A low-frequency radio halo associated with a cluster of galaxies
A low-frequency radio halo associated with a cluster of galaxies
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DOI：
10.1038/nature07379
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发表时间：
2008-10-16
期刊：
影响因子：
64.8
通讯作者：
Markevitch, M.
中科院分区：
文献类型：
--
作者：
Brunetti, G.;Giacintucci, S.;Markevitch, M.
星系团是宇宙中最大的引力束缚天体，包含约 10(15) 个太阳质量的热 (10(8) K) 气体、星系和暗物质，典型体积为 10 Mpc(3)。磁场和相对论粒子与气体混合，正如巨大的“射电晕”所揭示的那样，这些射电晕是由星团中心弥散的兆秒差距同步加速器辐射产生的(1,2)。射电晕要求发射电子在原位加速（通过湍流）(3-6)，或通过质子碰撞注入（作为次级粒子）到星系际介质中(7-10)。它们仅存在于具有复杂动力学的一小部分大规模星团中(11-14)，这表明这些机制与星团合并之间存在联系。在这里，我们报告了与合并星团 Abell 521 相关的低频射电晕。该晕具有极其陡峭的射电频谱，这意味着高频截止；这使得光晕很难通过 1.4 GHz（所有其他已知无线电光晕得到最好研究的频率）下的观测来检测。光晕的光谱与相对论电子的次生起源不一致，而是支持湍流加速，这表明宇宙中的许多射电晕应该主要以低频发射。
Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10(15) solar masses of hot (10(8) K) gas, galaxies and dark matter in a typical volume of 10 Mpc(3). Magnetic fields and relativistic particles are mixed with the gas as revealed by giant 'radio haloes', which arise from diffuse, megaparsec- scale synchrotron radiation at cluster centre(1,2). Radio haloes require that the emitting electrons are accelerated in situ ( by turbulence)(3-6), or are injected ( as secondary particles) by proton collisions into the intergalactic medium(7-10). They are found only in a fraction of massive clusters that have complex dynamics(11-14), which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut- off; this makes the halo difficult to detect with observations at 1.4 GHz ( the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.