Study of redshifted HI from the Epoch of Reionization Era

Abstract

The dark age of the universe ends with the formation of the first generation of galaxies. Ultraviolet radiation from these galaxies starts ionizing surrounding regions and eventually the ionized bubbles grow up in size and merge together until the hydrogen becomes fully ionized. This epoch marks a major phase transition of the universe and is known as ’Epoch of Reionization’ (z = 6 11). EoR signal being extremely weak (30 mK), detection through direct imaging is extremely difficult with present radio telescopes. The only way of detection is through statistical analysis which requires a stable instrument and large amount of data to reduce thermal noise, measure and subtract the foreground effects. The Murchison Widefield Array (MWA) is a low frequency radio telescope operating between 80 300 MHZ located in Western Australia. One of the key science project of this array is to detect fluctuations in the brightness temperature of the diffuse redshifted 21 cm line of neutral hydrogen from the epoch of reionization (EoR). In this thesis work we explore various modes of observations with MWA theoretically to maximize the signal to noise ratio. We have successfully developed a unique methodology to extract power spectra from radio interferometric tracking data. It has been suggested that the foregrounds can be isolated from the HI signal, yielding clean regions dominated by the HI signal. In this work, we propose a method to extract HI power spectra based on delay spectrum approach. This method is based on modeling the HI signal taking into account the impact of the ‘w term’ and the change in intensity pattern during a tracking run. The information of the decorrelation of the HI signal due to ‘w term’ variation is used as weights to cross-correlate the measured visibilities. Our method is applicable to radio interferometers with redundant baselines (e.g. PAPER) and non-redundant baselines (e.g MWA). We demonstrate our method by analysing 3 hours of MWA tracking data on the EoR1 field. We present both 2-dimensional and 1-dimensional (k) power spectra from the analysis and compare them with noise and point source simulation. The foreground free region (EoR window) is comparable to the expected thermal noise. Detection of the EoR in the redshifted 21 cm line is a challenging task. Here, we formulate the detection of the EoR signal using the drift scan strategy. In drift scan mode the pointing center is fixed at zenith and we observe a time dependent patch of sky.This method potentially has better instrumental stability compared to the case where a single patch of sky is tracked. In this work, we describe a methodology based on drift scans which exploits the correlation between visibilities measured at different times to estimate the EoR signal. In particular, our aim is to infer the efficacy of such a method for a wide field-of-view instrument such as MWA. We demonstrate that the correlation time between measured visibilities could extend up to 1 hr for an interferometer array such as the MWA, which has a wide primary beam. We estimate the EoR power based on a cross-correlation of visibilities over time and show that the drift scan strategy is capable of detecting the EoR signal with a signal to noise that is comparable/ better compared to the tracking case.We also estimate the visibility correlation for a set of bright point sources and argue that the statistical inhomogeneity of bright point sources might allow their separation from the EoR signal. 10