The origin of galactic magnetic fields

Abstract

Galactic magnetic fields and cosmic rays are tightly bound up together. The fact that the energy density of cosmic rays in the local region of the Galaxy is approximately the same order of magnitude as the energy density of the magnetic field has led many authors to postulate that this is a universal phenomenon, i.e., that there is equipartition. Gamma ray observations of the Magellanic Clouds lead us to doubt the presence of such equipartition there, however. Doubt is raised, therefore, about the application of equipartition, and a fixed electron to proton ratio, by radio astronomers to observations of synchrotron radiation from galaxies in general.A consequence of the lack of equipartition may be that in some galaxies the cosmic ray intensity is increasing (or decreasing). The positive link between the strength of galactic magnetic fields and the star formation rate implies that dynamo action may not be the only source of galactic magnetic fields.