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Abstract

The thermal evolution of the low density intergalactic medium (IGM) is a major diagnostic tool for understanding the nature and evolution of the predominant component of baryonic matter in the universe. In this study I present different approaches for measuring the thermal state of the IGM at different ages of the universe, in order to understand how it is affected by reionization processes. The main observable used to probe the thermal state of the gas is the so-called Lyman-alpha forest. This observable consists of a series of absorption lines in the spectra of distant quasi-stellar objects (QSOs) which arise due to the presence of residual intervening neutral hydrogen in the IGM between the observer and the QSO. Decomposing the Lyman-alpha forest into discrete absorption profiles allows one to explore how the distribution of Lyman-alpha absorption line widths and column densities (b-NHI distribution) depends on the thermal state of the gas, which is characterized by a temperature-density relation. In this thesis, I quantify the parameters of the temperature-density relation using high quality UVES and HIRES QSO spectra and state of the art cosmological hydrodynamic simulations. In the first part of this study, I apply a traditional cutoff Fitting method to the b-NHI distribution of the QSO spectra. Using simulations, I calibrate how the position of the cutoff in the b-NHI distribution relates to the thermal state of the IGM. I find that the thermal evolution of the IGM shows clear signatures of He II reionization at 2 < z < 3.4. In the second part of this thesis, I present a novel statistical method for constraining the thermal state of the IGM using the full shape of the b-NHI distribution. I show that this method is more accurate and precise than the traditional cutoff Fitting approach, by applying it to mock data realizations. I confirm this by applying it to observational data at z = 2. Finally, using this novel method, I quantify for the first time the parameters of the temperature-density relation at low redshift (z = 0.1) using the b-NHI distribution, and find broad agreement with theoretical expectations. Overall, this thesis demonstrates that the b-NHI distribution is a powerful statistical tool for studying the intergalactic medium and can place strong constraints on the evolution of its thermal state.