CLUSTERS OF GALAXIES-THERMAL AND NON-THERMAL PHENOMENA

The IRA scientists involved in this research study the physical conditions in the Intra-Cluster Medium (ICM) focussing on the interaction between the thermal and non-thermal components and on their evolution with cosmic epochs. Several studies on clusters and cluster galaxies are not confined to the radio domain but they use a multiband approach which includes the X-ray and optical regimes since in each band a different component of these large systems is dominant.

Clusters of galaxies are the largest gravitationally bound structures in the Universe. They form through energetic merging of substructures (with energy release of order 10^(64) erg/s) and accretion of matter, and thus they carry important information on the evolution of large scale structures in the Universe.

Clusters of galaxies are complex systems, which host a number of different components, such as individual galaxies, hot intracluster gas detectable through X-ray thermal Bremsstrahlung emission, and invisible dark matter. A non-thermal component is also present, in the form of magnetic fields and relativistic electrons, as clear from the existence of diffuse large-scale synchrotron radio emission in an increasing number of clusters. These diffuse radio sources, known as mini-halos, halos and relics, are of paramount importance in the study of galaxy clusters, as they are signatures of the cluster formation history. The IRA has been a leader for several years now in the study of the properties of halos and relics in the radio domain and in combined radio/ X-ray analysis.

The key studies in this area include: a) statistical investigations on the occurrence of radio halos, relics and mini-halos by means of deep radio observations of complete samples selected from the most recent X-ray and Sunyaev-Zeldovich cluster catalogues; b) detailed studies of the diffuse emission, in the form of halos, relics and mini-halos in individual clusters, with particular interest in the topology of the magnetic field, and in the distribution of the spectral index; c) development of the theory on the origin of mini-halos, halos and relics. In particular: (i) the study of the physical processes responsible for the acceleration of relativistic particles in the intracluster medium: shock and turbulent acceleration, magnetic reconnection. (ii) the modeling of the statistical properties of radio halos in galaxy clusters, following their cosmological evolution in connection with the cluster formation process. (iii) the use of numerical simulation to study the way non-thermal components are generated and evolve with the cluster properties.

Beyond the diffuse sources, radio emission in galaxy clusters may be associated to individual galaxies, as a result of an active nucleus. The radio galaxies associated with the brightest cluster galaxies (BCG) are particularly important, as they allow the study of feedback processes between the AGN and the intracluster thermal surrounding medium. Key projects in this area include: a) studies of the radio emission of BCGs, on small and large scale; b) complementary studies of the dynamical state of galaxy clusters by means of the analysis of distorted radio galaxies (such as narrow- and wide-angle tail; c) statistical studies of the properties of radio loud AGN in different environments and over different cosmic times, such as the radio luminosity function.