Periodic Reporting for period 2 - ClusterWeb (Unravelling the physics of particle acceleration and feedback in galaxy clusters and the cosmic web)
Reporting period: 2021-03-01 to 2022-08-31
The goal of the project is to unravel the origin of cosmic rays and magnetic fields in galaxy clusters and the lower density regions beyond clusters. In addition, the project aims to determine how supermassive black holes in galaxy clusters interact with their environment. To address these questions, the researchers make use of state-of-the-art radio telescopes. With the enormous gains in sensitivity, survey speed, and angular resolution of these radio telescopes - combined with recent breakthroughs that correct Earth’s distorting ionosphere - we can hope to take a big step in addressing these questions. LOFAR - a pan-European radio telescope plays a central role in this. LOFAR is an IT-telescope employing phased-array technology, where signals from small low-cost antennas are combined in a supercomputer to emulate a giant telescope. The sensitivity and angular resolution of LOFAR are more than an order of magnitude better than any other radio telescope operating in the low-frequency band. The LOFAR observations are complemented with data from the Chandra, XMM, and Planck satellites.
Galaxy clusters and cosmic web filaments are the Universe’s largest structures. Clusters grow through a sequence of mergers, generating shock waves and turbulence which heat the hot plasma in clusters. In these magnetized plasmas, cosmic rays are accelerated to extreme energies, producing megaparsec-size diffuse synchrotron emitting radio sources. It is still unclear how these cosmic rays are accelerated. For the project, a systematic study of these diffuse cluster radio will be carried out to understand how their properties vary with for example the mass of the host cluster. The lower density regions of the Universe, outside of clusters, should also be sites of particle acceleration since shocks are present. Ultra-deep low-frequency radio images will be made to detect this emission and understand the particle acceleration mechanisms operating in these environments.
During the last decade, it has become clear that supermassive black holes, associated with brightest clusters galaxies, play a crucial role in heating the central regions of clusters. Low-frequency observations are essential to study this process because they can trace old outburst of the supermassive black hole. Subarcsecond resolution LOFAR observations of clusters will be used to determine how this process of heating evolves over the lifetime of clusters.
Galaxy clusters and cosmic web filaments are the Universe’s largest structures. Clusters grow through a sequence of mergers, generating shock waves and turbulence which heat the hot plasma in clusters. In these magnetized plasmas, cosmic rays are accelerated to extreme energies, producing megaparsec-size diffuse synchrotron emitting radio sources. It is still unclear how these cosmic rays are accelerated. For the project, a systematic study of these diffuse cluster radio will be carried out to understand how their properties vary with for example the mass of the host cluster. The lower density regions of the Universe, outside of clusters, should also be sites of particle acceleration since shocks are present. Ultra-deep low-frequency radio images will be made to detect this emission and understand the particle acceleration mechanisms operating in these environments.
During the last decade, it has become clear that supermassive black holes, associated with brightest clusters galaxies, play a crucial role in heating the central regions of clusters. Low-frequency observations are essential to study this process because they can trace old outburst of the supermassive black hole. Subarcsecond resolution LOFAR observations of clusters will be used to determine how this process of heating evolves over the lifetime of clusters.
For the project, the researchers obtained observations of distant galaxy clusters with the radio telescope LOFAR. Galaxy clusters consist of thousands of individual galaxies, each containing billions of stars. However, most of the normal matter in galaxy clusters is in the form of a diffuse hot plasma. This plasma occupies the entire volume of a cluster. Within this plasma, magnetic fields are also present. However, the origin of these magnetic fields are not understood. The LOFAR observations of distant clusters detected the presence of cosmic rays residing in the hot plasma. Cosmic rays are extremely energetic particles that emit synchrotron radiation, which can be detected with radio telescopes. From the observations, the strength of the magnetic fields in the clusters could be estimated. The found magnetic field values turned out to be higher than expected, and this indicates the magnetic field strength grows quickly at the time the clusters formed.
Additionally, for the project the researchers developed new techniques to make high-resolution images at low radio frequencies. This is challenging because the Earth's ionosphere severely blurs the radio images. The ionosphere is a layer of ionized plasma about a hundred kilometers above the Earth's surface. This layer bends low-frequency radio waves from space, causing the blurring of images. The new techniques developed can correct for this effect, making use of stable compact beacons of radio emission which are often distant active galactic nuclei. Using these new techniques, the researchers studied how supermassive black holes at the center of galaxy clusters influence and interact with their environment.
Additionally, for the project the researchers developed new techniques to make high-resolution images at low radio frequencies. This is challenging because the Earth's ionosphere severely blurs the radio images. The ionosphere is a layer of ionized plasma about a hundred kilometers above the Earth's surface. This layer bends low-frequency radio waves from space, causing the blurring of images. The new techniques developed can correct for this effect, making use of stable compact beacons of radio emission which are often distant active galactic nuclei. Using these new techniques, the researchers studied how supermassive black holes at the center of galaxy clusters influence and interact with their environment.
Galaxy clusters are the largest structures in the Universe. When such clusters merge, the electrons between them are accelerated to almost the speed of light. The accelerated electrons emit radio waves when they come into contact with magnetic fields in the clusters. Until now, telescopes were not powerful enough to receive radio waves from distant colliding clusters. But thanks to the Dutch-European network of linked LOFAR antennas, the researchers were able to collect detailed data from distant clusters for the first time. The data shows, among other things, that the radio emission from distant colliding clusters is brighter than previously expected. According to theories, cluster radio emission originates from electrons that are accelerated by the turbulent motions. The magnetic fields in the distant clusters turned out to be about as strong as in previously investigated nearby clusters. This unexpected result indicates that magnetic fields grow quickly in strength during cluster formation. The researches have also used to LOFAR to make the first subarcsecond resolution images of radio sources in galaxy clusters. For this, they developed new calibration techniques to map the sky at unprecedented resolution at low radio frequencies using supercomputers.