Pascal Keller (Leiden Observatory)

Abstract: The Epoch of Reionisation (EoR) marks the cosmic period during which the neutral hydrogen that pervaded the Universe transitioned to an almost entirely ionised state. The most promising probe of the EoR is the redshifted 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. In this talk, I will discuss a calibration-independent method of detecting the cosmological signal fluctuations using the interferometric closure phase. I discuss recent developments in the interpretation of the closure phase delay power spectrum and present constraints on it obtained from a full season of HERA observations.

Additionally, I present the results of a VLA L-Band survey of 138 optically confirmed reionisation-era quasars. The survey aimed at finding quasars suitable for measuring the cosmological 21 cm signal in absorption to their bright radio emissions. The survey was further used to constrain the statistical properties of the high-redshift quasar population and their cosmic evolution.

Together, these efforts extend the required groundwork for detecting the cosmological 21 cm signal from the EoR.