The halo magnetic field of a spiral galaxy at z = 0.414

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
² Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
³ Thüringer Landessternwarte, Sternwarte 5, D-07778 Tautenburg, Germany
⁴ Research School of Astronomy & Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁵ Department of Astronomy and Astrophysics, University of California Santa Cruz, 1156 High St, Santa Cruz, 95064 CA, USA
⁶ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
⁷ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada

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

Received: 18 December 2024
Accepted: 14 July 2025

Abstract

Aims. Even though magnetic fields play an important role in galaxy evolution, the redshift evolution of galactic-scale magnetic fields is not well constrained observationally. In this paper we provide an observational constraint on the timescale of the mean-field dynamo, and derive the magnetic field in a distant galaxy at z = 0.414.

Methods. We obtained broadband spectro-polarimetric 1−8 GHz Very Large Array observations of the lensing system B1600+434, which is a background quasar gravitationally lensed by a foreground spiral galaxy into two images. We applied rotation measure (RM) synthesis and Stokes QU fitting to derive the RM of the two lensed images, which we used to estimate the lensing galaxy’s magnetic field.

Results. We measured the RM difference between the lensed images and detected Faraday dispersion caused by the magneto-ionic medium of the lensing galaxy at z = 0.414. Assuming that the RM difference is due to the large-scale regular field of the galaxy’s halo, we measured a coherent magnetic field with a strength of 0.2−3.0 μG at 0.7 kpc and 0.01−2.8 μG at 6.2 kpc vertical distance from the disk of the galaxy. We derive an upper limit on the dynamo e-folding time: τ_dynamo < 2.9 × 10⁸ yr. We find turbulence on scales below 50 pc and a turbulent field strength of 0.2−12.1 μG.

Conclusions. We measured the magnetic field in the halo of a spiral galaxy and find turbulence on scales of < 50 pc. If the RM difference is due to large-scale fields, our result follows the expectation from mean-field dynamo theory and shows that galaxies at z ≃ 0.4 already have magnetic field strengths similar to present-day galaxies. There is one caveat, however: we note the possibility of the turbulent field of the lensing galaxy contributing to the observed RM difference.