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Abstract

At high-redshifts z>= 6, the neutral hydrogen column densities in the IGM are so large that almost all of the Lyman-alpha photons around the resonance are absorbed. At those redshifts, the Lyman-alpha damping wing signals have proven to be instrumental in studying the epoch of reionization. With the recent advancements in the discovery and measurement of high-redshift sources (galaxies and quasars), it is becoming ever more crucial to explore what this new set of sources might reveal regarding the reionization history and its topology. Hence, in this thesis, we aim to comprehensively study signatures of astrophysical, IGM, and reionization source parameters on the reionization topology as seen through the spectra of an ensemble of damping wing profiles. For this, in chapter one, using 21cmFAST we generated the reionization models subjected to a large set of astrophysical parameters. We found that the neutral fraction, x_HI, quasar lifetime, t_q, quasar host halo mass, M_QSO, and minimum halo mass that can support star formation, M_min, significantly impact both the median signal and the scatter of the ensemble of damping wings. But just the idea of an ensemble is not sufficient. We need to quantize it and confirm if such an ensemble is observationally possible. That's why, in the second chapter, we studied the constraining power of damping wings over the parameters filtered in chapter one. We showed that the constraints provided by only 64 quasars at z= 7, x_HI = 0.5^(+/-0.02), log(M_min/ M_sun) = 8.78^(+/-0.53), log(t_q/yr) = 6.0^(+/-0.12), and log(M_QSO/ M_sun)= 11.52^(+/-0.32) are comparable to the results from other observables like 21cm signal, and our methodology works for as low as 32 quasars. Even though this proved the strength of our damping wing analysis, it did not show that of our suite of models. Hence, using the similar reionization models developed in chapters one and two, in chapter three, instead of calculating Lyman-alpha optical depth, we looked at the Lyman continuum optical depth at z = 6. Subsequently, we studied the ionizing photons' mean free path (MFP) dependency on the filtered set of parameters from chapter one. We then plotted our models of transmission flux in the Lyman continuum regime against the stacked quasar spectra to estimate the range of these parameters that best fit the spectra. Our results imply that to explain the short MFP from the late reionization models, we would need x_HI >= 0.35 and a very short t_q of 10^4 yrs, or x_HI ~ 0.5 for t_q = 10^6 yrs. Thus, indicating that the late reionization models are insufficient to explain the short MFP of ionizing photons, we need some additional Lyman limit systems within the quasar proximity zone to explain the short MFP of the ionizing photons at z = 6.