Clusters of galaxies : observational properties of the diffuse radio emission (1205.1919v1)

Abstract: Clusters of galaxies, as the largest virialized systems in the Universe, are ideal laboratories to study the formation and evolution of cosmic structures...(abridged)... 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.

• The paper systematically reviews the observational characteristics of radio halos, relics, and mini-halos in clusters.
• The paper finds that steep spectral signatures support turbulent reacceleration and shock acceleration mechanisms during cluster mergers.
• The paper underscores that next-generation telescopes like LOFAR and SKA will advance our understanding of cluster magnetic fields and non-thermal processes.

Analysis of Diffuse Radio Emission in Galaxy Clusters

The paper "Clusters of galaxies: observational properties of the diffuse radio emission," authored by Luigina Feretti and colleagues, presents an extensive review of the observational characteristics of diffuse radio emissions found within galaxy clusters. As an expert in the field, I will summarize the key findings and implications of this work, focusing on the underlying mechanisms driving these phenomena and their relevance to understanding cosmic structures.

Galaxy clusters represent the largest gravitationally bound systems in the universe. They consist predominantly of dark matter, with luminous matter composed of galaxies and hot, diffuse gas, known as the intracluster medium (ICM). This medium emits primarily in the X-ray regime, allowing its thermal properties to be studied extensively. However, non-thermal components—such as relativistic particles and large-scale magnetic fields—have been detected through radio emissions, signifying richness in cluster environments that goes beyond thermal content.

The paper thoroughly catalogs the diffuse radio emissions, which can be grouped into three main types: radio halos, relics, and mini-halos. These components vary significantly in morphology, location within the cluster, polarization, and spectrum:

1. Radio Halos: Typically found at cluster centers, these extended sources possess low surface brightness and are unpolarized. They are believed to arise from the turbulent reacceleration of relativistic electrons by cluster mergers, supported by correlations between halo occurrences and cluster X-ray luminosity.
2. Relics: These elongated sources are generally located on the peripheries of clusters and exhibit high degrees of polarization. Their origin is commonly associated with shock waves from dynamical processes like mergers, reaccelerating particles at the sites of high Mach number shocks.
3. Mini-Halos: Surrounding central dominant galaxies in relaxed, cool-core clusters, these sources have properties resembling larger halos but are generally smaller and less understood. Their formation likely involves reacceleration processes tied to the dynamics of cool cores or minor mergers, potentially linked with turbulent reacceleration or cosmic ray interactions.

A significant aspect of this paper lies in the exploration of the relationship between diffuse radio emissions and the magnetic fields within clusters. Faraday rotation measure studies indicate the presence of μG-level magnetic fields that are entangled with the cluster’s gas. The work implies these fields play key roles in shaping the observed radio properties, contributing to cluster dynamics and evolution.

One notable observation is the statistical association of radio halos with cluster mergers, while relics are often seen in both merging and relaxed clusters, suggesting a complex interplay of formation mechanisms influenced by cosmic events and environments.

The research further explores the spectral properties of these radio sources, with many radio halos and relics displaying steep spectra indicative of significant energy losses. Some exceptions exist in ultra-steep spectrum sources, likely extending our understanding of clusters across cosmic time.

The authors highlight ongoing challenges and future directions, emphasizing the need for deeper, multi-wavelength studies to unravel the subtleties of non-thermal processes in galaxy clusters. Instruments such as the LOFAR and SKA promise to extend radio studies to greater depths, unveiling faint sources and providing richer data for magnetic field analyses, thereby refining our knowledge of the vast structures in the universe.

In conclusion, the work by Feretti et al. encapsulates the current state and understanding of non-thermal radio phenomena in galaxy clusters, serving as a comprehensive resource for further theoretical and observational investigations aimed at uncovering the full dynamic history and evolution of the cosmos at the largest scales.