Published in "Rep. Prog. Phys.", 1994, Vol. 57, pp. 325-382.

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EXTRAGALACTIC MAGNETIC FIELDS

Philipp P Kronberg


Department of Astronomy, University of Toronto, 60 St. George Street, Toronto, Canada M5S IA7


Abstract: Recent advances in observational techniques reveal the widespread existence of magnetic fields in the Universe, and produce much firmer estimates of magnetic field strengths in interstellar and intergalactic space. Ordered, microgauss-level fields are common in spiral galaxy disks and halos, and appear to be a common property of the intra-cluster medium of clusters of galaxies, indeed well beyond the cluster core regions. Strengths of ordered magnetic fields in the intracluster medium of cooling flow clusters exceed those which are typical of the interstellar medium of the Milky Way, suggesting that galaxy formation, and even cluster dynamics are, at least in some circumstances influenced by magnetic forces, which also could possibly affect the global dynamics in areas of some galaxies, especially dwarf galaxies, which are rich in interstellar gas and cosmic rays.

Physical processes responsible for the regeneration of initial seed fields in galaxies, including mechanisms of magnetic diffusivity and dissipation which influence field amplification, are increasingly better, though far from completely, understood. We review the `conventional', slow mean field alpha - omega dynamo theory for disk galaxies, and more recent modifications to the theory. Fast-acting dynamo mechanisms appear to operate in galaxies, galaxy inflow and outflow, and in cooling flow clusters. Better understanding of the magnetic properties of extragalactic radio jets, including recent 3D numerical simulations, has shown how fast dynamo processes associated with the radio jet/lobe combination can effectivity magnetize large volumes of intergalactic space. Such processes, and starbust-driven outflow during the galaxy formation epoch, could have produced the microgauss level fields now commonly seen in galaxy systems - which would obviate the need for slow acting dynamos to build up field strength slowly over cosmic time.

The observational methods for detecting and measuring extragalactic magnetic fields are discussed, along with some new indirect methods which could be used for inferring field strengths at large redshifts which are otherwise beyond the reach of direct measurement.

Seed fields can be produced in battery-like processes in a variety of systems (stars, supernovae, and supernova winds), and are expelled into intergalactic space. Various cosmological seed field generation mechanisms are reviewed, which could generate seed fields for the subsequently formed galaxies. The question of whether the original seed fields were produced in galaxies, or the pre-recombination early Universe must await a clearer picture of how the first stars and galaxies formed, up to now, largely a `dark' era.

This review was received in its present form in September 1993


Table of Contents

INTRODUCTION
A little bit of history; how magnetic fields have been measured
The extent and implications of extragalactic magnetic fields

LARGE SCALE MAGNETIC FIELDS IN COSMOLOGICALLY NEARBY GALAXIES
Introduction
Observational overview of large scale magnetic fields in spiral galaxies
The physics of magnetic field amplification and evolution in spiral galaxies
How well does the galactic dynamo theory conform to the observations?
Where, and to what extent could magnetic forces compete with gravity in galaxy systems?

MAGNETIC FIELD OUTFLOW INTO THE ICM FROM GALAXY-GENERATED ACTIVITY
Evidence for the magnetic `seeding` of the intergalactic medium due to normal' activity of nearby galaxies
Injection of magnetic fields into the intergalactic medium by extragalactic radio source jets and lobes

MAGNETIC FIELDS ASSOCIATED WITH CLUSTERS OF GALAXIES
Faraday rotation and x-ray probes of galaxy cluster magnetic fields
Regeneration and amplification of magnetic fields in the intercluster medium
Low frequency radio emission as a tracer of extragalactic magnetic fields

THE DETECTION OF GALACTIC AND EXTRAGALACTIC MAGNETIC FIELDS AT EARLIER COSMOLOGICAL EPOCHS
Searches for a magnetic field in the widespread intergalactic medium up to the redshifts of quasars
Magnetic fields associated with absorption line system in quasars
Probes of cosmological magnetic field evolution using quasar Faraday rotation statistics
Indirect indicators of extragalactic magnetic fields
Seed field before the galaxy formation epoch

COSMOLOGICAL SEED FIELDS, OR GALAXY-GENERATED SEED FIELDS, OR BOTH?

CONCLUDING REMARKS

REFERENCES

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