High-redshift Quasars as Probes of Cosmic Reionization

Abstract

Cosmic reionization was the last major phase transition of atomic hydrogen in the IGM, empowered by the first luminous sources in the universe. With extensive studies over the past two decades, the timeline of the second half of reionization has been moderately constrained, with a midpoint at z~7-8, and a late end at z<6. High-redshift quasars are extremely powerful tools to probe the IGM during the late stage of reionization. In addition, observations of high-redshift quasars provide deep insights into the formation and growth of early supermassive black holes that emerged during reionization. In this dissertation, I use observations of high-redshift quasars to probe the late stage of reionization, including its timeline, topology, and dominant sources, and also study properties of early supermassive black holes during cosmic reionization. First, I measure the dark fractions in the quasar Ly-alpha forest and Ly-beta forest to derive model-independent upper limits on the IGM neutral fraction at z~5-7, filled the redshift gap at z~6-7 in the timeline of reionization inferred from quasars. Second, I use JWST/NIRCam WFSS surveys in quasar fields to investigate the relation between large-scale IGM transmission and galaxies, and probe the topology of reionization; Third, I use JWST NIRCam and NIRSpec observations in a quasar field to characterize the nature of ionizing sources and constrain their contribution to reionization. Together, the second and third studies reveal a complex relation between the IGM transmission and ionizing sources during reionization. Then I study quasar properties using Chandra, ALMA, and ground-based NIR spectroscopy, and investigate the relation between quasar spectral energy distribution and existence of fast outflows. I conclude by discussing future quasar observations in understanding the process of reionization using JWST.