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Unveiling the Origin of Non-Thermal Emission in Galaxy Clusters through Advanced Numerical Simulations in the LOFAR Era

Final Report Summary - GIANT RADIO HALOS (Unveiling the Origin of Non-Thermal Emission in Galaxy Clusters through Advanced Numerical Simulations in the LOFAR Era.)

Radio observations prove the existence of relativistic particles and magnetic fields associated with the intra-cluster-medium (ICM) through the presence of extended synchrotron emission in the form of radio halos and peripheral relics. This observational evidence has fundamental implications on the physics of the ICM. Nonthermal components in galaxy clusters are indeed unique probes of very energetic processes operating within clusters that drain gravitational and electromagnetic energy into cosmic rays (CRs) and magnetic fields. These components strongly affect the (micro-)physical properties of the ICM, including viscosity and electrical conductivities, and have also potential consequences on the evolution of clusters themselves. The nature and properties of CRs in galaxy clusters, including the origin of the observed radio emission on cluster-scales, have triggered an active theoretical debate in the last decade (Brunetti & Jones 2014 for a recent review).
The project is focussed on one of the most promising approaches to explain the origin of relativistic particles (CRe) responsible for giant radio halos. In this model, CRe are accelerated by ICM turbulence injected intermittently by major mergers into the halo. In the last decade the model has been pioneered by researchers at the hosting institute (Brunetti et al 2001, Cassano & Brunetti 2005, Brunetti & Lazarian 2007) and became the reference scenario in this field.
In the project we proposed for the first time to calculate in direct numerical simulations the reacceleration of CRe by merger-induced turbulence and their synchrotron emission in the radio band. A numerical approach provides a unique way to model the process of CRe acceleration during cluster mergers and the resulting complexity of the properties of the cluster-scale radio sources. It also offers a unique way to model the connection between thermal and non-thermal properties in galaxy clusters. On the other hand these numerical studies are extremely challenging because of the difficulties to measure turbulence in numerical simulations of galaxy clusters and of the complexity of the interaction between CRs and turbulence.

Specifically the project aimed at ambitious and innovative goals :
(1) IMPLEMENTATION OF TURBULENT CR ACCELERATION IN SIMULATIONS OF GALAXY CLUSTERS.
In particular, the main goal of the researcher was to implement in the MHD code GADGET the theoretical understanding of turbulence and turbulent acceleration of CRs. The goal was to achieve the implementation of this physics in the cosmological context. The main motivation was to open the possibility to model self-consistently the generation and evolution of radio halos (and radio relics) in the cosmological framework, both in binary simulations of cluster mergers and in full cosmological simulations.
(2) PREDICTION OF NON-THERMAL EMISSION IN GALAXY CLUSTERS.
The second goal of the project was to simulate samples of galaxy clusters with the novel implementation of CR physics and follow the evolution of non-thermal emission, including the radio and high-energy emission. The motivation was to explore the connection between thermal and non-thermal properties of galaxy clusters using the reference framework for CRs acceleration in these systems. This goal was considered particularly useful for the interpretation of the data from the next generation of radio telescopes (eg., LOFAR).
To achieve these goals we had to make innovations in two pivoting areas :
(i) NOVEL FORMULATIONS TO BETTER MODEL TURBULENCE IN SPH SIMULATIONS
During the project we found that the standard SPH simulations do not form a turbulent cascade over a wide range of scales and are hence not suitable for our research. This required us to significantly innovate on the subject testing a large variety of alternative SPH formulations and improving the existing schemes.
(ii) NOVEL NUMERICAL SCHEMES TO EFFICIENTLY SOLVE THE EQUATIONS FOR CR ACCELERATION AND EVOLUTION
The non-linear evolution of the spectrum of CRs that are accelerated by turbulence is complex and can be described by Fokker-Planck equations, which in general have to be solved numerically. A milestone of the project was the successfull implementation of a fast and numerically stable code to compute the evolution of a CR spectrum coupled to turbulence and subject to the most relevant energy losses. During the project we also discover that memory consumption is a challenge in the simulation of this kind of physics. Consequently to ease the memory demand in very large simulations we implement a succesful method to compress CR spectra.

The main goals of the project have been fully achieved pioneering the field and paving the way to more detailed and self-consistent explorations of the subject through a next generation of advanced numerical, "PByte-scale", simulations.
In Donnert et al. (2013) and Donnert & Brunetti (2014) we obtained the first simulations of turbulent acceleration of relativistic particles in galaxy clusters (FIGURE 1). Simulations confirmed the basic theoretical expectations, showing that Mpc-scale radio emission, consistent with giant radio halos, can be generated during clusters-clusters mergers and fade away when the hosting clusters become dynamically relaxed.
In addition these simulations allowed to explore the connection between non-thermal and thermal properties of galaxy clusters providing novel expectations for the spectral properties of radio halos and their time-evolution with clusters dynamics. These studies started to address a subject that is of great interest for the incoming radio observations with Low Frequency Array (LOFAR) and with SKA precursors and pathfinders. In particular, the European scientific community (NL, Germany, UK, France, Sweeden, Poland) has envisioned and built LOFAR to explore the Universe at low radio frequencies. LOFAR will soon provide a transformational new view on diffuse Mpc-scale radio emission in galaxy clusters. The achievements of our project now open the possibility to massive and self-consistent simulations in this field that will be vital for a full exploitation and interpretation of LOFAR data.

The advances obtained during the project into several areas (1-2, and i-ii) activated an intensive network of interational collaborations to extend our results through a next generation of advanced numerical simulations with a great benefit for several communities active in radioastronomy and cosmology, and putting the researcher in a strong position to develop a bright career in astrophysics.

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