Date of Award

Spring 1-1-2017

Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

J. Michael Shull

Second Advisor

James C. Green

Third Advisor

Mitchell C. Begelman

Fourth Advisor

Benjamin Oppenheimer

Fifth Advisor

Kevin Stenson


The majority of baryons in the low-redshift universe are found in the intergalactic medium (IGM) and circumgalactic medium (CGM). Understanding the thermal and ionization state of this gas can provide insights into numerous astrophysical problems, including the distribution of baryons in the universe, the intensity of the metagalactic ionizing UV radiation background (UVB), and the nature of the flows of gas that connect galaxies to their environments. This thesis uses cosmological simulations of the IGM and CGM to study two of these problems. The first part of the thesis focuses on constraining the nature of the low-redshift UVB. By comparing simulations run with a range of UVB prescriptions to observations of the column-density distribution of H I absorption lines, we find that the z= 0 UVB has approximately twice the intensity of the commonly used Haardt & Madau (2012) background. We begin to test the assumption of a spatially uniform UVB by post-processing one of our simulations to include the effects of local density variations on the UVB intensity. We find that these local variations are unimportant for lower column density absorption lines associated with the IGM and the CGM of low-mass galaxies, but they become significant for partial Lyman-limit systems and Lyman-limit systems associated with the most massive halos.

The second portion of the thesis traces the distribution of highly ionized oxygen in circumgalactic gas out to distances of five virial radii. Ionization models using both collisional ionization equilibrium (CIE) and collisional + photoioinization equilibrium (C+P) predict distributions of O VI within the virial radius that are consistent with observations. The overall spatial distributions of O VI differ significantly between the models, with high O VI column densities extending to impact parameters of 4rvir in the CIE model, but restricted to within 1.5rvir for C+P. The C+P model also predicts high column densities of

O VIII, potentially detectable by future X-ray observations, out to distances of 3rvir. The overall broad distribution of ionized oxygen indicates that metal-enriched galactic outflows play an important role in the evolution of the surrounding IGM.