Probing Cosmological Reionization through Radio-interferometric Observations of Neutral Hydrogen

TL;DR

This paper explores methods to study the Universe's reionization era using 21-cm hydrogen line observations, developing formalism and techniques to detect ionized bubbles and analyze the reionization history.

Contribution

It introduces a formalism for the Multi-frequency Angular Power Spectrum and a matched filter technique for detecting ionized bubbles in 21-cm maps, advancing observational strategies.

Findings

Predicted detectability of ionized bubbles with GMRT and MWA.

Analyzed impact of HI fluctuations on bubble detection.

Discussed optimal redshift and bubble size determination methods.

Abstract

The epoch of reionization is one of the least known chapters in the evolutionary history of the Universe. This thesis investigates two major approaches to unveil the reionization history of the Universe using HI 21-cm maps.The most discussed approach has been to study the global statistical properties of the reionization HI 21-cm. We develop the formalism to calculate the Multi-frequency Angular Power Spectrum (MAPS) and quantify the statistics of the HI signal as a joint function of the angular multipole l and frequency separation \Delta\nu. We adopt a simple model for the HI distribution which incorporates patchy reionization and use it to study the signatures of ionized bubbles on MAPS. We also study the implications of the foreground subtraction. A major part of the thesis investigates the possibility of detecting ionized bubbles around individual sources in 21-cm maps. We present a visibility based matched filter technique to optimally combine the signal from an ionized bubble and minimize the noise and foreground contributions. The formalism makes definite predictions on the ability to detect an ionized bubble or conclusively rule out its presence within a radio map. Results are presented for the GMRT and the MWA. Using simulated HI maps we analyzed the impact of HI fluctuations outside the bubble on its detectability. Various other issues such as (i) bubble size determination (ii) blind search for bubbles, (iii) optimum redshift for bubble detection are also discussed.