Observational Constraints on Global 21-cm Signal from the Epoch of Reionization

Abstract

One of the most promising probes of Cosmic Dawn (CD) and Epoch of Reionization (EoR) is the rest-frame global 21-cm signal of neutral hydrogen (HI) produced by the intergalactic medium (IGM) prior to the end of the EoR at z 6. The SARAS 2 radiometer was purposely designed to detect the global 21-cm signal. In this thesis, we describe the radiometer, data modeling for foregrounds and systematics, and derive constraints on EoR from night sky data from SARAS 2 observations. We also discuss alternative means of detecting the global 21-cm signal using interferometers. We begin with an overview of the ionization and thermal history of the Universe emphasizing the physics of dark ages, cosmic dawn and epoch of reionization. We present the design philosophy of the SARAS 2 system, followed by a discussion of the SARAS 2 architecture and dedicated algo- rithms developed for data processing. Laboratory tests with the antenna replaced with a variety of terminations, including a network model for the antenna impedance, show that the gain calibration and modeling of internal additive signals leave no residuals with Fourier amplitudes exceeding 2 mK. Thus SARAS 2 system is capable of detection of complex 21-cm pro les at the level predicted by currently favoured models for thermal baryon evolution. We use 63 hr night time observing of the radio background in the frequency band 110-200 MHz at Timbaktu Collective with the radiometer to derive constraints on EoR. We employ Bayesian and frequentist approaches, and reject the theoretically allowed 21-cm signals which are inconsistent with the SARAS 2 data. We have been able to rule out roughly 10% of the theoretically plausible models for the signal, and place limits on the X-ray efficiency of the first sources and the rate of reionization. All the rejected models share the scenario of inefficient heating of the primordial gas by the first population of X-ray sources along with rapid reionization. Interferometers provide an attractive possibility of detecting global signals. We infer that (a) it is indeed possible to measure the global 21-cm from EoR using interferometers, (b) a practically useful configuration is with omnidirectional antennas as interferometer elements, and (c) that the spatial coherence may be enhanced using, for example, a space beam splitter between the interferometer elements. We conclude the thesis with the current status of 21-cm cosmology. We then summarize the salient features of the SARAS 2 radiometer and its constraints on EoR via observations aimed at detecting the global 21-cm signal. We also describe the constraints on CD/EoR placed by other experiments targeting the global 21-cm signal. We end with an indication of the path forward for SARAS based on the experience gained in this thesis work.