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Abstract
This dissertation investigates gas surrounding galaxies. We started by running hydrodynamic simulations with the FLASH computational code. We modeled how cold, dense clouds mix with hot, diffuse ambient material, using a variety of initial parameters based on observations. Then, we developed a method to calculate the amount of material in observed clouds, from the observed O VI column density and our calculated O VI ionization fraction. We found that the O VI ionization fraction calculated with our non-equilibrium ionization methodology is much lower than that found from the standard approximation, which is the peak value from a collisional ionization equilibrium model. The lower O VI ionization fraction suggests that the clouds contain a higher amount of material than existing estimates. In addition, the simulations show that O VI exists in a wider temperature range than expected from the standard approximation. To further study the wide distribution of O VI over the temperature range, we examined spectra from our simulated clouds. We created synthetic O VI absorption spectra, from which we derived the Doppler broadening parameters (b values). We found a large fraction of relatively small b values, which implies a substantial amount of O VI in cool gas. This result is consistent with our earlier finding of O VI in cool regions. We also extended our analysis of ionization fractions by adding carbon and silicon to our simulations and found that the CIV and SiIV ionization fractions are also much lower than determined from the standard approximation. During this process, we also studied O VI emission from the edge region of starburst galaxies.