The cross-correlation between reionisation and galaxies

Abstract

The epoch of reionisation (EoR) is one of the major phase transitions in the universe’s history as the first UV sources – expected to be hosted by dark matter halos which are end points of structure formation – ionise the neutral hydrogen left after recombination. Theoretically, we expect bubbles of ionised hydrogen centred at UV emitting galaxies to grow and intersect, eventually reionising the inter-galactic medium. Recent measurements from the JWST-ASPIRE program have provided the first observational signatures of these ionised regions around [O III] emitting galaxies. In this thesis, we forward model observational complexities involved in the measurement of excess transmission of Ly-a flux around [O III] emitters with the BIBORTON semi-numerical simulations. Using these simulations that also capture Lyman-a opacity fluctuations at late times, we reproduce the observed [O III] emitters - IGM cross-correlation. We statistically compare our simulations with the observed excess transmission close to galaxies, from which we present constraints on the mean free path of ionising photons and the mean photoionisation rate. We present promising forecasts for extending this observational test with larger samples of quasar sightlines. Additionally, while other Ly-a measurements have indicated reionisation ending at z \approx 5.3, we find a mild preference in the data for reionisation ending at z > 5.3 which can be tested with additional data.