A thesis submitted September 3, 2013 for the degree of Doctor of Philosophy and defended December 2, 2013.
The PhD School of Science
Faculty of Science, Niels Bohr Institute, Dark Cosmology Centre, University of Copenhagen Denmark
Pieces to the puzzle of high-redshift galaxies falling into place
Our Universe is filled with a mind-blowing diversity and different types and appearances of galaxies. Finding out about how they formed and evolved is one of the most challenging tasks in astronomy. When looking about 10 billion years back, to an epoch about 3 billion years after the big bang, we can see galaxies at earlier stages of their lives. In this thesis studies of different kinds of galaxies in the early universe are presented. Two examples of the very intriguing population of massive quiescent z~2 galaxies were analyzed in terms of their stellar populations and morphologies. Although during the past decade big steps forward have been made in the study of this particular population of galaxies, key questions about their formation and evolution remain unsolved and the observational sample is still small, especially for galaxies at the faint end of the luminosity function. To make spectroscopic investigations easier for those typically very faint objects we make use of the biggest available “telescopes” in the universe: We search for red z~2 galaxies whose apparent brightnesses have been boosted by the Gravitational Lensing effect of intermediate redshift galaxy clusters with available mass models. Our findings indicate older ages for these galaxies than expected. Also, their remarkable compactness was corroborated. Further conclusions such as about the validity of the downsizing paradigm are not possible given the very small sample of just two such galaxies. Furthermore, we present a study of another class of high-redshift galaxies which came into the limelight during the last decade: Star-forming galaxies with relatively small stellar masses but large surrounding hydrogen reservoirs, which can only be found and studied against the background light of a luminous quasar in the line of sight. We studied a special case of these so-called Damped Lyman-α Absorbers (DLAs), with two intervening galaxies in the line of sight of a higher-redshift QSO, which is also one example of only about a dozen known galaxy counterparts of a DLA. It fits into the emerging paradigm that galaxies which are responsible for higher metallicity DLAs are more massive and luminous thantypical DLA galaxies. Finally, we present a study of another star-forming galaxy, which was selected as the host of a Gamma Ray Burst (GRB). It revealed a relatively high metallicity, contrary to the common perception of GRB hosts as metal-poor galaxies, indicating that GRBs are less biased tracers of star-formation at higher redshifts than previously thought. Each of the presented studies of high-redshift galaxies constitutes pioneering work, opening a window through which we might one day see a coherent picture of the formation and evolution of galaxies in the history of the universe. We can also conclude that although we gained valuable insights into both the star-forming as well as quiescent population of galaxies at high redshifts, new questions have emerged, which we will have to tackle in future studies.