The halo magnetic field of a spiral galaxy at

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: τdynamo < 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.