounding galaxy clusters, thereby enhancing our comprehension of cosmic magnetism and cluster physics.