Search

Ultra-diffuse galaxies (UDGs) are a class of diffuse, spatially-extended galaxies. Their existence challenges the conventional framework of galaxy evolution.Outside of an isolated handful of previously known examples of UDGs, the astronomy community in the 1980s and 1990s largely failed to uncover this hidden population of low surface brightness (LSB) galaxies, halting the progress of studying galaxy formation and evolution in this extremely LSB regime. However, over the past few decades, improvements in computing capabilities, telescope technologies, and data reduction techniques have enabled the development of extremely deep imaging surveys, covering large fractions of the sky. Such surveys have achieved sufficient depth to uncover a large number of UDGs in galaxy clusters and other environments, sparking questions regarding their unique properties, their survival in some of the harshest and most extreme environments, and their formation and evolutionary mechanisms. Due to the extremely low surface brightnesses of UDGs, obtaining spectra of (and thereby distances to) UDGs becomes an arduous task. Yet, acquiring accurate distance information is of utmost importance to identify UDGs, constrain their environments, and study their properties in context with their environment. In addition, understanding the environment's role in shaping their observed properties provides constraints on proposed UDG formation scenarios. As such, throughout the work presented in this dissertation, we focused our efforts on measuring and estimating redshifts to perform statistical analysis of UDGs. Initially, we acquired spectroscopic redshifts for 5 UDGs (when only one in Coma was available at the time). We showed that most of the Dragonfly UDG sample was indeed associated with the Coma cluster. Furthermore, we compared our spectra with stellar population synthesis models and showed that the spectra of UDGs were consistent with being composed of intermediate age, metal-poor stellar popul

Quasars allow to probe the formation and evolution of supermassive black holes throughout cosmic time. It is now understood that they are intimately related to properties of their host galaxies and thus are potentially important for galaxy evolution as well. The thesis at hand investigates the luminosity distribution of quasars during the phase of rapid black hole mass growth (z=2.5-4.5) and explores black hole host galaxy co-evolution on the basis of the quasar luminosity function (QLF). We designed the Extremely Luminous Quasar Survey (ELQS) to provide a highly complete sample of very luminous quasars at z=2.5-4.5. In order to achieve a high completeness for quasars at the targeted redshifts, we developed a highly inclusive infrared color cut using the J,K photometric bands of the Two Micron All Sky Survey (2MASS) as well as the W2 photometric band from the Wide Infrared Survey Explorer (WISE) AllWISE survey. With photometric redshifts and further star-quasar classification from Random Forests, a supervised machine-learning technique, the ELQS achieves a completeness of >=75% and a selection efficiency of 80%. We have followed up 184 promising quasar candidates and identified 109 new quasars at z=2.8-4.5 with dereddened SDSS i-band magnitudes of mi<=18.0, which make up ~26% of the full ELQS quasar sample (407 objects). We calculate the bright-end QLF on the basis of the full ELQS quasar sample and find the bright-end slope to be steep. Assuming a single power law with exponential density evolution, we find the data to be well represented by a slope of beta~-4.1 and an exponent of gamma~-0.4 for the density evolution. We constrain the bright-end slope to be steeper than beta<=-3.4 at 99% confidence. We further investigate BH and galaxy co-evolution of the aggregate galaxy population by deriving black hole mass densities (BHMDs) from QLFs and stellar mass densities (SMDs) from a variety of stellar mass functions and the cosmic star formation rate. We compare the BHM

Galaxies in transition from star-forming to quiescence are a natural laboratory for exploring the processes responsible for this evolution. Using a sample of post-starburst galaxies identified to have recently experienced a recent burst of star formation that has now ended, I explore both the fate of the molecular gas that drives star formation and the increased rate of stars disrupted by the central supermassive black hole. Chapter 1 provides an introduction to galaxy evolution through the post-starburst phase and to tidal disruption events, which surprisingly favor post-starburst galaxy hosts. In Chapter 2, I present a survey of the molecular gas properties of 32 post-starburst galaxies traced by CO (1--0) and CO (2--1). In order to accurately put galaxies on an evolutionary sequence, we must select likely progenitors and descendants. We do this by identifying galaxies with similar starburst properties, such as the amount of mass produced in the burst and the burst duration. In Chapter 3, I describe a method to determine the starburst properties and the time elapsed since the starburst ended, and discuss trends in the molecular gas properties of these galaxies with time. In Chapter 4, I present the results of followup observations with ALMA of HCN (1--0) and HCO+ (1--0) in two post-starburst galaxies. CO (1--0) is detected in over half (17/32) the post-starburst sample and the molecular gas mass traced by CO declines on ~100 Myr timescales after the starburst has ended. HCN (1--0) is not detected in either galaxy targeted, indicating the post-starbursts are now quiescent because of a lack of the denser molecular gas traced by HCN. In Chapter 5 I quantify the increase in TDE rate in quiescent galaxies with strong Balmer absorption to be 30-200x higher than in normal galaxies. Using the stellar population fitting method from Chapter 3, I examine possible reasons for the increased TDE rate in post-starburst galaxies in Chapter 6. The TDE rate could be boosted due to

It has become increasingly clear that a complete picture of galaxy evolution requires a better understanding of the role of Active Galactic Nuclei (AGN). In particular, they could be responsible for regulating star formation and galaxy growth via feedback processes. There are also competing views about the main modes of stellar growth and supermassive black hole growth in galaxies that need to be resolved. With high infrared luminosities (thus star formation rates) and a frequent occurrence of AGN, galaxies selected in the far-infrared wavebands form an ideal sample to search for a connection between AGN and star formation. The first part of this thesis contains a detailed analysis of the molecular gas properties of nearby infrared luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). We find that the enhanced molecular gas density in the most IR-luminous systems can be explained by major galaxy mergers, and that AGN are more likely to reside in higher-density systems. While the frequent concurrence of AGN and galaxy mergers in ULIRGs was already established, this work provides a coherent framework that explains trends observed with five molecular gas tracers with a broad range of critical densities, and a comparison with simulations that reproduce observed molecular line ratios without invoking AGN-induced chemistry. The second part of the thesis presents an analysis of the AGN content of intermediate redshift galaxies (0.3