ARlogo Annu. Rev. Astron. Astrophys. 1996. 34: 155-206
Copyright © 1996 by . All rights reserved

Reprinted with kind permission from , 4139 El Camino Way, Palo Alto, California, USA

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GALACTIC MAGNETISM: Recent Developments and Perspectives

Rainer Beck


Max Planck Institute for Radioastronomy, Auf dem Hügel 69, Bonn, D-53121 Germany

Axel Brandenburg 1


Nordita, Blegdamsvej 17, Copenhagen Ø, DK-2100 Denmark

David Moss


Mathematics Department, The University, Manchester M13 9PL, United Kingdom

Anvar Shukurov


Computing Center, Moscow University, Moscow, 119899 Russia

Dmitry Sokoloff
Department of Physics, Moscow University, Moscow, 119899 Russia


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.


KEY WORDS: radio syncrotron emission, radio polarization, spiral arms, galactic halos, dynamos


Table of Contents

INTRODUCTION

INTERPRETATION OF RADIO OBSERVATIONS
Field Strength Estimates
Large-Scale Field Patterns
Small-Scale Field Structures
Comparison with Optical Polarization Data

MAGNETIC FIELDS IN SPIRAL GALAXIES
Field Strengths
M31: The Nearly Perfect Magnetic Torus
Density-Wave Galaxies
IC 342 and NGC 6946: Magnetic Spiral Arms
ASS, BSS, or What?
Magnetic Fields in Galactic Halos
Magnetic Fields in High-Redshift Galaxies
The Milky Way

GALACTIC DYNAMO THEORY
Random Magnetic Fields
Large-Scale Fields
Treatment of Galactic Turbulence
Basic Galactic Dynamo Models
The Quenching Problem

ORIGIN OF GALACTIC MAGNETIC FIELDS
Cosmological Magnetic Fields
The Primordial Origin of Galactic Magnetic Fields
The Dynamo Origin of Magnetic Field

EFFECTS OF THE DYNAMO ENVIRONMENT
Starbursts
Galactic Encounters
Parametric Resonance with Spiral Arms
Contrast Structures
The Influence of Magnetic Fields on the Galactic Disk

MAGNETIC FIELDS IN HALOS
In Situ Generation
Transport Out of the Disk

MAGNETIC FIELD MODELS
The Parity
Large-Scale Azimuthal Patterns
Spiral Field Lines and Pitch Angles
Spiral Arms and Magnetic Fields
Dynamo Models for Individual Galaxies

LAST WORDS

REFERENCES



1 Now at Department of Mathematics and Statistics, University of Newcastle upon Tyne, NE1 7RU, United Kingdom.

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