Intracluster magnetic fields

Magnetic fields are ubiquitous in the Universe. They have been detected in planets, stars, galaxies, groups and clusters of galaxies, and recently also along filaments, in the large scale structure of the Universe. The aim of this PhD thesis is to study magnetic fields in clusters of galaxies in different evolutionary stages. The presence of magnetic fields in the intracluster medium is clearly indicated by the discovery of diffuse synchrotron emission in the central region and in the outskirts of a larger and larger number of galaxy clusters. These sources, called radio halos and relics, are generated by high energy relativistic electrons moving along the flux lines of weak magnetic fields. These magnetic fields are deeply linked to the dynamical state and the energy transport in the system, and the study of their power spectrum gives information about the energy, the capacity to diffuse, and the ordered motion in the plasma of the relativistic particles. The origin of intracluster magnetic fields is still debated. The most reliable theories suppose a cosmological or a galactic (at z 2-3) origin. In both cases, to match the strengths presently observed, an amplification of the field is required, probably due to cluster formation processes. From the present day observations, intracluster magnetic fields appear to fluctuate over a range of scales and their strength seems to decrease with the distance from the cluster center as a function of the thermal gas density. Merging clusters show low magnetic field central strengths (∼1 μG), while relaxed systems are characterized by higher values (up to some 10 μG). In the radio band, the investigation of the intracluster magnetic field power spectrum can be performed by means of two techniques. Large scale (∼ Mpc) studies can be carried out through the analysis of the emission of large-scale diffuse synchrotron sources. Higher resolution investigations (∼ kpc) are allowed by the analysis of the Faraday rotation of the signal coming from galaxies within or behind galaxy clusters. The coexistence in a galaxy cluster of small and large scales of fluctuation of the magnetic field requires a combination of the two approaches. In fact, in this way, it is possible to follow with continuity the power spectrum of the magnetic field fluctuations from Mpc scales down to kpc scales. Up to now, a detailed study of the magnetic field power spectrum has been performed just in few galaxy clusters. In this work we present the results we obtained by applying these different approaches and their contri- bution to the present day knowledge of intracluster magnetic fields. To perform this kind of studies, we analyzed Very Large Array (VLA, New Mexico, USA) multi- frequency polarimetric data of diffuse radio halos and of discrete sources within or behind galaxy clusters. To constrain the magnetic field fluctuation power spectrum, we compared the observations with the expectations of two- and three-dimensional simulations obtained with the FARADAY software package. The structure of this manuscript is the following. Three main sections can be identified: in Chapter 1 we introduce clusters of galaxies, with particular care to radio emission originating from galaxies and diffuse synchrotron sources (radio halos, relics and mini-halos); in Chapters 2 and 3 we present the contribution of this thesis to the still scanty radio halo statistics and to the study of radio halo properties. We describe the discovery of six new radio halos and the investigation of the X-ray -radio correlation in systems hosting radio halos both on an individual and on a statistical point of view; in Chapter 4 we give an overview about intracluster magnetic fields and the main techniques applied for their investigation and we present statistical studies based on the Faraday rotation effect on the polarimetric properties of radio galaxies. The modeling we adopt and its application to the detailed investigation of the magnetic field in two galaxy clusters are described in Chapters 5, 6, and 7. Finally in Chapter 8 we summarize our conclusions. Throughout this thesis we adopt a ΛCDM cosmology with H0 = 71 km s−1 Mpc−1 , Ωm = 0.27, and ΩΛ = 0.73.