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

¹ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
² Dipartimento di Fisica e Astronomia, Universitá di Bologna, via Gobetti 93/2, 40122 Bologna, Italy
³ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Departamento de Fisica de la Tierra y Astrofisica & IPARCOS-UCM, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁵ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁶ ATNF, CSIRO Space & Astronomy, P.O. Box 1130 Bentley, WA 6102, Australia
⁷ SKA Observatory, SKA-Low Science Operations Centre, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁹ School of Natural Sciences and Medicine, Ilia State University, 3-5 Cholokashvili St., 0194 Tbilisi, Georgia
¹⁰ ICRAR, The University of Western Australia, 35 Stirling Hw, 6009 Crawley, Australia

^⋆ Corresponding author; carretti@ira.inaf.it

Received: 1 July 2024
Accepted: 18 November 2024

Abstract

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.