Annu. Rev. Astron. Astrophys. 2002. 40: 319-348 Copyright © 2002 by Annual Reviews. All rights reserved

1. INTRODUCTION

Magnetic fields play an important role in virtually all astrophysical phenomena. Close to home, the Earth has a bipolar magnetic field with a strength of 0.3 G at the equator and 0.6 G at the poles. This field is thought to originate in a dynamo due to fluid motions within the liquid core (Soward 1983). With its faster angular rotation, Jupiter leads the planets with an equatorial field strength of ~ 4 G (1963Warwick , Smith et al. 1974). A similar mechanism produces the solar magnetic fields which give rise to spectacular sunspots, arches, and flares (Parker 1979). Within the interstellar medium, magnetic fields are thought to regulate star formation via the ambipolar diffusion mechanism (Spitzer 1978). Our own Galaxy has a typical interstellar magnetic field strength of ~ 2 µG in a regular ordered component on kiloparsec scales, and a similar value in a smaller-scale, random component (Beck et al. 1996, Kulsrud 1999). Other spiral galaxies have been estimated to have magnetic field strengths of 5 to 10 µG, with fields strengths up to 50µG found in starburst galaxy nuclei (Beck et al. 1996). Magnetic fields are fundamental to the observed properties of jets and lobes in radio galaxies (Bridle & Perley 1984), and may be primary elements in the generation of relativistic outflows from accreting, massive black holes (Begelman, Blandford, & Rees 1984). Assuming equipartition conditions apply, magnetic field strengths range from a few µG in kpc-scale extended radio lobes, to mG in pc-scale jets.

The newest area of study of cosmic magnetic fields is on larger scales still, that of clusters of galaxies. Galaxy clusters are the largest virialized structures in the universe. The first spatially resolving X-ray observations of clusters (Forman et al. 1972) revealed atmospheres of hot gas (10⁷ to 10⁸ K) which extend to Mpc radii and which dominate the baryonic mass of the systems (10¹³ to 10¹⁴ M). Soon thereafter came the first attempts to measure magnetic field strengths in the intracluster medium (ICM) (Jaffe 1977). Only in the last decade has it become clear that magnetic fields are ubiquitous in cluster atmospheres, certainly playing a critical role in determining the energy balance in cluster gas through their effect on heat conduction, and in some cases perhaps even becoming important dynamically.

Cluster magnetic fields have been treated as secondary topics in reviews of cluster atmospheres (Sarazin 1988, Fabian 1994), and in general reviews of cosmic magnetic fields (Kronberg 1996, Ruzmaikin, Shukurov, & Sokolov 1987). To date there has been no dedicated review on cluster magnetic fields.

The focus of this review is primarily observational. We summarize and critique various methods used for measuring cluster magnetic fields. In the course of the review we consider important effects of magnetic fields in clusters, such as their effect on heat conduction and gas dynamics, and other issues such as the lifetimes of relativistic particles in the ICM. We then attempt to synthesize the various measurements and develop a general picture for cluster magnetic fields, with the caveat that there may be significant differences between clusters, and even within a given cluster atmosphere. We conclude with a section on the possible origin of cluster magnetic fields.

We assume H₀ = 75 km s^-1 Mpc^-1 and q₀ = 0.5, unless stated otherwise. Spectral index, , is defined as S .