Simulated rotation measure sky from primordial magnetic fields

¹ Dipartimento di Fisica e Astronomia, Universitá di Bologna, Via Gobetti 92/3, 40121, Bologna, Italy
² School of Natural Sciences and Medicine, Ilia State University, 0194, Tbilisi, Georgia
³ INAF Istituto di Radioastronomia, Via Gobetti 101, 40129, Bologna, Italy
⁴ Department of Physics and Astronomy, University of California, Los Angeles, CA, 90095, USA
⁵ Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA, 91101, USA
⁶ Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM, Universidad Complutense de Madrid, 28040, Madrid, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 November 2025
Accepted: 27 February 2026

Abstract

Primordial magnetic fields (PMFs) – magnetic fields that originate in the early Universe and permeate the cosmological scales today – can explain the observed micro-gauss-level magnetisation of galaxies and their clusters. In light of current and upcoming all-sky radio surveys, PMFs have drawn attention not only as major candidates for explaining the large-scale magnetisation of the Universe, but also as potential probes of early-Universe physics. While much recent work focuses on constraining the strength of PMFs, it remains challenging to constrain their structure (coherence scale). In this paper, using cosmological simulations coupled with light-cone analysis, for the first time we study the imprints of the PMF structure on the mean rotation measure (RM) originating in the intergalactic medium (IGM), ⟨RM_IGM⟩. We introduce a new method for producing full-sky RM_IGM distributions. By analysing the autocorrelation of RM_IGM on small and large angular scales, we find that PMF structures show distinct signatures. The large-scale uniform model (characterised by an initially unlimited coherence scale) leads to correlations up to 90°, while correlations for small-scale stochastic PMF models drop by a factor of 100 at 0.17, 0.13, and 0.11° angular scales, corresponding to 5.24, 4.03, and 3.52 Mpc scales (at z = 2 redshift depths) for magnetic fields with comoving 3.49, 1.81, 1.00 h⁻¹ Mpc coherence scales, respectively; the correlation amplitude of the PMF model with comoving an ∼19 h⁻¹ Mpc coherence scale drops by only a factor of 10 at 1.0° (30.6 Mpc). These results suggest that improvements in the modelling of Galactic RM will be necessary to investigate the signature of large-scale correlated PMFs. A comparison of ⟨RM_IGM⟩ redshift dependence obtained from our simulations with that from the LOFAR Two-metre Sky Survey shows agreement with the our previous upper limits’ estimates of the PMF strength derived from RM-rms analysis.