Detection of microgauss coherent magnetic fields in a galaxy five
  billion years ago
  Magnetic fields play a pivotal role in the physics of interstellar medium in
galaxies, but there are few observational constraints on how they evolve across
cosmic time. Spatially resolved synchrotron polarization maps at radio
wavelengths reveal well-ordered large-scale magnetic fields in nearby galaxies
that are believed to grow from a seed field via a dynamo effect. To directly
test and characterize this theory requires magnetic field strength and geometry
measurements in cosmologically distant galaxies, which are challenging to
obtain due to the limited sensitivity and angular resolution of current radio
telescopes. Here, we report the cleanest measurements yet of magnetic fields in
a galaxy beyond the local volume, free of the systematics traditional
techniques would encounter. By exploiting the scenario where the polarized
radio emission from a background source is gravitationally lensed by a
foreground galaxy at z = 0.439 using broadband radio polarization data, we
detected coherent $\mu$G magnetic fields in the lensing disk galaxy as seen 4.6
Gyrs ago, with similar strength and geometry to local volume galaxies. This is
the highest redshift galaxy whose observed coherent magnetic field property is
compatible with a mean-field dynamo origin.
