Magnetic fields in nearby galaxies: Prospects with future radio telescopes

Abstract

The origin of magnetic fields in the Universe is an open problem in astrophysics and fundamental physics. Our present-day knowledge is limited to regions of strong magnetic fields and to starforming disks of galaxies. Cosmic-ray electrons emitting at high (GHz) radio frequencies can propagate only a few kpc from their places of origin due to their lifetime limited by energy losses. Low-energy electrons emitting at low frequencies suffer less from energy losses and can propagate further into the intergalactic medium. The prospects are threefold: Firstly, LOFAR will map the structure of weak magnetic fields in the outer regions and halos of galaxies and in the Milky Way. Polarized emission is an excellent tracer of past interactions with other galaxies and with the interstellar medium. Secondly, high-resolution polarization observations are needed at high frequencies with the EVLA and SKA to trace the structure of magnetic fields in the disks and central regions of galaxies in unprecedented detail. The SKA can also detect polarized emission from distant, unresolved galaxies. Thirdly, Faraday rotation measures (RM) are signatures of regular magnetic fields generated by the dynamo mechanism. All-sky surveys of Faraday rotation measures (RM) towards polarized background sources will be used to model the structure and strength of the regular magnetic fields in the Milky Way, the interstellar medium of galaxies and the intergalactic medium. The novel method of RM Synthesis, applied to spectro-polarimetric data cubes, is able to separate RM components from different distances and may allow 3-D Faraday tomography. This will open a new era in the observation of cosmic magnetic fields. “Key Science” Projects on cosmic magnetism are organized for the Low Frequency Array (LOFAR), the planned Square Kilometre Array (SKA) and the Australian SKA Pathfinder telescope (ASKAP).