UCLA

While star-forming galaxies are likely the dominant force behind cosmic reionization, the relative contributions of different galaxy populations remains uncertain. Understanding how ionizing photons are produced and subsequently escape from these galaxies in the early Universe is crucial for reducing such uncertainties in existing models. In this dissertation, I conduct four observational studies of star-forming galaxies at z~3-5 to build a more complete picture of ionizing-photon production and escape. These works utilize observations of the rest-UV portion of the electromagnetic spectrum to examine the interplay between massive stars, neutral gas, and dust in star-forming galaxies. These studies are conducted at the highest redshifts feasible for this type of analysis by avoiding complete attenuation of relevant observational signals from the neutral intergalactic medium. I present the redshift evolution of rest-UV spectral features out to z~5 using a data set from the DEIMOS 10K Survey, alongside comparisons to similar analyses performed at z~2-4. I find a relationship between Lya emission line strength and low-ionization absorption line strength and conclude that the neutral-gas covering fraction of a galaxy simultaneously modulates the two. The redshift evolution of this relation indicates that the ionizing photon production efficiency xi_ion of this z~5 sample is elevated as compared to z~2-4 samples. I examine the effects of foreground contamination in direct measurements of the Lyman continuum (LyC) at z~3 using data from the Keck Lyman Continuum (KLCS) Survey and novel Hubble Space Telescope imaging. After the removal of contamination, I measure an average escape fraction of ionizing radiation of f_esc=0.06+-0.01 at z~3, and present 13 confirmed, individual LyC leakers. Using this cleaned KLCS sample, I measure trends between f_esc and different galaxy properties in order to elucidate which galaxy populations contribute most strongly to the ionizing emissivity during reionization, while simultaneously determining which galaxy properties are most appropriate to use as a proxy of f_esc at z>6. I attempt to measure the correlation between f_esc and star-formation rate surface density, an important diagnostic of f_esc found in the local Universe, and make recommendations for the total sample size required to determine whether this correlation exists out to z~3. I present a negative correlation between f_esc and stellar mass, indicating that lower-mass galaxies (<~10^9 M_sun) likely drive reionization. I also find a negative correlation between f_esc and E(B-V), underlining the importance of the configuration of neutral gas and dust in determining the escape fraction of an individual galaxy. Finally, I present a lack of significant trend between f_esc and stellar age or specific star-formation rate, indicating that recent star-formation does not strongly affect the conditions of the neutral-phase interstellar and circumgalactic media at the masses of the KLCS sample. Continued explorations of the physics behind f_esc and xi_ion at z~3-5 are crucial for building a comprehensive understanding of the reionization process.