Magnetic fields in the outskirts of galaxy clusters: Insights from radio relics

在某些情况下，星系团碰撞产生的冲击波会将电子加速到相对论速度，形成所谓的无线电遗迹。遗迹是具有陡峭光谱指数的极化(通常在10%到30%的水平)来源。它们有细长的形状，大小在100到1000 kpc之间，通常位于集群的外围。一些观测表明，遗迹具有相对较大的磁场，典型的值为几微克。这些磁场是通过几种方法得出的，主要是均分论点、硬X射线和射电发射的比较、光谱老化和遗迹的边缘宽度。在这里，我们建议结合LOFAR偏振观测和磁流体力学模拟来研究这些场的起源。因此，遗迹成为探索星系团外部区域的重要探测器。最近，随着一系列X射线和Sunyaev-Zel‘dovich(SZ)观测显示，在一些外围星系团区域出现了显著的非热压分量，这一领域引起了人们的极大关注。随着LOFAR有望发现许多100年的新无线电遗迹，我们可以史无前例地进入宇宙的这些未知区域。

Under some circumstances, shock waves produced by the collision of galaxy clusters accelerate electrons to relativistic velocities to form so-called radio relics. Relics are polarised (generally at a level of 10 to 30 %) sources with a steep spectral index. They have elongated shapes, with sizes between 100 and 1000 kpc, and are usually located at the cluster periphery. Some observations suggest that relics host relatively large magnetic fields, with typical values of a few µG. These fields are derived using several methods, mostly equipartition arguments, comparison of hard X-ray and radio emission, the spectral aging and the width of relics seen edge-on. Here we propose a combination of LOFAR polarimetry observations with magneto-hydrodynamical simulations to investigate the origins of these fields. Relics thus become an important probe of the outer regions of galaxy clusters. This field has recently attracted a lot of attention as a flurry of X-ray and Sunyaev-Zel’dovich (SZ) observations have shown signatures of significant non-thermal pressure components in some peripheral cluster regions. With the expected discovery of many 100s new radio relics with LOFAR, we can thus get unprecedented access to these unknown areas of the Universe.