Letter to the Editor

Detection of magnetic fields in the circumgalactic medium of nearby galaxies using Faraday rotation

¹ Hamburg University, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: volker.heesen@hs.uni-hamburg.de
² School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin D09 W6Y4, Ireland
³ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁴ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁶ INAF, Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁷ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁸ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
⁹ Laboratoire de Physique École Normale Supérieure (LPENS), 24 rue Lhomond, 75005 Paris, France
¹⁰ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany
¹¹ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
¹² CSIRO Space and Astronomy, PO Box 1130 Bentley, WA 6102, Australia
¹³ Dept of Space, Earth and Environoment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden

Received: 27 January 2023
Accepted: 10 February 2023

Abstract

Context. The existence of magnetic fields in the circumgalactic medium (CGM) is largely unconstrained. Their detection is important as magnetic fields can have a significant impact on the evolution of the CGM, and, in turn, the fields can serve as tracers for dynamical processes in the CGM.

Aims. Using the Faraday rotation of polarised background sources, we aim to detect a possible excess of the rotation measure in the surrounding area of nearby galaxies.

Methods. We used 2461 residual rotation measures (RRMs) observed with the LOw Frequency ARray (LOFAR), where the foreground contribution from the Milky Way is subtracted. The RRMs were then studied around a subset of 183 nearby galaxies that was selected by apparent B-band magnitude.

Results. We find that, in general, the RRMs show no significant excess for small impact parameters (i.e., the perpendicular distance to the line of sight). However, if we only consider galaxies at higher inclination angles and sightlines that pass close to the minor axis of the galaxies, we find significant excess at impact parameters of less than 100 kpc. The excess in |RRM| is 3.7 rad m⁻² with an uncertainty between ±0.9 rad m⁻² and ±1.3 rad m⁻² depending on the statistical properties of the background (2.8σ–4.1σ). With electron densities of ∼10⁻⁴ cm⁻³, this suggests magnetic field strengths of a few tenths of a microgauss.

Conclusions. Our results suggest a slow decrease in the magnetic field strength with distance from the galactic disc, as expected if the CGM is magnetised by galactic winds and outflows.