The interaction between galaxies and the surrounding gas plays a key role in galaxy formation and evolution. Feedback processes driven by star formation and active galactic nuclei facilitate the exchange of mass and energy between the galaxy and the circumgalactic medium through inflowing and outflowing gas. These outflows have a significant impact on the star formation rate and metallicity of the galaxy. Observations of outflows have provided evidence that these outflows are multi-phase in nature, identifying both low energy ions such as Mg II and C III and high energy ions such as O VI. The underlying physics maintaining the two phases as well as the ionization mechanism for these phases remains unclear. In order to better understand galactic outflows, hydrodynamic simulations are used to study the evolution of wind-cloud interactions. In this work, I carried out a suite of magnetohydrodynamic simulations to characterize the influence of magnetic fields on the evolution and lifetime of cold clouds. I found magnetic fields either provided little improvement to cloud stability over other influences such as radiative cooling or accelerated cloud disruption by pushing cloud material in the direction orthogonal to the wind and magnetic fields. To investigate the ionization mechanism of the material within outflows I first considered estimating the column densities of various ions within wind-cloud simulations with the post-processing tool Trident. Under the assumption of ionization equilibrium, the simulations did not reproduce the observed absorption profiles demonstrating the need for a more detailed treatment of the ionization processes. I then performed a new set of simulations with the non-equilibrium chemistry solver, MAIHEM. The column densities produced in the non-equilibrium model alter the evolution of the cloud and highlight the increased ionization along the boundary of the cloud.

Reionization is the phase transition of intergalactic atoms from being neutral to

becoming fully ionized. This process began ∼400 Myr after the Big Bang, when the first

stars and black holes began emitting ionizing radiation from stellar photospheres and

accretion disks. Reionization completed when all of the neutral matter between galaxies

became ionized ∼1 Gyr after the Big Bang, and the Universe became transparent as

it is today.

Characteristics of the galaxies that drove reionization are mostly unknown. The

physical mechanisms that create ionizing radiation inside these galaxies, and the

paths for this light to escape are even more unclear. To date, only a small fraction of

the numerous searches for this escaping light have been able to detect a faint signal

from distant galaxies, and no consensus on how Reionization was completed has been

established.

In this dissertation, I discuss the evolution of the atomic matter between galaxies

from its initially ionized state, to its current re-ionized state, potential sources of

re-ionizing energy, and the theoretical and observational status of the characteristics of

these sources. I also present new constraints on what fraction of the ionizing radiation

escapes from galaxies using Hubble Space Telescope UV imaging, theoretical models

of the stellar and accretion disk radiation, and models of the absorption of ionizing

radiation by the intergalactic medium.