In this thesis we push the boundaries of high-resolution imaging at radio frequencies between 100 and 200 MHz. Using the International LOFAR Telescope that spans across Europe we achieve sub... Show moreIn this thesis we push the boundaries of high-resolution imaging at radio frequencies between 100 and 200 MHz. Using the International LOFAR Telescope that spans across Europe we achieve sub-arcsecond angular resolution. We demonstrate mapping of the full field of view of the telescope at its native resolution and use the resulting images to study active galactic nuclei in the Lockman Hole field. Show less
Within the field of astronomy, understanding how galaxies grow and evolve from the Big Bang to the present day is a challenging and complex question. Radio observations - unhindered by dust... Show moreWithin the field of astronomy, understanding how galaxies grow and evolve from the Big Bang to the present day is a challenging and complex question. Radio observations - unhindered by dust attenuation - are a powerful tool in studying the formation of stars and subsequent buildup of galaxies. In this thesis, the distant star formation is studied using radio observations from the VLA COSMOS-XS survey specifically. In Chapter 2, we discuss the details of the sensitive COSMOS-XS survey and present the derived Euclidean-normalized source counts. In Chapter 3, we derive the dust-unbiased star formation rate density out to high redshift and present evidence for a significant underestimation of the star formation rate density based on ultraviolet observations. In Chapter 4, the focus shifts to the ‘optically dark’ population: extremely dust-obscured sources that are invisible even in deep ultraviolet imaging. We identify these sources with the COSMOS-XS survey and use them to quantify their contribution to the total star formation rate density. In Chapter 5, we present new ALMA observations of ‘optically dark’ sources and confirm the cosmic importance of ‘optically dark’ sources at high redshift. Show less
Ignesti, A.; Brienza, M.; Vulcani, B.; Poggianti, B.M.; Marasco, A.; Smith, R.; ... ; Drabent, A. 2023
One of the key quests in astronomy is to study the growth and evolution of galaxies across cosmic time. Radio observations provide a powerful means of studying the formation of stars and subsequent... Show moreOne of the key quests in astronomy is to study the growth and evolution of galaxies across cosmic time. Radio observations provide a powerful means of studying the formation of stars and subsequent buildup of distant galaxies, in a way that is unbiased by the presence of dust. This thesis provides a detailed view of faint, star-forming galaxies in the early Universe through sensitive radio observations, and compiles several studies probing distant star formation with both radio synchrotron and free-free emission. In Chapter 2, we detect a large number of galaxies using sensitive new radio data from the Very Large Array, allowing us to separate radio emission from star formation and active galactic nuclei in the faint radio sky. In Chapter 3, we calibrate synchrotron emission as a tracer of star formation in distant starburst galaxies, while in Chapters 4 & 5 we turn towards radio free-free emission — a faint but very powerful tracer of star formation. Using sensitive new radio data at high frequencies, we perform the first detailed studies of free-free emission in distant galaxies. 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
This thesis focuses on the study of transient and variable radio sources in the universe. Transient signals, unlike a source constantly visible, change in their brightness (or flux) over a certain... Show moreThis thesis focuses on the study of transient and variable radio sources in the universe. Transient signals, unlike a source constantly visible, change in their brightness (or flux) over a certain length of time. While there are a plethora of transients over short intervals in radio astronomy that last less than a minute, this thesis will primarily cover those which play out over many years and decades. Show less
This thesis explores the theoretical and observational properties of distant massive galaxies that harbour active black holes in their centres and shine brightly at radio wavelengths. These radio... Show moreThis thesis explores the theoretical and observational properties of distant massive galaxies that harbour active black holes in their centres and shine brightly at radio wavelengths. These radio galaxies are some of the rarest objects in the Universe, and studying them reveals more about the formation and evolution of massive galaxies and black holes in the Universe. In Chapter 2, we model faint radio galaxies at record distances (high redshifts) using a simple model to characterise the overall evolution of radio galaxies. We find that our model replicates well the properties of radio galaxies that are nearer to us. This gives us confidence in the predictions at higher redshifts (larger distances), where data is scarce. In Chapter 3, we compile a list of promising distant radio galaxies using all-sky radio datasets. We present observations for these candidates at a high resolution. In Chapter 4, we report the discovery of the most distant radio galaxy observed till date, TGSS J1530+1049. The galaxy’s distance is confirmed using data from various telescopes. In Chapter 5, we present a detailed study of other distant radio galaxies from our initial sample, showing their properties to be different from brighter radio galaxies. Show less
Radio astronomy was born during the Second World War. The early post-war radio astronomy group in the Netherlands was one of the most important radio astronomy groups in the world. There are... Show moreRadio astronomy was born during the Second World War. The early post-war radio astronomy group in the Netherlands was one of the most important radio astronomy groups in the world. There are several reasons for this. Firstly: Dutch radio astronomers were trained as (optical) astronomers, while in most countries engineers and physicists with a background in wartime radar research were the first radio __astronomers__. This was because radio telescopes shared the technology of wartime radar installations. Because Dutch astronomers were not familiar with the new kind of instrumentation, they had to conclude strategic alliances with industrial partners such as Philips, the PTT and the KNMI. These alliances would offer much more than merely technical know-how, which means that the disadvantage would prove to be an advantage in the end. Secondly: astronomy was still a very small-scale undertaking in the early post-war period. Even so, ZWO was still a very small organisation. The fact that so few people were involved meant that the impact of a personal network could be enormous. Thirdly: the Dutch post-war context was remarkably favourable to science: it was considered to be a key factor in the rebuilding of the country. Show less
