Investigating the Epoch of Cosmic Reionisation with Radio Interferometers

Abstract

The Epoch of Reionisation (EoR) marks the cosmic period during which the neutral hydrogen (H I) that pervaded the Universe transitioned to an almost entirely ionised state. This process was driven by the energetic photons emitted by the first luminous sources. As such, the EoR encodes crucial information about the nature of the ionising sources, the early formation of structure, and the thermal history of the Universe. The most promising probe of the EoR is the redshifted H I 21 cm line, which has the potential to characterise the 3-dimensional distribution of neutral hydrogen. The statistical detection of its spatial fluctuations is a prime objective of modern low-frequency radio interferometers, though instrumental systematics and calibration errors pose an immense challenge. This thesis explores a method of detecting the fluctuations of the cosmological signal by means of the closure phase, a calibration-independent interferometric quantity. The existing theoretical framework of this technique is extended by a rigorous derivation of the relationship between closure phases and the power spectrum of the cosmological signal. Furthermore, the sensitivity of the approach is investigated and a treatment of the effects of foreground emissions and common instrumental systematics on the closure phase is provided. The technique is applied to a full season of HERA observations to obtain deep upper limits on the cosmological H I 21 cm power spectrum. The approach is further validated using simulations. Additionally, this thesis seeks reionisation-era quasars suitable for measuring the cosmological H I 21 cm signal in absorption to their bright radio emissions. Radio data from a sample of 138 optically confirmed high-redshift quasars observed by the VLA is imaged. This search yields one new radio detection, albeit one that is too weak for follow-up 21 cm spectroscopy. The survey is further used to constrain the fraction of radio-luminous quasars and investigate its evolution with redshift, and the average properties of the undetected population are characterised using a stacking technique. Together, these efforts extend the required groundwork for detecting the cosmological H I 21 cm signal from the EoR.