Galactic Dynamos and Galactic Winds

TL;DR

This paper reviews magnetic fields in spiral galaxies, their generation by dynamos, and their role in galactic winds and halos, highlighting current observations and future prospects with low-frequency radio telescopes.

Contribution

It provides a comprehensive overview of galactic magnetic fields, dynamo mechanisms, and the impact of galactic winds, emphasizing observational evidence and future research directions.

Findings

Regular magnetic fields are strongest in interarm regions.

Large-scale coherent fields are signatures of mean-field dynamos.

Radio halos trace galactic wind interactions and cosmic-ray transport.

Abstract

Spiral galaxies host dynamically important magnetic fields which can affect gas flows in the disks and halos. Total magnetic fields in spiral galaxies are strongest (up to 30 \muG) in the spiral arms where they are mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to 15 \muG). Faraday rotation measures of radio polarization vectors in the disks of several spiral galaxies reveal large-scale patterns which are signatures of coherent fields generated by a mean-field dynamo. -- Magnetic fields are also observed in radio halos around edge-on galaxies at heights of a few kpc above the disk. Cosmic-ray driven galactic winds transport gas and magnetic fields from the disk into the halo. The magnetic energy density is larger than the thermal energy density, but smaller than the kinetic energy density of the outflow. The orientation of field lines allows to estimate the wind speed and direction. There is no observation yet of a halo with a large-scale coherent dynamo pattern. A global wind outflow may prevent the operation of a dynamo in the halo. -- Halo regions with high degrees of radio polarization at very large distances from the disk are excellent tracers of interaction between galaxies or ram pressure of the intergalactic medium. The observed extent of radio halos is limited by energy losses of the cosmic-ray electrons. -- Future low-frequency radio telescopes like LOFAR and the SKA will allow to trace halo outflows and their interaction with the intergalactic medium to much larger distances.