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

TL;DR

This study measures magnetic fields in a spiral galaxy at redshift 0.414 using gravitational lensing and Faraday rotation, providing insights into galaxy magnetic field evolution and dynamo timescales.

Contribution

First observational constraint on magnetic field strength and dynamo timescale in a galaxy at z=0.414 using gravitational lensing and RM synthesis.

Findings

Detected Faraday dispersion indicating magnetic fields in the galaxy halo.

Estimated coherent magnetic field strength of 0.2-3.0 μG at 0.7 kpc.

Derived an upper limit on the dynamo e-folding time of 2.9×10^8 years.

Abstract

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 aim to provide an observational constraint on the time-scale of the mean-field dynamo, and derive the magnetic field in a distant galaxy at $z=0.414$. We obtained broadband spectro-polarimetric $1-8$ GHz Very Large Array observation of the lensing system B1600+434, which is a background quasar gravitationally lensed by a foreground spiral galaxy into two images. We apply Rotation Measure (RM) synthesis and Stokes $QU$ fitting to derive the RM of the two lensed images, which we use to estimate the lensing galaxy's magnetic field. 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 measure a coherent magnetic field with a strength of $0.2 - 3.0\,\mu$G at 0.7 kpc, and $0.01 - 2.8 \,\mu$G at 6.2 kpc vertical distance from the disk of the galaxy. We derive an upper limit on the dynamo e-folding time: $\tau_{\rm dynamo} < 2.9~\times 10^8$~yr. We find turbulence on scales below 50 pc, and a turbulent field strength of $0.2 - 12.1 \, \mu$G. We measure 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 $z \simeq 0.4$ already have magnetic field strengths similar to present-day galaxies. However, we note the caveat of the possibility of the turbulent field of the lensing galaxy contributing to the observed RM difference.