McQuinn, Matthew JUpton Sanderbeck, Phoebe Rae2018-11-282018-11-282018-11-282018UptonSanderbeck_washington_0250E_19260.pdfhttp://hdl.handle.net/1773/42955Thesis (Ph.D.)--University of Washington, 2018Ultraviolet light emitted by the first galaxies sparked the last major phase change of matter in the Universe. The bulk of gas in the Universe went from neutral to ionized, and as galaxies and quasars continued to replenish this light, the ionizing background settled in the space between these sources. This radiation background has the potential to impact structure in the Universe on larger scales than any other non-gravitational, astrophysical process. Because the ionizing background both affects and is sourced by galaxies and AGN, the properties of the ionizing background are tied to the history of cosmic star formation and supermassive black hole accretion. Despite its astrophysical impact, the properties and evolution of the ionizing background are poorly constrained. This thesis focuses on three different aspects of the ionizing background: how the ionizing background molds the thermal history of the intergalactic medium, the nature of the extreme ultraviolet and soft X-ray background, and the impact of fluctuations of the ionizing background on the clustering of galaxies.application/pdfen-USnonecircumgalactic mediumcosmologyintergalactic mediumionizing backgroundlarge-scale structure of universeradiative transferAstrophysicsAstronomyPhysicsAstronomyThesis