Clusters of galaxies: observational properties of the diffuse radio emission
Open accessAbstract
Clusters of galaxies, as the largest virialized systems in the Universe, are ideal laboratories to study the formation and evolution of cosmic structures. The luminous matter of clusters consists of galaxies and of an embedding intracluster medium (ICM), which has been heated to temperatures of tens of millions degrees, and thus is detected through its thermal emission in the soft X-ray regime. Most of the detailed knowledge of galaxy clusters has been obtained in recent years from the study of ICM through X-ray Astronomy. At the same time, radio observations have proved that the ICM is mixed with non-thermal components, i.e. highly relativistic particles and large-scale magnetic fields, detected through their synchrotron emission.The knowledge of the properties of these non-thermal ICM components has increased significantly, owing to sensitive radio images and to the development of theoretical models. Diffuse synchrotron radio emission in the central and peripheral cluster regions has been found in many clusters. Moreover large-scale magnetic fields appear to be present in all galaxy clusters, as derived from Rotation Measure (RM) studies. Non-thermal components are linked to the cluster X-ray properties, and to the cluster evolutionary stage, and are crucial for a comprehensive physical description of the intracluster medium. They play an important role in the cluster formation and evolution.We review here the observational properties of diffuse non-thermal sources detected in galaxy clusters: halos, relics and mini-halos. We discuss their classification and properties. We report published results up to date and obtain and discuss statistical properties. We present the properties of large-scale magnetic fields in clusters and in even larger structures: filaments connecting galaxy clusters. We summarize the current models of the origin of these cluster components, and outline the improvements that are expected in this area from future developments thanks to the new generation of radio telescopes.
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Double radio relics arise from diffusive shock acceleration of relativistic electrons at outward-propagating merger shocks and appear as Mpc-scale, arc-like structures at the cluster outskirts.
“They arise from diffusive shock acceleration (DSA) of relativistic electrons at outward-propagating merger shocks and appear as Mpc-scale, arc-like structures at the cluster outskirts.”
From A New Robust Constraint on the Self-interaction Cross Section of Dark Matter with Double Radio Relic Clusters (2026) · cites this paper as L. Feretti et al. 2012
Sections 5.1.3 and 9.1.2 describe paired, peripheral relics and favor diffusive acceleration at outgoing merger shocks; Sections 5 and 5.1.1 describe their Mpc-scale, elongated or curved appearance. The claim states the merger-shock explanation as settled for double relics, whereas Section 5.1.3 presents it as a model and discusses accretion shocks as a possible alternative. Page numbers are unavailable in our copy. (LLM, conf=0.88)
Experimental — accuracy not yet measuredDouble radio relics are diffuse radio sources that trace merger shock fronts, providing a direct measurement of the shock-to-shock distance.
“Double radio relics are diffuse radio sources that trace merger shock fronts, providing a direct measurement of the shock-to-shock distance (T. A. Ensslin et al. 1998;L. Feretti et al. 2012;R. J. van Weeren et al. 2012R. J. van Weeren et al. , 2019)).”
From A New Robust Constraint on the Self-interaction Cross Section of Dark Matter with Double Radio Relic Clusters (2026) · cites this paper as L. Feretti et al. 2012
Section 5.1.3 describes double relics on opposite sides of a cluster as signatures of merger shocks, and §9.1.2 says radio emission is produced close to shock waves. Together, these support using the relics to estimate the separation of the shocks, but the paper does not establish an unqualified direct measurement of the shock-to-shock distance. (LLM, conf=0.84)
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Jee, M. J., Park, D., & Lee, W. (2026) · The Astrophysical Journal · doi:10.3847/1538-4357/ae88fb · Cited by 1 · see citing works
- Authors
- Feretti, L., Giovannini, G., Govoni, F., & Murgia, M.
- Affiliations
- Istituto di Radioastronomia di Bologna (IT)
- Osservatorio Astronomico di Cagliari (IT)
- Corresponding author
- Feretti, Luigina
- Venue
- The Astronomy and Astrophysics Review
- Published
- 2012-05-19
- Volume
- 20
- Issue
- 1
- ISSN
- 0935-4956 (ISSN-L)
- 1432-0754
- OpenAlex
- W1994263939
- Cited by
- 667
- References
- 0
- Citations by year
- 2026: 22 · 2025: 49 · 2024: 42 · 2023: 31 · 2022: 25 · 2021: 41 · 2020: 52 · 2019: 57 · 2018: 53 · 2017: 65 · 2016: 75 · 2015: 55 · 2014: 49 · 2013: 45 · 2012: 6
- FWCI
- 26.26
- Type
- journal article
- Language
- en
- Open access
- hybrid
- doi.org
- Licence
- cc-by
- Locations
- The Astronomy and Astrophysics Review (publishedVersion, cc-by)
- arXiv (Cornell University) (submittedVersion)
- Archivio istituzionale della ricerca (Alma Mater Studiorum Università di Bologna) (submittedVersion)
- arXiv (Cornell University)
- Topics
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Galaxies: Formation, Evolution, Phenomena
- Physical Sciences › Physics and Astronomy › Nuclear and High Energy Physics › Astrophysics and Cosmic Phenomena
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysical Phenomena and Observations
- Keywords
- galaxy clusters, intracluster medium, diffuse radio emission, synchrotron radiation, relativistic particles, large-scale magnetic fields, rotation measure, radio halos, radio relics, minihalos, cosmic filaments, cluster formation and evolution, intracluster medium heating
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