Of all the mass in our Universe, 80% is thought to consist of a hypothetical and invisible substance called dark matter (DM). So far, all observations of DM are based on its gravitational... Show moreOf all the mass in our Universe, 80% is thought to consist of a hypothetical and invisible substance called dark matter (DM). So far, all observations of DM are based on its gravitational interaction, either through the dynamics of normal (baryonic) matter or through the deflection of light. The latter approach, called ‘gravitational lensing’, is a unique way to probe the distribution of DM without making any assumptions on its dynamical state, and on scales larger than the extent of baryons. Using weak gravitational lensing with the Kilo-Degree Survey (KiDS), we first study the relation between galaxies and their dark matter halos on the scale of individual galaxies and galaxy groups. We then attempt to measure the effect of the local and large scale (cosmic web) density distribution on galaxies and halos, and we measure the interplay between galactic and DM structures at the scale of the cosmic web. Finally, we perform the first test of Verlinde’s theory of Emergent Gravity, all with the ultimate goal of gleaning some insight into the possible nature of the elusive ‘missing mass’. Show less
The nature of the Dark Matter is one of the biggest open questions in modern cosmology and particle physics. The work in this thesis concerns a search for the observational effects of one... Show moreThe nature of the Dark Matter is one of the biggest open questions in modern cosmology and particle physics. The work in this thesis concerns a search for the observational effects of one particular class of hypothetical Dark Matter particles, namely those that are allowed to decay. In decaying, X-ray photons are emitted and should be observable. One part of the thesis details the discovery of a potential Dark Matter decay signal in X-ray spectra of galaxies and galaxy clusters, and the subsequent efforts to identify its origin. To this end archival data and new observations are compared to the respective Dark Matter masses of the observed objects. Interpretations of the signal as an instrumental effect, or due to regular astrophysical processes are unsatisfactory. Although the Dark Matter interpretation remains plausible, definitive conclusions about the origin of the signal can not be drawn yet and will require measurements by next generation observatories. The last chapter of the thesis contains the proof-of-concept of a novel technique to search for such weak signals that combines increased statistical power with the ability to determine the physical origin of a signal, while avoiding some of the disadvantages of traditional methods. Show less
Galaxy clusters are the largest reservoirs of matter in the Universe, and as such are unique laboratories to understand the connection between dark and luminous, 'normal' matter. We use... Show more Galaxy clusters are the largest reservoirs of matter in the Universe, and as such are unique laboratories to understand the connection between dark and luminous, 'normal' matter. We use several techniques and galaxy cluster samples to study this connection from various angles. In particular, we try to understand how does the motion of galaxies within clusters relate to their luminous mass content; how do the shapes of galaxies respond to the strong gravitational potential of their host cluster (analogous to tidal waves produced by the Earth-Moon gravitational interaction), and how much of their total mass are galaxies able to retain once they fall under the influence of their host cluster, in connection with the same interactions. Our results provide important information for models of galaxy formation and evolution, particularly their dark matter content, and for studies trying to link observations of galaxy clusters to the overall properties of the Universe such as its total matter content. 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
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
Our Universe is comprised not only of normal matter but also of unknown components: dark matter and dark energy. This Thesis recounts studies of dark matter haloes, using a technique known as weak... Show moreOur Universe is comprised not only of normal matter but also of unknown components: dark matter and dark energy. This Thesis recounts studies of dark matter haloes, using a technique known as weak gravitational lensing, in order to learn more about the nature of these dark components. The haloes analysed are both those surrounding individual galaxies and those encompassing massive galaxy clusters. In order to better study these haloes, new lensing software is developed and existing lensing theory is advanced in the context of this Thesis. As a result, new higher-order signals are detected and lenses are studied in greater detail than ever before. Show less
