The redshift evolution of extragalactic magnetic fields

arXiv (Cornell University)

Abstract

Faraday rotation studies of distant radio sources can constrain the evolution and the origin of cosmic magnetism. We use data from the LOFAR Two Metre Sky Survey: Data Release 2 (LoTSS DR2) to study the dependence of the Faraday rotation measure (RM) on redshift. By focusing on radio sources that are close in terms of their projection on the sky, but physically unrelated (random pairs), we measure the RM difference, $Δ$RM, between the two sources. Thus, we isolate the extragalactic contribution to $Δ$RM from other contributions. We present a statistical analysis of the resulting sample of random pairs and find a median absolute RM difference |$Δ$RM| $ = (1.79 \pm 0.09)$ rad/m$^{2}$ , with |$Δ$RM| uncorrelated both with respect to the redshift difference of the pair and the redshift of the nearer source, and a median excess of random pairs over physical pairs of $(1.65 \pm 0.10)$ rad/m$^{2}$. We seek to reproduce this result with Monte Carlo simulations assuming a non vanishing seed cosmological magnetic field and a redshift evolution of the comoving magnetic field strength that varies as $1/(1 + z)^γ$. We find the best fitting results $B_0 \equiv B_{\rm comoving}(z = 0) \lesssim (2.0 \pm 0.2)$ nG and $γ\lesssim 4.5 \pm 0.2$ that we conservatively quote as upper limits due to an unmodelled but non vanishing contribution of local environments to the RM difference. A comparison with cosmological simulations shows our results to be incompatible with primordial magnetogenesis scenarios with uniform seed fields of order nG.

Authors 10

1. Valentin Pomakov Aachen

   Dublin City University · RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University , Templergraben 55 , D-52062 Aachen , Germany

   School of Physical Sciences and Centre for Astrophysics & Relativity , Dublin City University , Glasnevin D09 W6Y4 , Ireland

2. S. P. O’Sullivan
3. M. Brüggen
4. F. Vazza
5. Ettore Carretti

   Istituto di Radioastronomia di Bologna · National Institute for Astrophysics

   Affiliation as printed

   INAF Istituto di Radioastronomia , Via Gobetti 101 , I-40129 Bologna , Italy

6. George H. Heald

   Commonwealth Scientific and Industrial Research Organisation

   Affiliation as printed

   CSIRO, Space and Astronomy , PO Box 1130 , Bentley WA 6102 , Australia

7. C. Horellou

   Chalmers University of Technology

   Affiliation as printed

   Department of Space, Earth and Environment , Onsala Space Observatory , Chalmers University of Technology , SE-43992 Onsala , Sweden

8. T. W. Shimwell Aachen Leiden Observatory

   Leiden University · Leiden Observatory · Netherlands Institute for Radio Astronomy

   Affiliation as printed

   ASTRON , the Netherlands Institute for Radio Astronomy , Postbus 2 , NL-7990 AA Dwingeloo , the Netherlands

   Leiden Observatory , Leiden University , PO Box 9513 , NL-2300 RA Leiden , the Netherlands

9. Aleksandar Shulevski Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , PO Box 9513 , NL-2300 RA Leiden , the Netherlands

10. Tessa Vernstrom

    Commonwealth Scientific and Industrial Research Organisation

    Affiliation as printed

    CSIRO, Space and Astronomy , PO Box 1130 , Bentley WA 6102 , Australia