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 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
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
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
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
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
This thesis explores the chemistry of interstellar and circumstellar molecules during star formation and death. From the perspective of chemical physics, the most important outcome of this thesis... Show moreThis thesis explores the chemistry of interstellar and circumstellar molecules during star formation and death. From the perspective of chemical physics, the most important outcome of this thesis lies in that the rates for two important reactions are determined accurately for the first time: N2 photodissociation and reaction rates (both state-to-state and thermal) of OH with H. In particular, accurate N2 photodissociation rate and shielding functions were calculated. The results are very useful in many astrophysical fields. In addition, a new method for accurately calculating molecular shielding functions in full 3D radiation field was proposed, and was employed to give new predictions for molecule distributions in a C-rich AGB star, IRC +10216, and an O-rich AGB star, IK Tau, based on the latest progress from both observations and simulations. By far, these results are the most accurate ones, and can be directly compared to the future observations. Show less
Using n-body and stellar evolution simulations I model different star systems. Through the use of a large number of n-body simulations I model the effect of subvirial initial conditions on the... Show moreUsing n-body and stellar evolution simulations I model different star systems. Through the use of a large number of n-body simulations I model the effect of subvirial initial conditions on the evolution of star clusters and shed light on a dynamical mechanism for rapid mass segregation in young clusters. Using stellar evolution simulations I estimate there to be 10^8 intermediate mass black holes in the local universe. Lastly, I explore the effect of a supernova in a triple star system Show less
