Simulating the distribution of cosmological neutral hydrogen over cosmic times

Abstract

The reionization of the intergalactic medium is a paramount event in the evolution of the universe. In this thesis, we have studied the distribution of the neutral hydrogen in the universe at two different epochs i.e. post- reionization era and during the reionization. We have used cosmological N-body simulations to make numerical and semi-numerical calculations. In introduction chapter, we introduce the concepts needed in the thesis. We describe basic cosmology, density perturbations, power spectrum and correlation functions. We also describe about the various methods used in cosmological N-body simulations and about different codes like GADGET-2, Friends-of-Friends, N-GenIC that we have used. Chapter 2 deals with the study of the distribution of the neutral hydrogen in post- reionization era. Redshifted 21-cm signal and Lyman-α forest are two signals which we get mainly from galactic and intergalactic neutral hydrogen respectively. Though these signals arise from different astrophysical phenomena, they are expected to trace the underlying dark matter distribution at large scales. We calculate cross power spectrum using two methods i.e. Multi-frequency Angular Power Spectrum and using colour plots in (k , k K ) plane. In chapter 3, we describe the study about Hii bubble distribution during the reion- ization. We first describe the analytical models that use ‘excursion set formalism’ to calculate it. We calculate the distribution using semi-numerical formalism and compare it with analytical calculations. We observed the presence of a large ionized bubble for particular parameters, which needs further investigation. We also visualized the ionized regions to check if they match with the halos and bubbles calculated. In chapter 4, we summarise and describe the future work to be done with these projects.