A low-frequency radio halo associated with a cluster of galaxies

Abstract

The next generation of radio telescopes will enter what is virtually uncharted territory, observing the Universe at low radio frequencies with unprecedented resolution and sensitivity. A glimpse of what they might find is provided by the discovery of a new class of low frequency galaxy clusters. Deep observations with the Giant Metrewave Radio Telescope, located just north of Pune in India, reveal a radio halo at low frequencies associated with the merging cluster Abell 521. The halo has an extremely steep radio spectrum that makes it difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been studied). The spectrum supports a turbulent acceleration mechanism, which predicts that many radio haloes in the Universe should emit mainly at low frequencies. Clusters of galaxies are the largest gravitationally bound objects in the Universe. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant 'radio haloes'. A radio halo at low frequencies associated with the merging cluster Abell 521 is reported. This halo has an extremely steep radio spectrum that makes it difficult to detect with observations at 1.4 GHz. The spectrum supports a turbulent acceleration mechanism, which predicts that many radio haloes in the Universe should emit mainly at low frequencies. 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.