Experience indicates that most of the matter in the Universe is composed of ionized or partially ionized gas permeated by magnetic fields. Celestial objects are magnetized and magnetic fields of significant strength are found everywhere in the interstellar space, and over small and very large scales, in the extragalactic universe. In general, small compact objects have the largest magnetic field strengths, while larger low-density objects have weaker magnetic fields.
The Earth has a bipolar field of about 0.5 G at its surface, originating from an idealized current due to the charged fluid motion going circularly in a ring inside the liquid molten metallic core. On the Jupiter surface the magnetic field is about 4 G, owing to the fast Jupiter rotation. In the interplanetary space of the solar system the magnetic fields are of the order of 50 µG.
On the Sun, the magnetic field is of 10 G at the poles, while localized sunspots on the surface near the equatorial zone of the Sun, and more generally of a star, can have magnetic field strengths of 2000 G. In protostellar envelopes and protostars, fields are of ~ 1 mG. A bipolar field is "frozen" into the gas of a star during the contraction from a normal star to a degenerate star. It will remain bipolar-shaped but its intensity will increase as r-2, thus magnetic fields of pulsars and neutron stars are of the order of 1012 G, those of white dwarfs are around 106 G.
A widespread field of ~ 5 µG, characterized by a spiral shape, is present in the Galaxy. At the Galaxy nucleus, highly organized filaments with strength of ~ 1 mG are detected. Fields in other spiral galaxies are of ~ 10 µG on average, with values up to ~ 50 µG in starburst galaxies and ~ 30 µG in massive spiral arms.
Fields of ~ µG are found in the radio emitting lobes of radio galaxies. Fields of similar or weaker strength are detected in the intracluster medium of clusters of galaxies, and in more rarefied regions of the intergalactic space. Upper limits of 10-8 - 10-9 G have been obtained for the cosmological fields at large redshift.
In this review large-scale magnetic fields in clusters of galaxies will be analyzed. In the last years the presence of cluster magnetic fields has been unambiguously proven and the importance of their role has been recently recognized. The study of cluster magnetic fields is relevant to understand the physical conditions and energetics of the intracluster medium. Cluster magnetic fields provide an additional term of pressure and may play a role in the cluster dynamics. They couple cosmic ray particles to the intracluster gas, and they are able to inhibit transport processes like heat conduction, spatial mixing of gas, and propagation of cosmic rays. They are essential for the acceleration of cosmic rays and allow cosmic ray electron population to be observed by the synchrotron radiation.
Despite many observational efforts to measure their properties, our knowledge on cluster magnetic fields is still poor. Overviews on observational and theoretical arguments can be found in the literature [1, 2, 3, 4, 5, 6].
The focus of this review is primarily observational, however, we present the basic theory needed for the interpretation of the data. We analyze some of the main issues that have led to our knowledge on magnetic fields in clusters of galaxies and discuss some of their limitations. An outline of the review is as follows: In Sec. 2 we summarize some general properties of clusters of galaxies. Sec. 3 is devoted to theoretical background related to the detection of cluster magnetic fields and to the estimate of their strengths. We recall the basic theory concerning synchrotron radiation, inverse Compton radiation and Faraday rotation. These are the main observed features which provide information on the cluster magnetic fields. The observational results of cluster magnetic fields through synchrotron radio and inverse Compton hard X-ray emissions are described in Secs. 4 and 5. In Sec. 6 we give the results obtained by analyzing rotation measures of radio galaxies located within or behind clusters of galaxies. In Sec. 7 we present cluster magnetic fields detected through the study of cold fronts. In Sec. 8 we report the evidence for a radial decline of cluster magnetic fields. In Sec. 9 we discuss how magnetic field values obtained with different approaches can be reconciled. In Sec. 10 we summarize the results of a numerical technique which can significantly improve our interpretation of the data and thus the knowledge of the strength and structure of magnetic fields. In Sec. 11 we briefly review the current knowledge on the cluster magnetic field origin and amplification.
Throughout this paper we assume the CDM cosmology with H0 = 71 km s-1 Mpc-1, m = 0.3, and = 0.7, unless stated otherwise.