Galaxies through cosmic time illuminated by gamma-ray bursts and quasars

Abstract

In the early Universe, most of the cold neutral gas that will later form into individual stars and galaxies is practically invisible to us. These neutral gas reservoirs can, however, be illuminated by bright cosmic lightsources such as gamma-ray bursts (GRBs) and quasars. The aim of this thesis is to use these luminous objects as tools to study the environments of intervening or host galaxy absorption systems through cosmic time. Part I of this thesis is dedicated to examining the gas, dust and metals in the immediate region surrounding GRBs. In Chapter 2, a comprehensive analysis of the host galaxy of GRB 140506A is presented. The optical afterglow spectrum of this GRB showed a very peculiar shape, likely related to an unusual dust reddening curve that extinguished the intrinsic afterglow spectrum. We were able to better characterize this unusual afterglow extinction by subtracting the host galaxy spectrum from the first epoch afterglow spectrum and showed that it can be modelled by a very steep reddening law (but not a 2175 Å dust bump as previously proposed). Since no such extinction component was observed for the average host galaxy spectral energy distribution (SED), we concluded that it was produced local to the GRB, and likely related to the interaction of the GRB with the surrounding circumburst dust. In Chapter 3, the high-ionization metal lines in a sample of GRB afterglows observed with the VLT/X-shooter are examined. We argue that in particular the N v absorption line feature is produced by the surrounding gas that has been highly ionized by the GRB. This argument is also based on the observed tentative corelation with the X-ray derived equivalent hydrogen column density, NH,X, that is also believed to originate from gas within ~10 pc from the GRB. The highly-ionized N v line transition therefore likely probes gas and metals in the circumburst environment of the GRB. Part II presents a search for and the study of cold and molecular gas in high-z GRB host galaxy absorption systems. Chapter 4 introduces a survey for neutral atomic-carbon (CI), which is used as a tracer for molecular hydrogen (H2) in the GRB host absorbers. Here, we also characterize the basic properties of the GRB CI absorbers in terms of their gas, dust and metal content and compare them to CI-bearing absorbers in quasar sightlines. We find that a higher HI column density and metallicity is required for GRB host galaxies to contain cold and molecular gas. In Chapter 5, we specifically examine the dust properties of the GRB CI absorbers and demonstrate that the characteristic 2175 Å dust extinction feature is likely produced by dust particles associated with the CI-bearing molecular cloud. Chapter 6 provides a detailed analysis of the presence of different probes of the molecular gas-phase in the ISM of GRB host galaxies such as CI and vibrationally-excited H2. Here, the typical physical conditions of the H2-bearing molecular clouds in GRB host galaxies are also derived. Part III focuses on using quasars to examine gas-rich intervening galaxies in the line of sight, with specific focus on absorption systems rich in dust and metals. It is now clear that several dust-reddened quasars have evaded identification in e.g. the extensive Sloan Digital Sky Survey (SDSS). To alleviate this dust bias we designed the new optical to near/mid-infrared selection criteria presented in Chapter 7, specifically tailored to select dust-reddened quasars at z > 2. In Chapter 8, one of the dusty quasar absorption systems identified in this survey is presented. Here, we also characterize the properties of this absorber and compare the few cases of dusty and metal-rich absorption systems to the general population of absorbers observed in quasar sightlines. Finally, in Chapter 9 we demonstrate that it is possible to define a complete and unbiased quasar selection using the astrometric data from the space-based Gaia mission. This thesis demonstrates the importance of observing large samples of GRB afterglows to 1) allow for statistical studies of the GRB phenomena itself and the associated host galaxy environments and 2) to obtain spectra of peculiar or unusual GRB afterglows, that is only observed rarely. In addition, it highlights that defining a complete and unbiased sample of quasars is vital to fully exploit the potential of quasars as probes of cosmic chemical evolution. In Chapter 10, the general work of this thesis is concluded and some future prospects are provided.

Megnið af hinu kalda, ójónaða efni í ungum alheimi, sem síðar myndar stjörnur og vetrarbrautir, er nánast ósýnilegt okkur. Þó geta björt fyrirbæri eins og gammablossar og dulstirni lýst upp þetta dimma efni. Markmið ritgerðarinnar er að nota þessi fyrirbæri sem nokkurs konar verkfæri til að fræðast um útgeimsefnið. Í fyrsta hluta ritgerðarinnar er gas, ryk og þungefni í nánasta nágrenni gammablossa kannað til hlítar. Í kafla 2 er kynnt yfirgripsmikil greining á hýsilvetrarbraut blossans GRB 140506A. Litróf sýnilegra glæða blossans var mjög óvenjulegt, líklega tengt sérstæðri geimroðnun. Okkur tókst að sýna fram á að þessi sérkennilega ljósdeyfing ætti upptök nálægt blossanum og væri afleiðing af víxlverkun blossans við sitt nánasta umhverfi. Í kafla 3 er safn blossa skoðað sem rannsakað var með nýlegum litrófsmæli á Very Large Telescope (VLT). Hér var áhersla lögð á gleypilínur frá mikið jónuðu gasi. Við sýndum fram á að þessar línur eru afleiðing blossans og notuðum auk þess röntgenmælingar til að styrkja þær niðurstöður. Í öðrum hluta er lögð áhersla á rannsóknir á sameindagasi við hátt rauðvik. Í 4. kafla er hlutlaust kolefni (Ci) kortlagt en það má nota sem nokkurs konar sporefni fyrir vetnissameindina H2. Grunneiginleikar viðkomandi gass voru rannsakaðir og meginniðurstaðan var sú að hýsilvetrarbrautir gammablossa verða að hafa hátt hlutfall þungefna til að geta innihaldið kalt sameindagas. Í 5. kafla er athyglinni beint að eiginleikum ryks í Ci kerfum og í 6. kafla er nákvæm greining á tilvist sporefna sameindagass í miðgeimsefni hýsilvetrarbrauta gammablossa. Í þriðja og síðasta hluta eru dulstirnin í aðalhlutverki. Þau eru hér notuð til að rannsaka kerfi sem eru rykrík og hafa hátt hlutfall þungefna. Kynnt er ný aðferð til að finna slík kerfi við hátt rauðvik sem fyrri athuganir hafa hingað til misst af. Fjallað er um slíka valforsendu, sem byggir á sýnilegum og innrauðum mælingum, í kafla 7. Eitt slíkt kerfi er svo rannsakað gaumgæfilega í kafla 8. Að lokum eru gögn frá Gaia geimsjónaukanum notuð í kafla 9 til að sýna fram á að hægt er að skilgreina óhlutdrægt safn dulstirna. Þessi ritgerð sýnir fram á mikilvægi þess að skoða stór söfn gammablossa til að 1) gera tölfræðilegar rannsóknir á þeim og hýsilvetrarbrautum þeirra sem og að 2) auka möguleikana á því að uppgötva sérkennilegt umhverfi sumra þeirra. Að auki þá undirstrika þessar rannsóknir brýna þörf á að skilgreina óhlutdrægt safn dulstirna. Þannig má nýta þau til að kortleggja aukningu þungefnis sem fall af tíma í alheiminum.