Understanding how galaxies form, interact, and evolve comes largely from comparing theory predictions with observational data. Numerical simulations of galaxies provide the most accurate approach... Show moreUnderstanding how galaxies form, interact, and evolve comes largely from comparing theory predictions with observational data. Numerical simulations of galaxies provide the most accurate approach to testing the theory, as they follow the non-linear evolution of gas and dark matter in great detail and incorporate numerous baryonic processes, among which are energy feedback from supernovae (SNe) and Active Galactic Nuclei (AGN). In this thesis, we show the results of the development of the new model COLIBRE for cosmological simulations of galaxy formation that include a cold interstellar medium. First, we present a new SN feedback recipe developed for COLIBRE, whereby SN energy is injected into the gas in thermal and kinetic forms, and the total energy and momentum of the system of gas and stars are exactly conserved. Second, we conduct a detailed comparison of different ways in which SN energy is distributed in the gas environment around young stellar populations. Third, by using our simulation setup originally developed to test COLIBRE’s SN feedback, we show that the radioactive isotope Fe60 that has been detected on Earth is likely of SN origin. Finally, we present the calibration of the SN and AGN feedback of the COLIBRE model using machine learning. 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
Galaxies in the Universe are distributed along the intricate framework of the Cosmic Web. Groups and clusters of galaxies comprise the densest regions in this network, and therefore, are excellent... Show moreGalaxies in the Universe are distributed along the intricate framework of the Cosmic Web. Groups and clusters of galaxies comprise the densest regions in this network, and therefore, are excellent cosmic laboratories to study different aspects of galaxy evolution in extreme environments. In this thesis, we explore a wide range of properties of cluster galaxies and their host systems, such as the spatial distribution of mass in galaxies within clusters, the faint and diffuse stellar halo in groups and clusters, and processes that quench massive galaxies in high-redshift clusters. For this exploration, we develop methods to enable and optimise detailed comparisons of state-of-the-art observations and cosmological hydrodynamic simulations over more than half of the age of the Universe. Through our carefully-designed analyses, we test the validity of the simulations for studying the low-surface-brightness and high-redshift Universe. We also demonstrate how such comparisons can provide novel insights and motivate new tests for understanding galaxy evolution in dense environments. Show less
Outflows are crucially important for the gas budget and evolution of luminous star-forming galaxies and AGNs, with observed mass outflow rates of the same order as the star formation rate. Greater... Show moreOutflows are crucially important for the gas budget and evolution of luminous star-forming galaxies and AGNs, with observed mass outflow rates of the same order as the star formation rate. Greater star formation and black hole growth lead to more intense feedback and outflows, resulting in self-regulated galaxy growth. Multi-phase observations show that the cool molecular and atomic gas dominate the mass and momentum budget of massive galaxy outflows which additionally remove the direct fuel for star formation. In this thesis we target the molecular and atomic outflows at cosmic noon and dawn where the most extreme star formation and black hole activity is found but where current observations are severely lacking. Techniques commonly used to detect outflows in the nearby universe with emission lines are, however, challenging or impossible with current technology at the high-redshifts of this thesis. Molecular absorption lines provide a powerful and reliable alternative which is demonstrated with the OH+ and OH molecules in this thesis. With observations from the Atacama Large Millimeter/submillimeter Array (ALMA), this thesis provides cutting-edge comparisons of molecular/neutral outflows at cosmic dawn/noon between star-forming galaxies and dusty quasar hosts. Show less
Galaxies in the local Universe fall into two main categories of spirals and ellipticals. In this Thesis, we explore the structural evolution of galaxies into this bimodal distribution. To do so, we... Show moreGalaxies in the local Universe fall into two main categories of spirals and ellipticals. In this Thesis, we explore the structural evolution of galaxies into this bimodal distribution. To do so, we study galaxies in the context of the Fundamental Plane, the tight scaling relation between galaxy size, kinematics and luminosity, which connects the structural and stellar population properties of galaxies. This work is built on a combination of observational data and theoretical models. Large spectroscopic surveys are used to construct a representative sample of massive quiescent and star-forming galaxies across 8 Gyr of cosmic time. We hence show that there is strong variation and evolution in the mass-to-light ratios of galaxies, due to evolution in the stellar populations. However, surprisingly, all galaxies lie on a single mass Fundamental Plane, which does not evolve with time. Cosmological simulations are used to assess the structural properties that may underlie the observed mass Fundamental Plane. Based on the simulations, we propose that this relation may originate from a systematic variation in the central dark matter content within galaxies as a function of their size and mass. Show less
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
The formation and evolution of galaxies is fundamentally driven by the formation of new stars out of cold gas. Observations of young stars in distant galaxies in the early universe, such as we can... Show moreThe formation and evolution of galaxies is fundamentally driven by the formation of new stars out of cold gas. Observations of young stars in distant galaxies in the early universe, such as we can see in the Hubble Ultra Deep Field, have unveiled how the cosmic star formation rate density evolves. Yet, while the effect of star formation—the young stars—has been mapped in ever-increasing detail, the cause—the cold molecular gas that fuels star formation—has been elusive. This thesis presents an observational study of the cold interstellar medium of distant galaxies in the early universe, using the most sensitive submillimeter telescope to date, the Atacama Large Millimeter Array, together with new integral-field spectrographs, such as the Multi Unit Spectroscopic Explorer on the Very Large Telescope. It unveils the physical properties of star-forming galaxies and their molecular gas reservoirs, and describes the evolution of the cosmic molecular gas density—the fuel for star formation. Show less
This thesis presents pioneering work on the panchromatic emission of some of the most luminous galaxies in the early Universe: star forming galaxies and active galactic nuclei. Using state-of... Show moreThis thesis presents pioneering work on the panchromatic emission of some of the most luminous galaxies in the early Universe: star forming galaxies and active galactic nuclei. Using state-of-the-art statistical methods and new-generation radio-to-X-rays instruments, this thesis expands the parameter space covered by current multi-wavelength studies of galaxy evolution. In particular, this thesis pushes three different frontiers. The statistical frontier is explored by developing a sophisticated statistical tool to robustly recover the parameters required to model multi-wavelength emission. The wavelength frontier is pushed forward by exploring galaxy evolution from the new spectral window at low-frequency radio, opened by the LOFAR instrument. Finally, the resolution frontier will be pushed by exploring the distribution of emission components across the spectrum using a combination of high-resolution ALMA and HST imaging. Show less
One of the major unresolved questions in astronomy is: how do galaxies form and evolve? In the local universe we can distinguish between actively star-forming and quiescent galaxies. Quiescent... Show moreOne of the major unresolved questions in astronomy is: how do galaxies form and evolve? In the local universe we can distinguish between actively star-forming and quiescent galaxies. Quiescent galaxies are typically the most massive, with elliptical morphologies and red optical colors. The mechanisms that cause star-formation in galaxies to be turned off, so that star-forming galaxies become quiescent, are not yet well understood. Using the FourStar Galaxy Evolution Survey (ZFOURGE), comprising near-infrared data of over seventy thousand galaxies, we aim to find and study the first quiescent galaxies. First we describe the data products of ZFOURGE. Then we present the discovery of 15 very massive quiescent galaxies over 12 billion years ago, when the universe was only 1.6 billion years old. The implication is that they must have formed extremely rapidly, with explosively high star-formation rates. They are very compact, and much smaller than nearby quiescent galaxies as well as equally distant star-forming galaxies. Considering number counts and average properties of star-forming galaxies at even earlier times, we speculate that their formation history may have included a dust-obscured star-burst, possibly also forming a dense stellar core. Finally, we present a study of star-forming galaxy kinematics 11 billion years ago. Show less
Ten billion years ago the Universe was at the peak of its star formation activity, which has been declining since then. This thesis investigates, with novel spectroscopic data from Hubble Space... Show moreTen billion years ago the Universe was at the peak of its star formation activity, which has been declining since then. This thesis investigates, with novel spectroscopic data from Hubble Space Telescope, the evolution of the galaxy population from that particular period, the so-called "Cosmic Noon", to the present epoch. The main topics addressed are the contribution of emission lines to the optical light of galaxies through cosmic time, the star formation rates of actively star-forming galaxies and quenched galaxies, and the evolution of the stellar ages of galaxies from 10 billion years ago to the current time. 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
The question of how the first stars formed and assembled into galaxies lies at the frontier of modern astrophysics. The study of these first sources of cosmic illumination was transformed by the... Show moreThe question of how the first stars formed and assembled into galaxies lies at the frontier of modern astrophysics. The study of these first sources of cosmic illumination was transformed by the installation of new instrumentation aboard the Hubble Space Telescope during one of the final Space Shuttle missions in 2009. Hubble has since unveiled a population of ultra-faint galaxies seen just a few hundred million years after the Big Bang, an epoch often termed the Cosmic Dawn. This thesis presents pioneering observational studies of the first generations of galaxies, enabling an examination of their properties and the physics that governed the illumination of the early cosmos. Show less
Understanding how galaxies form in our dark matter dominated Universe is a key goal of extragalactic astronomy. Both the stellar mass function and the spatial distribution of galaxies provide... Show moreUnderstanding how galaxies form in our dark matter dominated Universe is a key goal of extragalactic astronomy. Both the stellar mass function and the spatial distribution of galaxies provide insights in the connection between dark matter and the stellar component. This thesis presents measurements on the stellar component in the most massive structures formed in the Universe, with the potential to test and further expand current physical models and thus our understanding of the cosmos. After studying 10 clusters in the distant Universe (Chapter 2-4), and 10 clusters in the local Universe (Chapter 5), we consider different evolutionary scenarios to explain the observed trends. Chapter 6 presents measurements on the very distant Universe, probing a time at which these massive structures have not yet formed. Show less
Galaxies have changed drastically over the past 10 billion years. This thesis deals with these changes, focusing on evolution in the structure of very massive galaxies with a range of stellar... Show moreGalaxies have changed drastically over the past 10 billion years. This thesis deals with these changes, focusing on evolution in the structure of very massive galaxies with a range of stellar population properties. The main subjects addressed are the rapid changes in the sizes of old galaxies, the gradients in stellar population content within galaxies, and the predictions from theoretical models regarding these properties. Show less
Galaxies with all their varieties, have been home to billions of stars during their life. It is because of the presence of these shining stars that we are able to observe them through the cosmic... Show moreGalaxies with all their varieties, have been home to billions of stars during their life. It is because of the presence of these shining stars that we are able to observe them through the cosmic time. Although we observe galaxies mostly through the light emitted by their stars, we cannot resolve these stars individually unless they are very close by. Because of this, the cumulative light from billions of stars in every galaxy is analyzed using stellar population models to extract information about the evolution of galaxies. Stellar light does not reach us without passing through the interstellar medium (ISM) which contains clouds of gas and dust particles. Gas and dust can absorb and re-emit the light from stars, or scatter it towards us and make interpreting what we observe in galaxies very complicated. Despite all these difficulties, just by analyzing the total light from galaxies, we can constrain the global physical properties of galaxies such as stellar mass, star formation rate and age, based on the stellar population models. By combining stellar population models and photoionization models we can further analyze the emission line spectrum of star-forming galaxies coming from ionized gas around young stars which provide us with a wealth of information about the small-scale properties of galaxies e.g., the ISM. This thesis is an attempt in understanding the relation between these small-scale properties and global properties of star-forming galaxies over cosmic time using stellar population synthesis models and photoionization models. Show less
One of the important properties of galaxies is their sizes which correlate with their stellar masses. Evidence is provided by many recent studies that the sizes of galaxies were smaller at higher... Show moreOne of the important properties of galaxies is their sizes which correlate with their stellar masses. Evidence is provided by many recent studies that the sizes of galaxies were smaller at higher redshifts compared to galaxies of similar mass in the local Universe. It is essential to understand which physical processes could explain the evolution of galaxy properties and the growth of the galaxies with time. Among different proposed scenarios, each mechanism could affect differently on the growth of stellar mass and hence sizes of galaxies. Consequently, determining these properties is essential to constrain the plausible model(s) for describing galaxy evolution. In this thesis, I will present the study of the mass-size evolution of galaxies from z=7 to z=1. I will also present the stellar mass-size relation for nearby galaxies (z=0), for samples selected using different criteria down to the stellar mass of 10^{9} solar mass. The robustness of size measurement techniques for z=0 and z=1 galaxies is also studied in details. Show less
Galaxies grow by accreting gas, which they need to form stars, from their surrounding haloes. These haloes, in turn, accrete gas from the diffuse intergalactic medium. Feedback from stars and black... Show moreGalaxies grow by accreting gas, which they need to form stars, from their surrounding haloes. These haloes, in turn, accrete gas from the diffuse intergalactic medium. Feedback from stars and black holes returns gas from the galaxy to the halo and can even expel it from the halo. This cycle of gas inflow and outflow, its impact on star formation, and the detectability of the gas outside of galaxies are discussed in this thesis. The growth of galaxies and their gaseous haloes depends strongly on their mass, the age of the Universe, and the inclusion of feedback processes, as do their physical and observational properties. Show less
Galaxy clusters are the densest large scale structures in the Universe. This makes them ideal laboratories for studying the effect of environment on the formation and evolution of galaxies. To... Show moreGalaxy clusters are the densest large scale structures in the Universe. This makes them ideal laboratories for studying the effect of environment on the formation and evolution of galaxies. To truly understand the role of environment in galaxy evolution it is essential that galaxy clusters are studied across cosmic time, back to when the Universe was still relatively young and galaxy clusters were still in the process of formation. Finding these forming galaxy clusters (or protoclusters) in the early Universe is difficult. One of the ways to do this is by looking at the environment of high-z radio galaxies. These special, very massive galaxies are often at the centre of overdensities that are expected to grow into galaxy clusters. In this thesis I have studied these protoclusters in order to determine what the nature of these structures is and how they evolve and whether there is any indication of environmental in fluence on the protocluster galaxies at such early times. I have also attempted to identify new protoclusters for future research. Show less
Although we have a broad picture of how the universe has evolved from the Big Bang more than 13.7 billion years ago, the details of the formation and evolution of galaxies are still not well... Show moreAlthough we have a broad picture of how the universe has evolved from the Big Bang more than 13.7 billion years ago, the details of the formation and evolution of galaxies are still not well understood. Using a combination of optical and infrared observations called the SIMPLE survey, it is possible to picture galaxies at different stages in their evolution. It appears that the star formation rate per unit mass, the specific star formation rate, has been steadily decreasing for over the last 10 billion years. During this period it was always lower for the most massive galaxies but the rate of change does not seem to be a strong function of mass. Using the specific star formation rate, it is possible to determine the fraction of galaxies that are passively evolving. It appears that 30% of the most massive galaxies already formed the bulk of their stars 10 billion years ago and have been passively evolving at least since then. This measurement can be used as a constraint for models of galaxy evolution. A comparison between such a model and the SIMPLE observations, shows that the models can not predict the correct growth rate through star formation well enough yet. Show less
The first stars formed a few hundred million years after the Big Bang, when the Universe was only a small fraction of its present age. Their radiation transformed the previously cold and neutral... Show moreThe first stars formed a few hundred million years after the Big Bang, when the Universe was only a small fraction of its present age. Their radiation transformed the previously cold and neutral hydrogen that filled intergalactic space into the hot and ionised cosmic plasma that is observed today. This milestone in the history of the Universe is called the epoch of reionisation. Much about reionisation is still unknown. Computer simulations are one of the most promising theoretical tools to study reionisation. The wealth of high-quality data that will soon be provided by the next generation of telescopes, specifically designed to observe the reionisation event, make it a particularly exciting time to perform such simulations. The thesis "Simulating Cosmic Reionisation" presents TRAPHIC, a novel method to include the transport of ionising radiation emitted by the first stars in simulations of reionisation. TRAPHIC (TRAnsport of PHotons In Cones) is one of the first of a new type of radiative transfer methods that allow the accurate and efficient computation of the growth of ionised regions in representative models of the Universe that contain hundreds of millions of stars. First simulations that employ TRAPHIC on the Dutch national supercomputer Huygens demonstrate the importance of the concepts that underly its design. Show less
