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
Butler, K.M.; Werf, P.P. van der; Rybak, M.; Costa, T.; Cox, P.; Weiß, A.; ... ; Valtchanov, I. 2021
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
Pannella, M.; Schreiber, C.; Elbaz, D.; Ciesla, L. 2017
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
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
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
