Quasars and H II regions during reionization

The reionization history of the intergalactic medium (IGM) at high redshift (z ≳ 6) is just beginning to be probed by observational astronomy. In this dissertation, I discuss three projects related to the epoch of reionization. The first project explores how the evolution of the IGM neutral fraction was likely shaped by feedback processes. Because the earliest ionizing sources formed at the locations of the rare density peaks, their spatial distribution was strongly clustered. By building a semi-analytical model which includes feedback and clustering simultaneously and applying this model to the suppression of star formation in minihalos due to photoionization, I demonstrate that this clustering significantly boosts the impact of feedback processes operating at high redshift.;The second project exploits the fact that high-redshift quasars drive ionization fronts (I-fronts) into the IGM, with the thickness of the front generally increasing with the hardness of the ionizing spectrum. If the thickness of the front can be measured, it can provide a novel constraint on the ionizing spectral energy distribution (SED). I simulate the propagation of an I-front into a uniform IGM, and compute its thickness for a range of possible quasar spectra and ages, and IGM neutral hydrogen densities and clumping factors. With a sufficiently high intrinsic hydrogen column density obscuring the source or a sufficiently hard power-law spectrum and some obscuration, the thickness of the front exceeds ∼ 1 physical Mpc and may be measurable from the three-dimensional morphology of its redshifted 21cm signal.;In the third project, I investigate one promising method for probing the tail end of reionization, through the detection and characterization of the Gunn-Peterson damping wing absorption of the IGM in bright quasar spectra. However, the use of quasar spectra to measure the IGM damping wing requires a model of the quasar's intrinsic Lyman-alpha emission line. I quantify uncertainties in the intrinsic line shapes and the effect of these uncertainties on the determination of the IGM neutral fraction, concluding that if the redshift of the Lyman-alpha emission line is presumed to coincide with the quasar redshift determined from metal lines, the inferred IGM neutral fraction is biased to low values due to a systematic blueshift of the Lyman-alpha line relative to the metal lines. If a similar blueshift persists in quasars at z > 6, this bias strengthens previous claims of a significant neutral hydrogen fraction at z ≈ 6.