Understanding cosmic reionisation

This proposal is focused on improving our understanding of a crucial epoch in the early history of the Universe, referred to as cosmic

reionisation. This is a period in the Universe's history when the neutral Hydrogen gas that permeates inter-galactic space was fully

ionised by high-energy ultraviolet (UV) photons. The source of the UV photons is believed to be luminous, hot young stars within the first generation of galaxies. Consequently, studying the epoch of reionisation and studying the formation and evolution of the first generation of galaxies, is one and the same.

Although we have a rough understanding of when reionisation occurred, and the sources responsible, there are still many unanswered questions regarding exactly how reionisation progressed. For example, we are still unsure about whether reionisation was driven by a multitude

of faint galaxies, or a much smaller number of rarer, massive galaxies. This proposal aims to tackle these issues by identifying and studying galaxies within the epoch of reionisation itself, together with studying similar galaxies at later stages in the Universe's history, which are easier to study in fine detail.

By using the latest data from the James Webb Space Telescope (JWST), in combination with the highest-quality ground-based data, the project aims to firstly improve our knowledge of how many ionising UV photons early galaxies can produce. Secondly, and just as importantly, the project also aims to determine the fraction of these ionising photons that are able to escape their host galaxy and ionise the inter-galactic gas (the so-called escape fraction).

Finally, and in tandem with the first two science goals, the project also aims to better understand the impact of dust attenuation in the early universe, a key systematic uncertainty in current efforts to pin-down the UV photon production efficiency and escape fraction. By tackling these three issues together, the overall goal is to use the latest observational data to reach an improved understanding of the physics of cosmic reionisation.