Abstract
Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 1015 solar masses of hot (108 K) gas, galaxies and dark matter in a typical volume of 10 Mpc3. 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 centre1,2. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence)3,4,5,6, or are injected (as secondary particles) by proton collisions into the intergalactic medium7,8,9,10. They are found only in a fraction of massive clusters that have complex dynamics11,12,13,14, 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.
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Acknowledgements
We acknowledge partial support from ASI-INAF I/088/06/0 and PRIN-INAF 2007. G.B. and R.C. acknowledge the Harvard-Smithsonian Center for Astrophysics, the US Naval Research Laboratory and the National Radio Astronomy Observatory for hospitality and partial support during the preparation of the manuscript. Basic research in radio astronomy at the Naval Research Laboratory is supported by 6.1 base funding. The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. We thank R. Athreya and the staff of the GMRT for their helping during the observations. The GMRT is run by the National Centre for Radio Astrophysics, Tata Institute of Fundamental Research.
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Brunetti, G., Giacintucci, S., Cassano, R. et al. A low-frequency radio halo associated with a cluster of galaxies. Nature 455, 944–947 (2008). https://doi.org/10.1038/nature07379
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DOI: https://doi.org/10.1038/nature07379
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