Detection of microgauss coherent magnetic fields in a galaxy five billion years ago

摘要

Magnetic fields play a pivotal role in the physics of interstellar medium ingalaxies, but there are few observational constraints on how they evolve acrosscosmic time. Spatially resolved synchrotron polarization maps at radiowavelengths reveal well-ordered large-scale magnetic fields in nearby galaxiesthat are believed to grow from a seed field via a dynamo effect. To directlytest and characterize this theory requires magnetic field strength and geometrymeasurements in cosmologically distant galaxies, which are challenging toobtain due to the limited sensitivity and angular resolution of current radiotelescopes. Here, we report the cleanest measurements yet of magnetic fields ina galaxy beyond the local volume, free of the systematics traditionaltechniques would encounter. By exploiting the scenario where the polarizedradio emission from a background source is gravitationally lensed by aforeground galaxy at z = 0.439 using broadband radio polarization data, wedetected coherent $\mu$G magnetic fields in the lensing disk galaxy as seen 4.6Gyrs ago, with similar strength and geometry to local volume galaxies. This isthe highest redshift galaxy whose observed coherent magnetic field property iscompatible with a mean-field dynamo origin.