LOFAR observations of SZ-selected galaxy clusters, and high-fidelity imaging

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Giant radio halos in galaxy clusters provide the best evidence for the existence of a relativistic plasma (particles and magnetic fields) in the intergalactic medium (IGM). The nature of these Mpc-sized, steep spectrum synchrotron sources that are not clearly associated with individual galaxies, is not yet known. In this project, we pursue a two-pronged approach.In the first sub-project, we aim to investigate whether such halos are a common phenomenon in galaxy clusters and whether their radio power is related to their mass. The predominant baryonic component, namely the hot gas, can be traced both, in X-rays as well as via the Sunyaev-Zel'dovich (SZ) effect. The X-ray emission of clusters is mapped in various surveys conducted with XMM and Chandra and will also be a prime goal of the upcoming eRosita mission. The SZ signal has been measured for a large cluster sample and compiled in the Planck All-Sky Cluster Catalog. A large sample of galaxy clusters will be observed in the framework of the Tier 1 Survey, a complete survey of the northern sky to be conducted with the new LOFAR radio telescope. This systematic low-frequency survey of galaxy clusters at the lowest radio frequencies will remedy what has been a major obstacle in comparing the thermal and non-thermal components in galaxy clusters: the lack of a homogeneous low-frequency data set that can map the population of non-thermal electrons in a large sample of clusters to unprecedented depth. The radio maps can be compared directly to the X-ray and SZ data that yield the pressures, densities and temperatures of the thermal component. Finally, the technique of three-dimensional "Faraday Synthesis", developed in the framework of our Research Unit (Project A 8), will help to investigate the magnetic-field structure in cluster radio halos and shed light of their low degrees of linear polarization. The thus measured relationship between the radio emission, the magnetic field structure as well as the density distribution of the thermal gas, will give essential clues to the origin of radio halos.In a second sub-project, we strive to establish tools that will finally facilitate the discrimination between many of the models for cluster halos and relics, which is hampered by the many observational artefacts of radio maps. The aim is to produce high-fidelity radio maps of a number of prominent galaxy clusters using existing archival data and complement them with any new data from LOFAR (and other telescopes). These cutting-edge quality maps are to be combined into intensity, spectral index and spectral curvature maps using multi-frequency imaging methods. In addition, we will process polarimetric data of the selected galaxy clusters, making use of novel methods: RM Synthesis yields spectral data cubes containing three-dimensional information about the magnetic field, and Faraday Synthesis, which combines Aperture and RM Synthesis in a single three dimensional Faraday cube reconstruction step. With these tools at hand, one can overcome Faraday depolarization effects which erase the intrinsic polarization of the synchrotron emission of cluster radio haloes.