GALACTIC MAGNETISM: Recent Developments and Perspectives

TLDR

Although dynamo theory has its own problems, the general form of the dynamo equations appears quite robust. The paper reviews current observational and theoretical knowledge of galactic magnetic fields, focusing on large‑scale structure in disks and halos and on unresolved questions about field enhancement in interarm regions and magnetic spiral arms. The authors examine how global magnetic fields are enhanced in interarm regions and magnetic spiral arms, model them as superpositions of azimuthal modes, and present detailed models that include spiral structure, starbursts, galactic winds, and fountains, comparing them with observations. They argue that a turbulent hydromagnetic dynamo with an inverse cascade of magnetic energy best explains regular galactic magnetic fields, while primordial cosmological fields are inadequate as dynamo seeds, and they confront detailed generation models with observations.

Abstract

▪ Abstract We discuss current observational and theoretical knowledge of magnetic fields, especially the large-scale structure in the disks and halos of spiral galaxies. Among other topics, we consider the enhancement of global magnetic fields in the interarm regions, magnetic spiral arms, and representations as superpositions of azimuthal modes, emphasizing a number of unresolved questions. It is argued that a turbulent hydromagnetic dynamo of some kind and an inverse cascade of magnetic energy gives the most plausible explanation for the regular galactic magnetic fields. Primordial theory is found to be unsatisfactory, and fields of cosmological origin may not even be able to provide a seed field for a dynamo. Although dynamo theory has its own problems, the general form of the dynamo equations appears quite robust. Finally, detailed models of magnetic field generation in galaxies, allowing for factors such as spiral structure, starbursts, galactic winds, and fountains, are discussed and confronted with observations.