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

星系团中的巨大射电晕为星系际介质（IGM）中存在相对论性等离子体（粒子和磁场）提供了最好的证据。这些mpc大小的陡谱同步加速器源的性质尚不清楚，它们与单个星系没有明显的联系。在这个项目中，我们采用了双管齐下的方法。在第一个子项目中，我们的目标是研究这种光晕是否是星系团中的常见现象，以及它们的无线电功率是否与它们的质量有关。主要的重子成分，即热气体，可以通过x射线和Sunyaev-Zel'dovich （SZ）效应来追踪。星团的x射线辐射在XMM和钱德拉进行的各种调查中被绘制出来，这也将是即将到来的eRosita任务的主要目标。SZ信号是在一个大的星团样本中测量的，并被收录在普朗克全天空星团目录中。在一级巡天（Tier 1 Survey）的框架内，将观察到大量的星系团样本。一级巡天是用新的LOFAR射电望远镜对北方天空进行的一次全面巡天。这种系统的以最低无线电频率对星系团进行的低频调查将弥补比较星系团中热和非热成分的主要障碍：缺乏均匀的低频数据集，可以将大量星系团中的非热电子数量映射到前所未有的深度。射电图可以直接与x射线和SZ数据进行比较，这些数据可以产生热成分的压力、密度和温度。最后，在我们的研究单元（a8项目）的框架下开发的三维“法拉第合成”技术将有助于研究星系团射电晕中的磁场结构，并揭示它们的低线偏振度。由此测量出的射电辐射、磁场结构以及热气体密度分布之间的关系，将为射电晕的起源提供重要线索。在第二个子项目中，我们努力建立工具，最终促进许多星团晕和遗迹模型之间的区分，这受到许多射电地图观测人工制品的阻碍。其目的是利用现有的档案数据制作一些突出的星系团的高保真无线电地图，并与来自LOFAR（和其他望远镜）的任何新数据进行补充。这些尖端的高质量地图将使用多频成像方法组合成强度、光谱指数和光谱曲率图。此外，我们将处理所选星系团的极化数据，使用新的方法：RM合成产生包含磁场三维信息的光谱数据立方体，而法拉第合成将孔径和RM合成结合在一个三维法拉第立方体重建步骤中。有了这些工具，就可以克服法拉第去极化效应，这种效应会消除星系团射电晕同步辐射的固有极化。

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.