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

By Gianfranco Brunetti, S. Giacintucci, R. Cassano, Wendy Lane, D. Dallacasa, T. Venturi, N. Kassim, G. Setti, W. Cotton, M. Markevitch. Nature, 2008

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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 centre. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, 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.

Subfields: Aerospace Engineering, Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/6ac019a1da45252279e730de533587c16c077403

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Citations in the corpus

Shock waves in Eulerian cosmological simulations: main properties and acceleration of cosmic rays (2008, background): “Large-scale radio emission in the form of giant radio haloes may be powered by particle re-acceleration by magnetohydrodynamics (MHD) turbulence injected in the ICM during energetic merging events (Brunetti et al. 2001, 2004, 2008; Petrosian 2001).”
ON THE CONNECTION BETWEEN GIANT RADIO HALOS AND CLUSTER MERGERS (2010, background): “The discovery of RHs with very steep spectrum supports the scenario of particle reacceleration by merger-driven turbulence (e.g., Brunetti et al. 2008).”
The Very Large Array Low-frequency Sky Survey Redux (VLSSr) (2014, background): “Brunetti et al. 2008; Kothes et al. 2008; Argo et al. 2013).”
Diffuse Radio Emission from Galaxy Clusters (2019, background): “While indirect arguments against the hadronic model can be drawn from the integrated radio spectral (Brunetti et al. 2008) and spatial characteristics of halos, and from radio–X-ray scaling relations (for a review see Brunetti & Jones 2014), only gamma-ray observations, which will be discussed in…”
Massive and refined - II. The statistical properties of turbulent motions in massive galaxy clusters with high spatial resolution (2010, background): “…Sarazin 1999; Dolag & Ensslin 2000; Brunetti et al. 2001) Turbulence in the magnetized ICM may power large scale radio emission in galaxy clusters, provided that relativistic electrons can couple with the MHD modes excited during merger events (e.g. Brunetti et al. 2008 and references therein).”
Turbulent Motions and Shocks Waves in Galaxy Clusters simulated with AMR (2009, background): “In addition, the phenomenology of diffuse radio halo emission suggests a scenario in which turbulent MHD modes, excited during cluster mergers, may re-accelerate the relativistic emitting particles (e.g. Ferrari et al.2008; Brunetti et al.2008; Cassano 2009 and references therein).”
The LOFAR Two-metre Sky Survey. I. Survey description and preliminary data release (2016, background): “If it is a radio halo then it falls into the category of ultra-steep radio halos and these objects, of which only a few are known, are predicted by some models describing the origin of radio halos (see Cassano, Brunetti, & Setti 2006 and Brunetti et al. 2008).”
Science with a lunar low-frequency array: From the dark ages of the Universe to nearby exoplanets (2009, background): “hown that the steep spectra of cluster halos mean that observations at lower frequencies are much more likely to detect the cluster halos, speciﬁcall y for low-mass and high-redshift objects. Indeed, Brunetti et al. (2008) report the detection of diﬀuse steep-spectrum emission from the cluster Abell”
Acceleration of primary and secondary particles in galaxy clusters by compressible MHD turbulence: from radio haloes to gamma-rays (2010, background): “…synchroton emission in clusters without radio halos constrain the energy density of relativistic protons in the central Mpc regions of galaxy clusters to less than a few percent of that of the IGM (Reimer et al 2004; Brunetti et al 2007, 2008; Aharonian et al 2009a,b; Ackermann et al 2010).”