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Final Activity Report Summary - GALAXY CLUSTER SAMPL (Multi-wavelength analysis of merging galaxy clusters)

The present project is a study of the complex physics of formation and evolution of galaxy clusters, whose knowledge is essential for cosmological theories.

Galaxy clusters are the only systems small enough to have achieved dynamical equilibrium during the Universe lifetime, and large enough to let us estimate the average ratio of baryonic to dark matter in the Universe. While dark matter, whose composition is still unknown, cannot be detected directly, baryons emit electromagnetic radiation at different wavelengths, depending on their nature. Galaxy clusters, in particular, can be observed at optical, X-ray and radio wavelengths due to the presence of galaxies, hot and diffuse gas that fill the intra-cluster medium (ICM), and a non-thermal component, i.e. magnetic fields and relativistic particles. Multi-wavelength observations of galaxy clusters reveal that more than 80% of their total mass is in dark matter, 16% is in gas, while stars and galaxies represent only the 3%. In the framework of the hierarchical model of structures formation, galaxy clusters are supposed to form by accretion and merging of smaller units. By studying lumpy clusters one has then the opportunity to understand the gravitational and non-gravitational processes acting during the formation of the large scale structures of the Universe.

In this picture, during my Marie Curie Fellowship I worked on an international project to study the formation of galaxy clusters and the complex physical processes that are driving their evolution. A joint observational and numerical approach was applied, which combined optical, X-ray and radio observations of a sample of galaxy clusters with N-body and hydrodynamic simulations of structures formation and evolution. The complex dynamics and morphologies of the clusters analysed are observational evidence of the hierarchical model of structure formation. We found evidences that the merging event can significantly affect the physical properties of the different cluster components.

In addition to its well-known effects on the velocity and density distribution of galaxies, and on the temperature and density distribution of the gas, our combined observational and numerical analysis showed that mergers seem to trigger star formation in galaxies, accelerate the relativistic particles observed at radio wavelengths, and strongly affect the presence and distributions of heavy elements in the ICM.

Our analysis also showed that the ICM metallicity distribution is a more powerful diagnostic to determine the dynamical state of clusters than the former methods developed and used up to now. This combined observational and numerical analysis has therefore given new insights to the understanding of the complex physics of merging clusters. New perspectives on the study of galaxy clusters have also been opened.

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