ClusterWeb

The ERC-StG ClusterWeb project aims to unravel the origin of cosmic rays and magnetic fields in galaxy clusters and the lower-density regions beyond them. It also seeks to understand how supermassive black holes in galaxy clusters interact with their environment. To tackle these questions, ClusterWeb researchers use cutting-edge radio telescopes with significantly improved sensitivity, survey speed, and angular resolution. Recent advances in correcting for Earth's ionospheric distortions further enhances the capabilities of these telescopes.

LOFAR, a pan-European radio telescope, plays a central role in the project. As an IT-telescope employing phased-array technology, LOFAR combines signals from numerous low-cost antennas in a supercomputer to emulate a giant telescope. Its sensitivity and angular resolution are more than an order of magnitude better than any other low-frequency radio telescope. The LOFAR observations are complemented by data from the Chandra, XMM-Newton, and Planck satellites, providing a comprehensive multi-wavelength view of these cosmic structures.

ClusterWeb is organized along three scientific themes further detailed below.

MERGING GALAXY CLUSTERS AND PARTICLE ACCELERATION

Radio observations of galaxy clusters have revealed the presence of large, megaparsec-scale diffuse synchrotron-emitting sources known as radio halos and relics, depending on whether they are located near the cluster center or in the outskirts. The detection of synchrotron radiation indicates the presence of cosmic rays and magnetic fields within the intracluster medium (ICM). With their vast extent, these sources trace some of the largest particle accelerators in the Universe.

Radio halos and relics are thought to form in response to shock waves and turbulence generated during galaxy cluster collisions and mergers. At these shocks, particles are accelerated to relativistic energies, and in the presence of magnetic fields, these high-energy particles emit synchrotron radiation detectable by radio telescopes. However, the physics of the acceleration processes and the origin of cluster-wide magnetic fields remain poorly understood.

To advance our understanding, a large sample of radio halos and relics will be compiled, selecting clusters from the Planck satellite cluster catalog. LOFAR observations of these clusters will be analyzed to search for halos and relics, and their occurrence rates and properties will be compared to models of particle acceleration. Additionally, the spectral properties of diffuse cluster emission will be studied to pinpoint sites of particle acceleration and assess the role of AGN in contributing radio plasma.

AGN FEEDBACK AND THE INTRACLUSTER MEDIUM ENERGY BUDGET

Gas in the central regions of many relaxed galaxy clusters has a cooling time much shorter than the age of the Universe. This rapid cooling is expected to result in a cooling flow, where gas in the cluster’s core cools and flows inward. However, X-ray observations reveal far less cool gas than predicted in these cool-core clusters, indicating that some form of heating must counterbalance the radiative losses.

AGN radio lobes, associated with the central brightest cluster galaxy (BCG), have been identified as the primary source of energy input. X-ray observations frequently reveal cavities in cool-core clusters that coincide with the lobes of the BCG. In these regions, radio plasma displaces the X-ray-emitting gas, creating low-density bubbles that rise buoyantly and expand, distributing energy to the surrounding intracluster medium (ICM). This process, known as radio-mode feedback, plays a critical role in regulating cooling.

In some clusters, extended cavity systems are observed where the inner cavities are filled with radio plasma, but the outer cavities appear empty. These outer ghost cavities are likely remnants of earlier AGN outbursts. Inside these old cavities, the relativistic electrons have lost most of their energy, making them undetectable at GHz frequencies and complicating the study of past AGN activity. Furthermore, the evolution of radio-mode feedback during the early stages of cluster formation remains poorly understood.

To address these gaps, clusters will be observed with LOFAR to detect emission from past outbursts. The energy input into the ICM will be estimated based on the thermal pressure and the volume of radio lobes and cavities. By combining these observations with higher-frequency data, the relative ages of cavities can be mapped, providing insights into how AGN feedback regulates thermal balance in clusters over time. Additionally, by imaging distant clusters with LOFAR’s long European baselines, achieving sub-arcsecond resolution, we will obtain the first detailed view of radio-mode feedback during the early stages of cluster formation.

MAGNETOGENESIS AND THE WARM-HOT INTERGALACTIC MEDIUM

Compared to the intracluster medium, the intergalactic medium within galaxy filaments has significantly lower density and temperature. Approximately half of the Universe’s baryons reside in this warm-hot intergalactic medium (WHIM). Galaxy filaments and cluster outskirts are expected to be surrounded by strong accretion shocks, where plasma is initially shock-heated. However, studying the WHIM and its associated shocks is challenging due to the lack of sufficiently sensitive observational tools.

Magnetic fields are also expected to be present in the WHIM, but their strength is likely much weaker than in galaxy clusters. Because of the long dynamical timescales governing the evolution of filaments, the WHIM magnetic fields should still closely reflect the properties of the primordial seed field. Understanding the nature of this seed field is essential for unraveling the origin of magnetic fields in galaxies and clusters, a process known as magnetogenesis. However, it remains unclear whether the seed field was established in the early Universe or generated later through processes such as jets and magnetized winds during galaxy formation.

To make progress, we will use LOFAR to produce ultra-deep images of galaxy filaments and the regions surrounding galaxy clusters. If radio emission is detected, it can be used to probe the magnetic fields and physical conditions in these regions, as well as to trace cosmic rays and particle acceleration within this diffuse medium. These observations will provide crucial insights into the nature and evolution of magnetic fields beyond clusters and help address fundamental questions about magnetogenesis.

CLUSERWEB PUBLICATIONS

Publications resulting from  the ClusterWeb project can be found here.