Following the Big Bang, structure in the Universe started collapsing under the force of gravity. This resulted in the formation of the first stars, galaxies and clusters of galaxies. The majority... Show moreFollowing the Big Bang, structure in the Universe started collapsing under the force of gravity. This resulted in the formation of the first stars, galaxies and clusters of galaxies. The majority of the baryonic mass in a galaxy cluster is part of the hot intracluster medium, which permeates the entire cluster As this medium cools down, it accretes onto the central galaxies where it triggers the formation of new stars. However, it also feeds the central supermassive black hole, creating an active galactic nucleus (AGN) that injects a large amount of energy into the intracluster medium again, resulting in a feedback cycle. This feedback cycle is an essential ingredient in the formation and evolution of galaxies. Using new high-resolution radio observations taken with the LOw Frequency ARray (LOFAR), we study AGN feedback in galaxy clusters in unprecedented detail, gaining insight into both the duty cycle of the AGN as well as the energy budget of the feedback cycle. Show less
On the largest scale, the Universe resembles a cosmic spiderweb. Most galaxies coexist in small groups within the threads of this web. At the nodes of the threads are enormous groups of galaxies... Show moreOn the largest scale, the Universe resembles a cosmic spiderweb. Most galaxies coexist in small groups within the threads of this web. At the nodes of the threads are enormous groups of galaxies forming the largest structures in the universe still held together by gravity: clusters of galaxies.Clusters of galaxies consist of thousands of galaxies, although the galaxies constitute only a few per cent of the total cluster mass. The majority of the (non-dark) mass of a cluster is in a hot and dilute gas that resides in the space between galaxies and is permeated by magnetic fields. Clusters grow by collisions with other clusters, shocking and heating the gas causing amplification of magnetic fields and acceleration of particles to near the speed of light. This makes clusters a source of radio synchrotron radiation.This thesis investigates the particle acceleration process and the magnetic fields of merging clusters using the LOFAR and VLA radio telescopes. The thesis presents, among other things, one of the few radio maps of clusters at ultra-low frequencies and examines clusters of lower mass than usual. Additionally, the thesis includes observations of a sample of over a hundred clusters to statistically determine the properties of the magnetic field in clusters in a novel way. Show less
Three major cosmology-focused missions are planned for the next decade: the Euclid space telescope, the Vera C. Rubin Observatory in Chile, and the Nancy Grace Roman Space Telescope. Their surveys... Show moreThree major cosmology-focused missions are planned for the next decade: the Euclid space telescope, the Vera C. Rubin Observatory in Chile, and the Nancy Grace Roman Space Telescope. Their surveys will detect billions of galaxies over more than a third of the celestial sphere and out to redshifts of z~2 to constrain the evolving matterdistribution using weak gravitational lensing. This thesis focusses on ways of disentangling the contribution of normal matter, baryons for cosmologists, from the measured weak lensing signal for cosmic shear analyses and galaxy cluster mass calibrations. Combining the predictions of computer-simulated universes, evolving billions of particles in time, with simplified models that reproduce the observeddistribution of hot gas inside clusters of galaxies, but that freely vary the amount of matter where no observations are available, we have quantified how strongly our ignorance of the relation between ordinary and dark matter will affect the analysis of the planned surveys. Additionally, we have studied how baryons affect cluster massdeterminations and we suggest a new analysis method for cluster abundance studies that is less sensitive to our lack of knowledge of the exact distribution of normal matter in the outskirts of galaxy clusters. Show less
The intracluster medium (ICM) is a hot, tenuous and X-ray emitting gas that pervades galaxy clusters. Meanwhile, extended synchrotron sources, i.e., radio relics and radio halos are frequently... Show moreThe intracluster medium (ICM) is a hot, tenuous and X-ray emitting gas that pervades galaxy clusters. Meanwhile, extended synchrotron sources, i.e., radio relics and radio halos are frequently observed in galaxy cluster, for which the two widely accepted particle acceleration are shock acceleration and turbulent acceleration, respectively. Both of these scenarios are due to ICM motions, which means that X-ray observations of the ICM are fundamental for quantifying shock wave and turbulence properties to test the acceleration models. This thesis focuses on X-ray observations of galaxy clusters, and covers topics from ICM shock - radio relic connection to turbulence - radio halo connection. The new observational evidences further support the two acceleration scenario. Show less
At the largest scales, two ingredients dictate the distribution of matter in the Universe. The first is dark matter, acting as an invisible scaffolding held together by gravitational forces. The... Show moreAt the largest scales, two ingredients dictate the distribution of matter in the Universe. The first is dark matter, acting as an invisible scaffolding held together by gravitational forces. The second is dark energy, an enigmatic component responsible for the accelerated expansion of the Universe. Under these two forces, matter in the Universe organizes itself in the so-called cosmic web. The nodes of this network are large dark matter haloes, and this thesis explores how their boundaries provide information about the nature of dark energy and cosmology. Chapters 3 and 4 present robust theoretical predictions for this titular edge and discuss its simple physical interpretation. Chapters 2 and 5 corroborate these results by presenting measurements of this feature in weak-lensing data. The last scientific chapter of this thesis is a collection of studies in gravitational-wave physics. This chapter explores how these spacetime ripples observed from across the cosmos can be used to detect alternative theories of gravity. Show less
In this thesis, I studied the origin and evolution of the non-thermal radiation in merging galaxy clusters. Cluster mergers are the among the most energetic events in the Universe. Part of this... Show moreIn this thesis, I studied the origin and evolution of the non-thermal radiation in merging galaxy clusters. Cluster mergers are the among the most energetic events in the Universe. Part of this energy is released in turbulence and shock waves which can (re-)accelerate cosmic rays and amplify magnetic fields in the intra-cluster medium (ICM). The interaction of these results in large-scale synchrotron radiation, which is visible through radio observations. In addition, X-ray observations of the ICM were carried out to investigate the dynamical state of the clusters, and the properties of cluster shocks. Show less
It has now been well established that shocks and turbulent motions in the intra-cluster medium (ICM) generated through cluster mergers can produce large-scale synchrotron emission. However, the... Show moreIt has now been well established that shocks and turbulent motions in the intra-cluster medium (ICM) generated through cluster mergers can produce large-scale synchrotron emission. However, the underlying particle acceleration mechanisms responsible for the origin of these faint diffuse radio sources are not well understood. With the advent of new generation low-frequency radio telescopes and better calibration techniques it has now become possible to unveil low-frequency radio sky with unprecedented depth and sensitivity, that is full of potential for new discoveries. The first part of this dissertation presents the study to better understand the nature of revived fossil (radio) plasma sources in galaxy cluster and establish them as a distinct class of radio sources. The complexity of objects, observed with these new instruments are just tip of the iceberg of the population, and will challenge the traditional taxonomy of diffuse radio sources. The last part of the dissertation focuses on deriving the deepest radio source-counts at 150 MHz. This is one of the several cosmological tests to check feasibility of new cosmological models and a sanity check for the data reduction. The derived source-counts are also compared with the other existing determinations, as well as with state-of-the-art evolutionary models. Show less
In this thesis, I study the formation of large-scale structure and the physics of particle acceleration at large scales (~Mpc). The object of my study is clusters of galaxies that are... Show moreIn this thesis, I study the formation of large-scale structure and the physics of particle acceleration at large scales (~Mpc). The object of my study is clusters of galaxies that are in the formation process. Galaxy clusters form through a sequence of mergers of sub-clusters. During the formation of galaxy clusters, shocks and turbulence are generated in the intra-cluster medium (ICM). These shocks and turbulence (re-)accelerate cosmic ray (CR) particles to relativistic speeds. In the presence of magnetic fields, the CR particles emit synchrotron radiation that we can observe with radio telescopes. I also combine radio data with X-ray and optical data to study these clusters. Studies of these merging galaxy clusters help us to understand the physics of particle acceleration at cosmic scales Show less
Galaxy clusters are the largest reservoirs of matter in the Universe, and as such are unique laboratories to understand the connection between dark and luminous, 'normal' matter. We use... Show more Galaxy clusters are the largest reservoirs of matter in the Universe, and as such are unique laboratories to understand the connection between dark and luminous, 'normal' matter. We use several techniques and galaxy cluster samples to study this connection from various angles. In particular, we try to understand how does the motion of galaxies within clusters relate to their luminous mass content; how do the shapes of galaxies respond to the strong gravitational potential of their host cluster (analogous to tidal waves produced by the Earth-Moon gravitational interaction), and how much of their total mass are galaxies able to retain once they fall under the influence of their host cluster, in connection with the same interactions. Our results provide important information for models of galaxy formation and evolution, particularly their dark matter content, and for studies trying to link observations of galaxy clusters to the overall properties of the Universe such as its total matter content. Show less
Galaxy clusters mainly grow through mergers with other clusters and groups. Major mergers give rise to cluster-wide traveling shocks, which can be detected at radio wavelengths as relics: elongated... Show moreGalaxy clusters mainly grow through mergers with other clusters and groups. Major mergers give rise to cluster-wide traveling shocks, which can be detected at radio wavelengths as relics: elongated, diffuse synchrotron emitting areas located at the periphery of merging clusters. The 'Sausage' cluster hosts an extraordinary Mpc-wide relic, which enables us to study to study particle acceleration and the effects of shocks on cluster galaxies. We derive shock properties and the magnetic field structure for the relic. Our results indicate that particles are shock-accelerated, but turbulent re-acceleration or unusually efficient transport of particles in the downstream area are important effects. We demonstrate the feasibility of high-frequency observations of radio relics, by presenting a 16 GHz detection of the 'Sausage' relic. Halpha mapping of the cluster provides the first direct test as to whether the shock drives or prohibits star formation. We find numerous galaxies in close proximity to the radio relic which are extremely massive, metal-rich, star-forming with evidence for gas mass loss though outflows. We speculate that the complex interaction between the merger, the shock wave and gas is a fundamental driver in the evolution of cluster galaxies from gas rich spirals to gas-poor ellipticals. Show less
In this thesis an interferometric study of diffuse radio sources, so-called halos and relics, in and around galaxy clusters is performed. These sources are thought to trace shocks and turbulence... Show moreIn this thesis an interferometric study of diffuse radio sources, so-called halos and relics, in and around galaxy clusters is performed. These sources are thought to trace shocks and turbulence generated by galaxy cluster merger events. GMRT, WSRT and VLA observations are analyzed to measure the spectral and polarimetric properties of the diffuse cluster emission. In addition, a search for new relics and halos is carried out based on existing radio surveys. Numerical simulations of cluster mergers are described, which have the aim of constraining the cluster mergers parameters from observations of double radio relics. The first LOFAR observation of cluster-scale diffuse radio emission is presented. LOFAR is a new pan-European radio telescope that operates at the lowest radio frequencies accessible from the surface of the Earth. Show less
The study of cosmic large-scale structure formation benefits from radio observations, because it provides an unbiased view on the early Universe. Distant radio galaxies and diffuse cluster sources... Show moreThe study of cosmic large-scale structure formation benefits from radio observations, because it provides an unbiased view on the early Universe. Distant radio galaxies and diffuse cluster sources generally have a steep spectrum, which implies an increased brightness towards lower frequencies (below 300 MHz). The quality of low-frequency radio observations is compromised by the propagation effects on cosmic radio waves passing through the ionosphere. In this thesis, we present a calibration method for low-frequency radio interferometric observations. This method significantly improves the quality of radio maps from archival observations as compared to other existing calibration methods. The method was used to produce one of the deepest high-resolution surveys at 153 MHz to date, including the detection of 16 candidate distant radio galaxies. Furthermore, the method was used in a study of the diffuse radio sources in the merging galaxy cluster Abell 2256. These observations support the theory of revival of old radio sources through cluster merger shock compression. Finally, we present a study of the cosmic large-scale structure near a radio galaxy in the early universe by using an optical selection technique for galaxies. The projected galaxy distribution appears to trace the cosmic structure during the assembly of galaxy clusters. Show less
