nOn-thermal Processes in gALaxy cluStErs – OPALES
Deep radio observations of the sky have revealed the presence of diffuse radio sources in about 10% of known galaxy clusters. Their emission is not associated to single galaxies, but to the presence of intracluster relativistic particles and magnetic fields. One of the main open questions of modern astrophysics is to understand how this non-thermal intracluster component originates and if it affects the thermo-dynamical evolution of galaxy clusters. A deep understanding of the complex evolutionary physics of these systems is indeed essential if we want to exploit them as cosmological tools through ongoing and upcoming multiwavelength surveys. For this, we need a detailed knowledge of the nature of all the different cluster components (galaxies, thermal and non-thermal intracluster medium - ICM) and of their mutual interactions. After the huge progress in the last 15 years of our knowledge of galaxy and thermal ICM physics, we are now living in the 'golden age' for non-thermal cluster studies: the opening of the few spectral windows largely unexplored by astronomical observations (i.e. the low-frequency radio band, as well as hard X-rays and Gamma-rays) will allow to study the non-thermal physics of galaxy clusters with unprecedented statistics and thoroughness. The main aim of this project is to test the different theoretical models about the origin of the non-thermal intracluster component and to analyze its effects on the evolutionary physics of galaxy clusters. It is firstly strongly debated how the observed cosmic-rays are accelerated. Two main classes of models have been proposed: primary and secondary models. The former predict that cluster mergers (i.e. the process by which clusters form and evolve) power the mechanisms responsible for the acceleration of intracluster electrons. The latter propose that relativistic electrons are the secondary product of hadronic collisions between relativistic protons and ICM ions. Even more debated are the intracluster magnetic field origin and the effects of the non-thermal intracluster component on the transport processes and pressure of the ICM. At present we lack the observational means to test these different theories: the low surface brightness and steep power-law spectra of diffuse cluster radio sources make them more easily detectable at low-frequencies and difficult to be imaged accurately with current instruments, mainly operating in the GHz regime. Actually, only recent developments in technology and computing power are allowing observations of the low frequency window of the electromagnetic spectrum through a new generation of radio telescopes that are the technical and scientific pathfinders of one of the most important future international project: the Square Kilometre Array (SKA). Among the different SKA pathfinders, two have been projected to have the analysis of diffuse emission in clusters among the science drivers, and thus are most adapted for these kind of studies: the Low Frequency Array (LOFAR, in Europe) and the Long Wavelength Array (LWA, in the U.S.). These instruments will allow to map diffuse radio emission in clusters through deep and extended surveys. In this work we will exploit the capabilities of modern low frequency radio telescopes, in particular of the European project LOFAR. Radio data will be combined with complementary observations (in the X-ray and possible Gamma-ray bands), extremely important to fully characterize the non-thermal physics of galaxy clusters. With this project C. Ferrari, expert in multi-wavelength observations of galaxy clusters and member of the science team of the LOFAR 'Surveys' Key Project, will open a new field of research at the Côte d'Azur Observatory (OCA), where she has just got a permanent position. On the other side, the researchers involved in this project will provide all the complementary competences necessary to this work: C. Benoist and S. Maurogordato have been studying since more than ten years the large scale structures of the Universe, T. Passot and D. Laveder have a deep theoretical knowledge of magneto-hydrodynamic processes, E. Slezak and A. Bijaoui are specialist in image processing, fundamental expertise to develop non-existent tools for diffuse radio sources detection in all-sky survey radio maps. Actually, the successful development of our project strongly depends on the proper physical characterization of statistical samples of diffuse cluster radio sources. A consistent part of the project will thus be devoted to the development of tools for an efficient and automatic detection and spectral characterization of diffuse radio sources in LOFAR multi-frequency maps.
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
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