Revived Fossil Plasma Sources in Galaxy Clusters

Abstract: It is well established that particle acceleration by shocks and turbulence in the intra-cluster medium can produce cluster-scale synchrotron emitting sources. However, the detailed physics of these particle acceleration processes is still not well understood. One of the main open questions is the role of fossil relativistic electrons that have been deposited in the intra-cluster medium by radio galaxies. These synchrotron-emitting electrons are very difficult to study, as their radiative life time is only tens of Myrs at GHz frequencies, and are therefore a relatively unexplored population. Despite the typical steep radio spectrum due to synchrotron losses, these fossil electrons are barely visible even at radio frequencies well below a GHz. However, when a pocket of fossil radio plasma is compressed, it boosts the visibility at sub-GHz frequencies, creating so-called radio phoenices. This compression can be the result of bulk motion and shocks in the ICM due to merger activity. In this paper, we demonstrate the discovery potential of low frequency radio sky surveys to find and study revived fossil plasma sources in galaxy clusters. We used the 150~MHz TGSS and 1.4 GHz NVSS sky surveys to identify candidate radio phoenices. A subset of three candidates were studied in detail using deep multi-band radio observations (LOFAR and GMRT), X-ray (\textit{Chandra} or \textit{XMM-Newton}) and archival optical observations. Two of the three sources are new discoveries. Using these observations, we identified common observational properties (radio morphology, ultra-steep spectrum, X-ray luminosity, dynamical state) that will enable us to identify this class of sources more easily, and help to understand the physical origin of these sources.

• The paper identifies and characterizes "radio phoenices," revived fossil plasma sources in galaxy clusters, using low-frequency radio surveys and multi-wavelength observations.
• Identified radio phoenices are diffuse structures lacking point source association and exhibit ultra-steep spectra, often ormalfont$\alpha \leq -2$ ormalfont{} below a GHz.
• The visibility of these sources is linked to dynamically disturbed host clusters where mergers potentially compress fossil plasma, reviving low-frequency emission.

Revived Fossil Plasma Sources in Galaxy Clusters

The paper "Revived Fossil Plasma Sources in Galaxy Clusters" explores the complex physical phenomena of galaxy clusters, focusing specifically on the identification and analysis of revived fossil plasma sources, colloquially known as "radio phoenices." These sources are derived from fossil relativistic electrons deposited by radio galaxies in the intracluster medium (ICM). This study highlights the potential insights offered by low-frequency radio surveys in understanding such phenomena.

Key Findings

The paper's primary endeavor is to delineate the characteristics and potential origins of radio phoenices within galaxy clusters. Utilizing radio frequency surveys, particularly the 150 MHz TGSS and 1.4 GHz NVSS, the study identifies candidate sources through low frequency emissions. The researchers examined three principal candidates with comprehensive multi-band radio observations from LOFAR and GMRT, as well as X-ray data from Chandra and XMM-Newton complemented with optical observations. These analyses produced notable new discoveries from two of the three identified sources.

Observation Highlights

• Radio Emission Morphology: The morphology of radio phoenices is observed as filamentary with a distinct lack of association with point sources, indicating these are diffuse, aged structures not immediately connected to active AGN emissions.
• Spectral Characteristics: The identified radio sources are characterized by ultra-steep radio spectra, often more pronounced at higher frequencies, indicative of synchrotron aging and Inverse Compton losses from fossil relativistic electrons. Such sources show spectral indices of $\alpha \leq -2$ at frequencies below a GHz.
• Cluster Host Dynamics: The X-ray observations reveal somewhat disturbed morphologies, suggesting dynamical disturbances within the clusters, potentially due to minor mergers or interactions which might trigger compressions leading to radio phoenix activation.

Implications of Fossil Plasma Studies

The identification and understanding of fossil plasma are crucial for advancing our comprehension of particle acceleration in galaxy clusters. The presence of radio phoenices offers a unique lens into past AGN activity and its long-term effects on the ICM. Given their visibility primarily at low frequencies, radio phoenices underscore the importance of sub-GHz observations to uncover the full extent of cluster dynamics and their role in large-scale structure formation.

The study establishes a critical connection between relic radio emissions and the large-scale environment of galaxy clusters, implying that historical AGN activities leave a persistent trace observable under specific conditions. This research supports the view that shock waves and bulk motions, resulting from cluster mergers, compress fossil radio plasma, thereby reviving these sources to a visible state at low radio frequencies.

Future Research Directions

The findings advocate further detailed multi-frequency observations to explore the physical origins and evolution of radio phoenices across a larger cluster sample. The study hints at pervasive relic plasma in galaxy clusters and calls for additional high-resolution surveys to map these phenomena globally.

Advancements in low-frequency radio astronomy are poised to significantly enhance our understanding of the interconnections between galaxy clusters and non-thermal cosmic phenomena. Future studies are likely to benefit from even more sensitive instruments, allowing deeper insights into the dynamics and evolution of large-scale structures in the universe.