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  • staa1395

    Final published version, 1.63 MB, PDF document

  • S. P. O'Sullivan
  • M. Brüggen
  • F. Vazza
  • E. Carretti
  • N. Locatelli
  • C. Stuardi
  • V. Vacca
  • T. Vernstrom
  • G. Heald
  • C. Horellou
  • T. W. Shimwell
  • M. J. Hardcastle
  • C. Tasse
  • H. Röttgering
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Original languageEnglish
Article numberstaa1395
Pages (from-to)2607–2619
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Early online date21 May 2020
Publication statusPublished - 1 Jul 2020


Measuring the properties of extragalactic magnetic fields through the effect of Faraday rotation provides a means to understand the origin and evolution of cosmic magnetism. Here we use data from the LOFAR Two-Metre Sky Survey (LoTSS) to calculate the Faraday rotation measure (RM) of close pairs of extragalactic radio sources. By considering the RM difference ($\Delta$RM) between physical pairs (e.g. double-lobed radio galaxies) and non-physical pairs (i.e. close projected sources on the sky), we statistically isolate the contribution of extragalactic magnetic fields to the total RM variance along the line of sight. We find a difference in the rms of $\Delta$RM between non-physical and physical pairs of 0.4$\pm$0.3 rad/m$^2$, and a difference in the corresponding median |$\Delta$RM| values of 0.3$\pm$0.4 rad/m$^2$. This enables us to place an upper limit on the co-moving cosmological magnetic field strength of $B <2.5$ nG on Mpc scales. This limit is obtained by exploring a wide range of input magnetic field strengths in a model of cosmic over-densities that realistically reflects the observed matter inhomogeneities on large scales. We also compare the LOFAR RM data with a suite of cosmological MHD simulations, that explore different magnetogenesis scenarios. Both magnetization of the large scale structure by astrophysical processes such as galactic and AGN outflows, and simple primordial scenarios with seed field strengths of $B\lesssim0.5$ nG cannot be rejected by the current data, while stronger primordial fields or models with dynamo amplification in filaments are disfavoured. In general, LOFAR polarized sources are typically located in regions of the Universe with low RM variance, making them excellent probes of the weak magnetization of cosmic filaments and voids far from galaxy cluster environments.


This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 495, Issue 3, July 2020, Pages 2607–2619: © 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

ID: 19694324