The nature of LOFAR rotation measures and new constraints on magnetic fields in cosmic filaments and on magnetogenesis scenarios

The measurement of magnetic fields in cosmic web filaments can be used to reveal the magnetogenesis of the Universe. In previous works, we produced the first estimates of the field strength and its redshift evolution using the Faraday rotation measure (RM) catalogue of extragalactic background sources at a low frequency obtained with LOFAR observations. For this work, we refined our analysis by selecting sources with a low Galactic RM, which reduces its residual contamination. We also conducted a comprehensive analysis of the different contributions to the extragalactic RMs along the line of sight, and confirm that they are dominated by the cosmic filaments’ component, with only 21 percent originating in galaxy clusters and the circumgalactic medium (CGM) of galaxies. We find a possible hint of a shock at the virial radius of massive galaxies. We also find that the fractional polarisation of background sources might be a valuable CGM tracer. The newly selected RMs have a steeper evolution with redshift than previously found. The field strength in filaments ( B f ) and its evolution were estimated assuming B f evolves as a power law B f = B f , 0 (1 + z ) α . Our analysis finds an average strength at z = 0 of B f , 0 = 11–15 nG, with an error of 4 nG, and a slope α = 2.3–2.6 ± 0.5, which is steeper than what we previously found. The comoving field has a slope of β = [0.3, 0.6]±0.5 that is consistent with being invariant with redshift. Primordial magnetogenesis scenarios are favoured by our data, together with a sub-dominant astrophysical-origin RM component increasing with redshift.