Magnetic Fields in Galaxy Clusters

Magnetic fields in galaxy clusters can be investigated using a variety of techniques. Recent studies including radio halos, Inverse Compton hard X-ray emissions and Faraday rotation measure, are briefly outlined. A numerical approach for investigating cluster magnetic fields strength and structure is presented. It consists of producing simulated rotation measure, radio halo images, and radio halo polarization, obtained from 3-dimensional multi-scale cluster magnetic field models, and comparing with observations. 1. Magnetic Field Measures A complete description of the astrophysical processes in cluster of galaxies requires knowledge of magnetic fields. The most detailed evidence for this component comes from the radio observations: • Some clusters of galaxies exhibit diffuse non-thermal synchrotron radio halos, associated with the intra-cluster medium, which extend up to Mega-parsec scales. Using minimum energy assumptions, it is possible to estimate an equipartition magnetic field strength averaged over the entire halo volume. These estimates give equipartition magnetic field strengths of ≃0.1 to 1 μG (e.g. Bacchi et al. 2003). • In a few cases, clusters containing a radio halo show an hard X-ray excess emission. This emission could be interpreted in terms of Inverse Compton scattering of the cosmic microwave background photons with the relativistic electrons responsible for the radio halo emission. In this case, the measurements of the magnetic field strength (e.g. Fusco-Femiano et al. 1999, Rephaeli et al. 1999) inferred from the ratio of the radio to X-ray luminosities are consistent with the equipartition estimates. • Indirect measurements of the magnetic field intensity can also be determined in conjunction with X-ray observations of the hot gas, through the study of the Faraday Rotation Measure (RM) of radio sources located inside or behind clusters. By using a simple analytical approach, magnetic fields of ∼5− 30 μG, have been found in cooling flow clusters (e.g. Allen et al. 2001; Taylor & Perley 1993) where extremely high RMs have been revealed. On the other hand, significant magnetic fields have also been detected in clusters without cooling flows: