标题： 合并星系团MACS J0717.5+3745中的弥散射电辐射：最强大的射电晕的发现 显示英文标题

标题： Diffuse radio emission in the merging cluster MACS J0717.5+3745: the discovery of the most powerful radio halo

摘要： 分层结构形成模型预测，星系团通过较小星系团和星系群的合并以及持续吸积气体而增长。MACS J0717.5+3745是一个X射线明亮且复杂的合并星系团，位于红移0.55处。在这里，我们展示了使用巨型米波射电望远镜（GMRT）在610 MHz下对该星系团的射电观测。观测的主要目的是搜索与正在进行的合并相关的星系团MACS J0717.5+3745内的弥散射电辐射。这些GMRT观测结果由甚大阵列（VLA）存档观测结果补充，观测频率为1.4、4.9和8.5 GHz。我们在这个星系团MACS J0717.5+3745中发现了一个尺寸约为1.2 Mpc的射电晕。射电功率P_1.4 GHz为5 x 10^25 W/Hz，这使其成为目前已知最强大的射电晕。一个700 kpc的射电结构，我们将其归类为射电遗迹，位于该系统的合并子结构之间。该遗迹的位置大致与钱德拉探测到的星系团内介质（ICM）中温度显著增强的区域重合。该遗迹的主要轴也大致垂直于合并轴。这表明该遗迹可能是与合并相关的冲击波的结果，其中粒子通过弥漫冲击加速（DSA）机制被加速。或者，该遗迹可能追踪了向西南方向的大尺度星系丝状结构的吸积冲击。在4.9 GHz和610 MHz之间，星系团内射电辐射的全局光谱指数为-1.24 +/-0.05。我们在射电晕位置推导出等离子体磁场强度为5.8微高斯。[简写]。 显示英文摘要

摘要： Hierarchical models of structure formation predict that galaxy clusters grow via mergers of smaller clusters and galaxy groups, as well as through continuous accretion of gas. MACS J0717.5+3745 is an X-ray luminous and complex merging cluster, located at a redshift of 0.55. Here we present Giant Metrewave Radio Telescope (GMRT) radio observations at 610 MHz of this cluster. The main aim of the observations is to search for diffuse radio emission within the galaxy cluster MACS J0717.5+3745 related to the ongoing merger. These GMRT observations are complemented by Very Large Array (VLA) archival observations at 1.4, 4.9 and 8.5 GHz. We have discovered a radio halo in the cluster MACS J0717.5+3745 with a size of about 1.2 Mpc. The radio power P_1.4 GHz is 5 x 10^25 W/Hz, which makes it the most powerful radio halo known till date. A 700 kpc radio structure, which we classify as a radio relic, is located in between the merging substructures of the system. The location of this relic roughly coincides with regions of the intra-cluster medium (ICM) that have a significant enhancement in temperature as shown by Chandra. The major axis of the relic is also roughly perpendicular to the merger axis. This shows that the relic might be the result of a merger-related shock wave, where particles are accelerated via the diffuse shock acceleration (DSA) mechanism. Alternatively, the relic might trace an accretion shock of a large-scale galaxy filament to the south-west. The global spectral index of radio emission within the cluster is found to be -1.24 +/-0.05 between 4.9 GHz and 610 MHz. We derive a value of 5.8 microGauss for the equipartition magnetic field strength at the location of the radio halo. [abridged].