We live in a magnetic universe with magnetic fields spanning an enormous range of spatial and temporal scales. In particular, magnetic fields at the scale of a galaxy are known as galactic magnetic... Show moreWe live in a magnetic universe with magnetic fields spanning an enormous range of spatial and temporal scales. In particular, magnetic fields at the scale of a galaxy are known as galactic magnetic fields and are the focus of this PhD thesis. These galactic magnetic fields are very important since they affect the dynamics of the interstellar gas as well as the gas distribution. The presence of these magnetic fields induces a certain type of radiation to occur at radio frequencies known as synchrotron radiation. The observed polarization properties of this synchrotron radiation then serves to record the imprint of these magnetic fields. The goal of this thesis has been to infer the structure of the magnetic field across various spatial scales in our own Galaxy as well as the strength and structure of the magnetic field in other galaxies using radiopolarimetric observations. 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
Radio observations provide a unique view of black holes in the Universe. This thesis presents low frequency radio images and uses the radio sources in those images to study the evolution of black... Show moreRadio observations provide a unique view of black holes in the Universe. This thesis presents low frequency radio images and uses the radio sources in those images to study the evolution of black holes and galaxies through the age of the Universe. Show less
In this thesis we used numerical simulations to explore the role that chemistry plays in galaxy formation. Simulations of galaxies often assume chemical equilibrium, where the chemical reactions... Show moreIn this thesis we used numerical simulations to explore the role that chemistry plays in galaxy formation. Simulations of galaxies often assume chemical equilibrium, where the chemical reactions between ions and molecules have reached a steady state. However, this assumption may not be valid if the physical conditions of the gas are evolving rapidly. Therefore, we developed a chemical model to follow the non-equilibrium evolution of ions and molecules. We then incorporated this model into hydrodynamic simulations of galaxies. We ran simulations with different metallicities (i.e. different proportions of heavy elements) and UV radiation fields, first using our full non-equilibrium chemical model and then assuming chemical equilibrium. We found that the total star formation rate is higher at higher metallicity and for weaker radiation fields. In contrast, non-equilibrium chemistry does not strongly influence the total star formation rate or outflow properties of the galaxy. However, it does affect the abundances of individual chemical species, for example in molecular outflows. Finally, we explored the properties of molecular clouds in our simulations. At low metallicity, the molecular fraction of young clouds tends to be below equilibrium, as the molecules are still forming. This also affects the observable CO emission from young clouds. Show less
The gas around galaxies provides fuel for star formation, playing a crucial role in the formation and evolution of galaxies. However, because the gas is very diffuse, it is difficult to observe in... Show moreThe gas around galaxies provides fuel for star formation, playing a crucial role in the formation and evolution of galaxies. However, because the gas is very diffuse, it is difficult to observe in emission, so in this work we examine it by analyzing absorption lines in the spectra of bright background sources. Our observational results are also compared with current cosmological simulations. Show less
De meeste materie in ons Universum is donker. Deze donkere materie vormt de bouwsteen van de grootschalige, kosmische structuren, waarin sterrenstelsels leven. Door zijn botsingloze natuur is... Show moreDe meeste materie in ons Universum is donker. Deze donkere materie vormt de bouwsteen van de grootschalige, kosmische structuren, waarin sterrenstelsels leven. Door zijn botsingloze natuur is donkere materie namelijk beter in staat structuren te vormen dan normale (__baryonische__) materie. Deze structuren bestaan uit vlakken, filamenten en knopen, die samen ook wel het kosmisch web worden genoemd. Sterrenstelsels bewonen de centra van grotere "halo__s" van donkere materie. Deze halo__s zijn zelf niet zichtbaar en het licht uitgezonden door sterrenstelsels kan ons alleen iets vertellen over het binnendeel van deze halo__s. In dit proefschrift trachten we meer over halo__s te weten te komen. Hiertoe maken we gebruik van kosmologische, hydrodynamische simulaties, waarin we niet alleen de donkere maar ook de zichtbare materie meenemen, alsmede alle processen die gedacht worden belangrijk te zijn voor de vorming en groei van sterrenstelsels. Dergelijke simulaties bieden ons de mogelijkheid om het verband tussen zichtbare en donkere materie te verkennen, aangezien beide componenten tegelijk en zelfconsistent worden gesimuleerd. In waarnemingen kan dit verband onderzocht worden door gebruik te maken van zwaartekrachtlenzen. De werking van dergelijke lenzen is gebaseerd op de afbuiging van fotonen (lichtdeeltjes) wanneer deze door een zwaartekrachtspotentiaal reizen. Zodoende ondervindt licht dat van ver in het heelal naar ons toe reist, onderweg verschillende kleine afbuigingen. Als gevolg hiervan zien wij het beeld van de bron als verplaatst, vergroot en verstoord. Het zwaartekrachtlenseffect kan gebruikt worden om verschillende eigenschappen van (materie in) het Universum te meten, waaronder de totale massa en het massaprofiel van halo__s, de vormen van halo__s, de effici_ntie van de vorming van sterrenstelsels en uiteindelijk ook de fundamentele kosmologische parameters van ons Universum. Door gebruik te maken van kosmologische, hydrodynamische simulaties kunnen we ook mogelijke effecten onderzoeken die ons ervan weerhouden om zwaartekrachtlenswerking te gebruiken om de fundamentele eigenschappen van de structuren waaruit ons Universum is opgebouwd, te meten. Show less
This thesis presents a study of how science can most effectively be used to engage and educate the global public and specifically describes the role of astronomy in doing this. Astronomy has a... Show moreThis thesis presents a study of how science can most effectively be used to engage and educate the global public and specifically describes the role of astronomy in doing this. Astronomy has a special place in the field of science education and public engagement with science. It has great appeal for large sections of the public for several reasons. We shall use astronomy as a case study to consider the effect and impact of transnational collaborations with innovative approaches and centralised coordination in science education and public outreach. The thesis is based on eight years of designing, implementing and evaluating transnational collaborative programmes in astronomy education and public outreach, from the perspective of the practitioner. We shall also show that large global science EPO projects can result in sustainable outcomes that outlive the projects themselves and analyse the various aspects of global science communication project that are necessary for their success. Show less
Ever since Isaac Newton in 1687 posed the N-body problem, astronomers have been looking for its solutions in order to understand the evolution of dynamical systems, such as our own solar system,... Show moreEver since Isaac Newton in 1687 posed the N-body problem, astronomers have been looking for its solutions in order to understand the evolution of dynamical systems, such as our own solar system, star clusters and galaxies. The main difficulty is that small errors grow exponentially, so that numerical solutions diverge easily from the mathematical solution. This thesis presents two new state of the art N-body algorithms, one of which is designed for high precision (Brutus) and the other for speed (Sakura). The assumption that N-body results are accurate in a statistical sense, is put to the test for three-body configurations. Finally, a new mathematical model is constructed that describes the origin of chaos in a dynamical systems, and explains the short Liapounov time of Comet Halley's orbit. Show less
This thesis discusses the structure of gas and dust in protoplanetary disks around young stars, in which the planets are formed, using ALMA (Atacama Large Millimeter/submillimeter Array)... Show moreThis thesis discusses the structure of gas and dust in protoplanetary disks around young stars, in which the planets are formed, using ALMA (Atacama Large Millimeter/submillimeter Array) observations. Primary targets of this study are the so-called 'transition disks', with a central cavity in the dust disk. A possible explanation for the presence of this cavity is the recent formation of a young planet which has cleared its own orbit. ALMA can for the first time zoom in onto the structure of both gas and dust and answer this question. The thesis presents the first ALMA observations of cold molecular gas and dust in transition disks. These data show that millimeter-dust grains are concentrated in a 'dust trap', allowing the dust particles to grow to larger sizes, an important step in the planet formation process. Also, it turns out that gas is still present in the dust cavity of the disks in this study, its structure points indeed towards the planet clearing mechanism. These discoveries form a giant leap in our understanding of planet formation. In the coming years, ALMA will be completed and allow us to see even smaller details in these disks, possibly even the planets itself. 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
This thesis focus on the study of the Interstellar Medium (ISM) of the Milky Way and consists of two parts: in the first one we present a study of the dust properties in HII regions and their... Show moreThis thesis focus on the study of the Interstellar Medium (ISM) of the Milky Way and consists of two parts: in the first one we present a study of the dust properties in HII regions and their surrounding PDRs. We focus our studies on two compact HII regions: W3(A) and the Orion Nebula (Chapters 2 and 3, respsectively). Using SOFIA/FORCAST observations, we determine the properties of dust in the ionized gas, their surrounding PhotoDissociation Regions and their parent molecular clouds. One of the most important conclusions of this thesis is that the grain growth can affect the dust size distribution in regions of massive star formation. The large amount of archival data available for Orion allow us to study other important phenomena related to dust in this region, such as the photoelectric heating effect and Lyman alpha heating. The second part of this thesis presents theoretical studies of the properties of the cold neutral medium (CNM) using carbon radio recombination lines (CRRL). Chapters 4 and 5 consists on the complete theory of CRRLs from the level population equation to the radiative transfer equation. Using these models and observations of CRRLs the physical parameters of the CNM can be determined. Show less
The space between stars is filled with a dilute mixture of atoms, molecules, and dust grains, which we call the interstellar medium (ISM). The physics of the ISM is a crucial part in many areas of... Show moreThe space between stars is filled with a dilute mixture of atoms, molecules, and dust grains, which we call the interstellar medium (ISM). The physics of the ISM is a crucial part in many areas of astronomy, such as the formation and evolution of stars and entire galaxies. It regulates molecule- and dust grain synthesis, which together constitute the very building blocks of planetesimals required to form planetary systems and, ultimately, life itself. Recent observations have revealed that a significant fraction of the ISM is dynamic and filamentary, likely caused by radiation, winds, and supernova explosions from massive stars that constantly stir the material that resides in the ISM. However, the exact mechanisms and contributions of these interactions remain poorly understood. To advance our knowledge of the ISM of galaxies, in first principle, we need to acquire a deep understanding of the interplay between stars and their surroundings. In this thesis, I investigate the interactions between gas, dust, and stars in the ISM, by using the Orion region as a benchmark model. Show less
This thesis presents the results from the analysis and characterisation of the water and mid-J (J<11) 12CO, 13CO and C18O observations for a large sample of low-, intermediate-, and high-mass... Show moreThis thesis presents the results from the analysis and characterisation of the water and mid-J (J<11) 12CO, 13CO and C18O observations for a large sample of low-, intermediate-, and high-mass young stellar objects (YSOs). The studied molecular transitions have been observed with the HIFI instrument on board of Herschel Space Observatory and within the context of the Herschel key programme __WISH__. These species and transitions constitute unambiguous tracers of specific physical conditions within the inner and warmer regions of the YSO environment. The sample of sources, composed by more than 120 YSOs, covers a large range of bolometric luminosities, several evolutionary stages within the embedded phase, and different physical scales. The aim of this work is to explore the differences and similarities between low- and high-mass star-forming regions. In particular, this study focuses on investigating the physical and dynamical structure of dense warm gas within protostellar environments by characterising the velocity-resolved H2O and CO spectra in terms of line profile and line luminosity. The ultimate goal is to contribute to the understanding of the star formation process without imposing luminosity boundaries, and to put in context these processes on Galactic and extragalactic scales. Show less
In this thesis we present research on the stellar halo of our Galaxy. In particular we focus on measuring the shape and profile of the stellar halo through photometric techniques and main sequence... Show moreIn this thesis we present research on the stellar halo of our Galaxy. In particular we focus on measuring the shape and profile of the stellar halo through photometric techniques and main sequence turnoff point star counts. We also present a cross-correlation algorithm that can detect stellar overdensities in the halo (streams or satellites) in Colour Magnitude Diagrams with only two photometric filters and no control field. We use this algorithm to characterize different streams and the vicinity of Globular clusters in search for associated stellar populations. Finally we also search for new substructure in the Kilo Degree Survey fields of Data Releases 1 and 2 and find signatures of the major substructures known in the halo. Show less
In this thesis the molecular emission of species such as CO, HCN and HNC and HCO+ are used to probe and quantify mechanical heating in star-forming galaxies. In the first part of the thesis photo... Show moreIn this thesis the molecular emission of species such as CO, HCN and HNC and HCO+ are used to probe and quantify mechanical heating in star-forming galaxies. In the first part of the thesis photo-dissociation models are used to find a diagnostic of mechanical heating at the level of molecular clouds. It was shown that mechanical heating leaves a strong signature in diagnostics that involve high-J to low-J transitions. In the second part of the thesis , synthetic line emission maps of molecular species are computed for model star-forming galaxies. The emission from these maps are used as input to photo-dissociation models in-order to constrain the physical properties, such as gas density and visual extinction, of star-forming galaxies. It was demonstrated that it is essential to consider mechanical heating in modelling the emission of star-forming galaxies using photo-dominated models. Show less
This thesis is devoted to the study of regular and deuterated water in ices and on surfaces against an interstellar background. A large network for the formation of regular water has been studied... Show moreThis thesis is devoted to the study of regular and deuterated water in ices and on surfaces against an interstellar background. A large network for the formation of regular water has been studied with the use of a Kinetic Monte Carlo model. A specific reaction has been investigated as well: H2 + O -> OH + H. Furthermore, in the light of deuterium fractionation, a thermal study on deuteron scrambling in the ice has been performed: H2O + D2O -> 2 HDO. Finally, two low-temperature routes relevant to HDO formation have been investigated: H2O + OD -> OH + HDO and D2O + OH -> OD + HDO. Show less
With over 1800 exoplanets detected, we now believe the majority of stars is orbited by one or more planets. If we want to understand the observed abundance and diversity of planetary systems, we... Show moreWith over 1800 exoplanets detected, we now believe the majority of stars is orbited by one or more planets. If we want to understand the observed abundance and diversity of planetary systems, we have to understand their formation history. The first step in the planet-formation process is the collisional coagulation of microscopic dust grains into kilometer-size planetesimals (the building blocks of planetary systems). The goal of this thesis has been to achieve a better understanding of the microphysics that govern this early growth. 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
