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Abstract

The lifetime of quasars is a fundamental parameter for understanding the physical processes governing the growth of supermassive black holes, their co-evolution with galaxies, and the reionization history of the intergalactic helium. However, despite cosmological importance, current estimates of quasar lifetime are uncertain by at least two orders of magnitude, preventing significant progress in our understanding of the above questions. This thesis presents theoretical progress toward constraining quasar lifetimes and properties of helium reionization. A combination of cosmological hydrodynamical simulations and 1D radiative transfer algorithm is used to investigate the structure and evolution of the He II Lya proximity zones around quasars at redshift z = 3-4 and draw the following conclusions.

In the first part of this thesis I show that the time evolution of the proximity zones can be described by a simple analytical model for the approach of the fraction of singly ionized helium (He II fraction) to ionization equilibrium, and use this picture to illustrate how the transmission profile depends on the quasar lifetime, quasar luminosity, and the the average He II fraction (or equivalently the metagalactic He II ionizing background). Due to density fluctuations I advocate stacking existing He II quasar spectra at z ∼ 3, and show that the shape of this average proximity zone pro- file is sensitive to lifetimes as long as ~ 30 Myr. I find that at higher redshift z ~ 4 where the He II fraction is poorly constrained, degeneracies will make it challenging to determine these parameters independently.

In the second part of this thesis I describe the method to constrain the quasar lifetime and the He II fraction (redshifts of the He II reionization) at z ~ 4 using the thermal state of the intergalactic medium around quasars. I show how the temperature of intergalactic gas increases due to quasar activity, producing thermal proximity effect in the Lyα absorption spectra. I investigate how the amplitude and the extent of thermal proximity effect depend on the amount of singly ionized helium in the IGM prior to quasar activity and on quasar lifetime by measuring the power spectrum of the H I Lya absorption spectra. I propose to use Markov Chain Monte Carlo algorithm to estimate the accuracy of the power spectrum measurement, and illustrate how with the mock sample of 50 high-resolution quasar spectra the He II fraction and quasar lifetime can be estimated with ~ 5 - 10% and 2 - 5% precision, respectively.