Our knowledge of the magnetic field properties in galaxy clusters has significantly improved in recent years, owing to the improved capabilities of radio and X-ray telescopes. It is well established that µG level magnetic fields are widespread in the ICM, regardless of the presence of large-scale diffuse radio emission. The magnetic field strengths show almost an order of magnitude scatter between clusters, or within a given cluster, and are extreme in cluster cooling cores. For such large fields the magnetic pressure is comparable to or larger than the gas pressure derived from X-ray data, suggesting that magnetic fields may play a significant role in the cluster dynamics.
The observations are often interpreted in terms of the simplest possible model, i.e. a constant field throughout the whole cluster. However, a decline with radius is expected if the intensity of the magnetic field results from the compression of the thermal plasma during the cluster gravitational collapse. Observational evidence of magnetic field profiles has been derived in some clusters. Moreover the magnetic field could show complex structure with a range of coherence scales.
The study of cluster magnetic fields has gained a big interest in recent years, leading to several new observations as well as simulations. There are, however, still many questions to answer: are the fields filamentary, what are the coherence scales, to what extent do the thermal and non-thermal plasmas mix in cluster atmospheres, how do the fields extend, what is the radial trend of the field strength, how does the field strength depend on cluster parameters such as the gas temperature, metallicity, mass, substructure and density profile, how do the fields evolve with cosmic time, and finally how were the fields generated?
New generation instruments in the radio band, as the EVLA, LOFAR and SKA, are rather promising and will establish a clear connection between radio astronomical techniques and the improvement in the knowledge of the X-ray sky. There are various satellite missions, as ASTRO-E2, XEUS and Constellation X, which will map the X-ray sky at low and high energies in the next years. These will provide a more precise knowledge of the X-ray surface brightness of clusters, i.e. of their thermal gas density, allowing a more accurate and correct interpretation of the sensitive RM measurements. The detection of HXR non-thermal emission will provide independent measurements of the magnetic fields. The accurate experimental determination of large-scale magnetic fields in the intracluster medium will thus be possible. The detection of synchrotron radiation at the lowest possible levels will allow the measurement of magnetic fields in even more rarefied regions of the intergalactic space, and the investigation of the relation between the formation of magnetic fields and the formation of the large-scale structure in the universe.
We are grateful to Rainer Beck, Gianfranco Brunetti, Tracy Clarke, Klaus Dolag, Torsten Enßlin, Roberto Fanti, Roberto Fusco-Femiano, Gabriele Giovannini, Dan Harris, Melanie Johnston-Hollitt, Phil Kronberg, Matteo Murgia, Greg Taylor, and Corina Vogt for several fruitful discussions on this topic, and for suggestions. We are indebted to Klaus Dolag for supplying Fig. 9.