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Galaxies are thought to grow through star formation that is fueled by the accretion of gas in dark matter halos and from intergalactic space. In turn, star formation and active galactic nuclei drive outflows that regulate gas cooling and accretion while enriching the surrounding gas with heavy elements. The resulting enriched gaseous halos that surround galaxies are thought to dominate the baryon reservoirs capable of fueling future growth. Developing an empirical understanding of the relationship between galaxies and this surrounding gas consequently represents a key step toward a more complete understanding of galaxy evolution. The diffuse nature of galaxy halo gas renders direct emission too faint to be detected with current facilities except in extreme cases, but the gas can be readily observed as absorption features in the spectra of UV-bright background sources such as quasars. This thesis presents a series of observational studies that provide new insights into the relationship between galaxies and surrounding diffuse baryon reservoirs through combined galaxy redshift and quasar absorption line surveys. The majority of the thesis focuses on results from deep and highly complete galaxy redshift surveys with the Magellan Telescopes in fields with high quality quasar absorption spectra from the Hubble Space Telescope that probe highly ionized diffuse gas through absorption from the H I Lyman series and metal ion transitions such as the O VI doublet. The thesis begins in Chapter 1 with historical background and motivation. Chapter 2 then presents a detailed analysis of the absorbing gas and corresponding galactic environments in the field of PKS 0405−123 which has the highest signal-to-noise ratio absorption spectrum available among intermediate redshift quasars. Though based on a single sightline, these survey data provide evidence that highly ionized gas observed as O VI absorption systems primarily trace the extended gaseous halos of gas-rich galaxies and low-mass galaxy “groups” signified by the presence of nearby star-forming galaxies. Chapter 3 presents the discovery of an unusually transparent sightline at small projected distance from a pair of interacting galaxies. The H I absorption associated with this galaxy pair falls more than two orders-of-magnitude below expected levels suggesting that interactions can significantly deplete cool halo gas reservoirs through stripping or heating. Chapter 4 presents a study of the spatial extent of H I and O VI absorbing gas as a function of both star-formation activity and galaxy environment out to 10× the galaxy host halo virial radius. Galaxies with nearby neighbors exhibit more extended O VI absorbing gas than isolated galaxies while no excess HI is observed suggesting that environment plays a role in distributing heavy element enriched gas beyond the gaseous halos of individual galaxies. In addition, Chapter 5 presents a survey of cool Mg II absorbing gas around quasar host galaxies to shed light on the relationship between cool halo gas and quasar activity while drawing insights into the environments of quasar hosts from the Magellan galaxy survey data. Together, these survey results highlight the role of environment in galaxy evolution and its possible relation with extended halo gas reservoirs. Finally, Chapter 6 summarizes the galaxy redshift and quasar absorption survey status, overviews data analysis techniques, highlights recent work confirming the results from Chapter 2 with a larger sample, and motivates future studies.


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