Detection of magnetic fields in superclusters of galaxies

Context. The properties of magnetic fields in large-scale structure filaments, far beyond galaxy clusters, are still poorly known. Superclusters of galaxies are laboratories for investigating low-density environments, which are not easily identified given the low signals and large scales involved. The observed Faraday rotation measure (RM) of polarised sources along the line of sight of superclusters allows us to constrain the magnetic field properties in these extended environments. Aims. The aim of this work is to constrain the magnetic field intensity in low-density environments within the extent of superclusters of galaxies using the Faraday RM of polarised background sources detected at different frequencies. Methods. We selected three rich and nearby (z < 0.1) superclusters of galaxies for which polarisation observations were available at both 1.4 GHz and 144 MHz: Corona Borealis, Hercules, and Leo. We compiled a catalogue of 4497 polarised background sources that have RM values either from the literature or derived from unpublished observations at 144 MHz. For each supercluster we created a 3D density cube in order to associate a density estimate with each RM measurement. We computed the median absolute deviation (MAD) variance of the RM values grouped in three density bins that correspond to the supercluster outskirts (0.01 < ρ/ρc < 1), filaments (1 < ρ/ρc < 30), and nodes (30 < ρ/ρc < 1000) regimes to investigate how variations in the RM distribution are linked to the mean density crossed by the polarised emission. Results. We find an excess ΔσMAD2RRM = 2.5 ± 0.5 rad2 m−4 between the lowest-density regions (outside supercluster boundaries) and the low-density region inside the supercluster. This excess is attributed to the intervening medium of the filaments in the supercluster. We modelled the variance of the RM distribution as being due to a single-scale, randomly oriented magnetic field distribution and therefore as being dependant upon the magnetic field intensity along the line of sight, the magnetic field reversal scale, and the line-of-sight path length. Our observations do not constrain the latter two parameters, but if we marginalise over their respective prior range, we constrain the magnetic field to $ B_{||}=19^{+50}_{-8} $ nG. Conclusions. Our findings are consistent with several other works that studied filaments of the large-scale structure. The results suggest that the purely adiabatic compression of a primordial magnetic field, which would imply observed magnetic fields of the order of B|| ∼ 2 nG, is not the only mechanism playing a role in amplifying the primordial seeds in superclusters of galaxies.