UCLA

Understanding the nature and evolution of the high-redshift universe is crucial in forming complete models of galaxy evolution and large-scale structure formation. In this dissertation I investigate several aspects of the high-redshift universe, including the structure of a massive galaxy protocluster, understanding the nature and interplay between massive stars and ionized gas in high-redshift galaxies. First, I present an analysis of densely sampled spectroscopic observations of galaxies within the SSA22 protocluster at z~3.09 which reveal two distinct structures separated in redshift space and segregated on the sky. An analysis of similar structures within cosmological N-body simulations reveals that such a distribution of galaxies can only be explained as two nearby overdensities which will remain distinct as they evolve to z=0. Based on the N-body simulations, I find that the opportunity to observe such a phenomenon is incredibly rare, with an occurrence rate of 7.4 h^3 Gpc^−3. In this dissertation I also investigate the differences between local and high-redshift galaxies suggested by the offset towards higher [OIII]5007/Hb and [NII]6584/Ha on the `BPT' diagram. I analyze combined rest-UV and rest-optical spectra of high-redshift galaxies. Crucially, rest-UV spectra provide a powerful constraint on the population of massive stars within high-redshift galaxies, which is an important driver powering the observed rest-optical emission lines. To investigate this origin of the offset on the BPT diagram, I construct two composite spectra composed of high-redshift galaxies at different locations on the BPT diagram. This analysis demonstrates that high-redshift galaxies that are more offset typically have younger stellar ages, lower stellar metallicities, higher ionization parameters, and are more alpha-enhanced compared to high-redshift galaxies that lie along the local sequence. In addition, this analysis reveals that even galaxies that are entirely consistent with the local nebular excitation sequence appear to be alpha-enhanced. This suggests that a similarity in the location of high-redshift and local galaxies in the BPT diagram may not be indicative of a similarity in their physical properties. I further expand upon this analysis by fitting the joint rest-UV and rest-optical properties of individual galaxies in the sample. An important aspect of analyzing individual galaxy spectra is a quantitative limit on how well we can fit the spectra. By introducing noise to model galaxy spectra which has known properties, and binding its best-fit properties, I determine that galaxy properties can be accurately reproduced if the spectrum has a SNR>4. The best-fit properties of individual galaxies in our sample reveals that they have comparable ionization parameters to those of local HII regions the share the same nebular metallicity. In addition, I find that all galaxies in the sample show evidence for being alpha-enhanced resulting in harder ionizing spectra compared to local galaxies. These results point toward the observed offset on the BPT diagram being primarily caused by a harder ionizing spectra at fixed nebular metallicity.