Intergalactic baryon enrichment and implications for galaxy evolution at high redshift

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 187-194).
In this thesis we present several surveys of heavy element absorber characteristics at high redshift, gauging properties of diffuse intra- and intergalactic gas in the first several Gyr of the Universe. At z ~ 3.5, we study chemical abundances of Lyman limit systems (LLSs) and evaluate their potential to represent expected reservoirs of cold, low-metallicity gas whose accretion onto galaxies is necessary to maintain star formation. In an initial survey focused only on LLSs identified as potential lowmetallicity absorbers, based on the absence of metal lines in low-resolution spectra, we indeed found the selected systems have low abundances, with a median of [M/H]~ -2.2 and several systems at [M/H]</~ -3, comparable to the intergalactic medium. While this result is tempting to interpret as evidence that a sizable fraction of LLSs are candidates of low-metallicity gas accreting onto galaxies, in a follow-up study of all LLSs at z ~ 3.5 we found that LLS abundances can be reasonably described by a unimodal distribution with [M/H]~ -2.5. Additional diagnostics are hence needed to evaluate if (and how many) low abundance LLSs arise from accretion flows, since the overall LLS abundance distribution overlaps heavily with the IGM at this epoch. In a seperate work that constitutes the largest study of multiple ions in individual metalline absorption systems at z >/~ 6, we find that the bulk of high-redshift absorption systems are low-ionization and low-metallicity. Since H i absorption cannot be seen at z >/~ 6, we further argue from incidence rates and absorber characteristics that these absorbers are analogous to strong neutral hydrogen systems seen at lower redshift. We conclude that the non-detection of weaker H i absorption systems (such as LLSs) is consistent with lower metallicities and lower ionization states in the circumgalactic medium than is seen at later times.
by Thomas Cooper.
Ph. D.