Tracing the evolution of the molecular gas content in galaxies is critical for a complete understanding of galaxy formation and evolution, as it provides the direct fuel for star formation. Studies... Show moreTracing the evolution of the molecular gas content in galaxies is critical for a complete understanding of galaxy formation and evolution, as it provides the direct fuel for star formation. Studies of high-redshift (z>1) molecular gas reservoirs, most commonly traced by carbon monoxide (CO), have seen substantial growth in recent years thanks to state-of the-art sub-millimeter interferometers such as the Jansky Very Large Array or the Atacama Large Millimeter Array. However, these studies have been largely based on detections of the bright mid/high-J transitions of CO (J>=3), which have been shown to give a biased and incomplete view of the total cold molecular gas content in galaxies. This thesis presents an observational study of the cold interstellar medium of massive galaxies in the distant universe. Chapter 2 presents one of the highest—resolution studies of low-J CO in a high—redshift galaxy to-date, allowing us to map its total cold gas reservoir in unprecedented detail. Chapter 3 presents a survey targeting low-J CO emission in high-redshift unobscured quasar hosts, finding direct evidence for high gas fractions but very short depletion times. Chapter 4 introduces first results from the VLA Legacy Library of Molecular Gas at High Redshift project, which significantly expands the number of high-redshift galaxies with cold gas detections. Finally, Chapter 5 shows ALMA observations targeting emission from atomic carbon in a subset of the VLA targets, and demonstrates how this alternative gas tracer complements our view of cold molecular gas reservoirs at high—redshift. Show less
High-angular-resolution observations of the circumstellar material have uncovered numerous and very diverse substructures in protoplanetary discs, raising the question of whether they are caused by... Show moreHigh-angular-resolution observations of the circumstellar material have uncovered numerous and very diverse substructures in protoplanetary discs, raising the question of whether they are caused by forming planets or other mechanisms. This dissertation focuses on interpreting gas substructures in discs in the context of disc winds and planet-disc interactions. A special focus is put on transition discs, which show dust (and gas) depleted inner regions and represent ideal laboratories to observe planet formation in action and test disc evolution models. Radiative transfer and hydrodynamical models are combined to investigate photoevaporative winds acting in discs in which volatile carbon is reduced. Compared to solar metallicity discs, photoevaporative winds are stronger in such carbon-depleted discs, resulting in higher mass-loss rates and profiles that extend to larger radii. This may explain more of the observed transition disc population. Furthermore, a large number of transition discs are analysed through CO ALMA observations in terms of substructures in the kinematics and brightness temperatures. In particular, two sources, CQ Tau and HD 100546, are studied in detail and the analysis reveals prominent spiral features in both discs. Together with other substructures, these point towards ongoing planet formation. Show less