UC Riverside

The reionization of neutral hydrogen in the intergalactic medium (IGM) represents the last major phase transition in our universe's history. The timing and sources responsible for reionization carry important implications for the formation and evolution of early galaxies. This dissertation focuses on the IGM during the later stages of reionization using hydrogen absorption lines in spectra of QSOs at redshift 5.0 ≲ z ≲ 6.5. In our search of 55 high-quality QSO spectra from Keck/ESI and VLT/X-Shooter for dark gaps (where >95% of Lyα photons are absorbed), we discovered that long dark gaps (length L≥30h−1Mpc) extend down to z≈5.4. Our comparison of the dark gap distribution in observed spectra with mock data suggests that the scenario where reionization completes by z=6 with a homogeneous ultraviolet background (UVB) is disfavored. However, models featuring neutral islands and/or large-scale UVB fluctuations persisting down to z≲ 5.5 align with our findings. We also report the discovery of an extremely long dark gap in the Lyβ forest, persisting down to z∼5.5. We derive the upper limits on IGM neutral fraction from dark gap length distributions based on a conservative model. The high fraction of QSO spectra exhibiting long Lyβ dark gaps further suggests that the IGM could remain considerably neutral (approximately 15%) as late as z=5.8. We also directly measure the mean free path of ionizing photons (mfp) over 5