School of Physics - Theses
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ItemHydrogen in the first billion years: a study of the 21-cm signal from the high-redshift Universe( 2024-01)The history of our Universe is reflected in the state of its hydrogen (HI) atoms. After recombination (redshift z ~ 1000), the intergalactic medium (IGM) is composed primarily of neutral hydrogen (HI). The formation of the first stars and the first galaxies in the early Universe during Cosmic Dawn (z ~ 30) triggered the last major phase change of the HI. During the Epoch of Reionisation (EoR), the intense ultraviolet (UV) and X-ray radiation emitted by the first luminous sources carve out ionised hydrogen (HII) bubbles in the IGM. These HII bubbles expand and fill the whole Universe (z ~ 5). By altering the thermal and ionisation state of the IGM, the EoR directly impacts the subsequent formation and evolution of galaxies in the Universe. The 21-cm hyperfine spin-flip of HI is the primary probe of this period, and dedicated observational campaigns are ongoing/under construction to observe this redshifted 21-cm emission. Theoretical models must be on hand to interpret current upper limits as well as future observations. Semi-analytical models (SAMs) are well-suited for this purpose because of their computationally efficient and physically motivated prescriptions of relevant physics. Galaxy formation SAMs typically work by post-processing the dark-matter halo merger trees from dark-matter-only N-body simulations. This thesis updated the Meraxes SAM of coupled galaxy formation and reionisation in this thesis. Specifically, the explicit calculation of the spin temperature of the HI gas was implemented. This involves tracking the thermal state of the IGM, which is influenced primarily by the X-rays. This updated version of Meraxes was deployed on an N-body simulation of side 210 h^(-1) Mpc. Such large cosmological volumes are necessitated by the long mean free paths, ~ O(100 Mpc), of X-rays in the early Universe. At the same time, for a given number of particles, the mass resolution of an N-body simulation is inversely proportional to its volume. Hence, the simulations will not capture the full source population. To overcome this, the dark matter merger trees are augmented by introducing low-mass haloes into the simulations. The augmented simulation is one of the large-volume simulations in the literature that is simultaneously capable of resolving all atomically cooled haloes down from z = 20 and is sufficiently large enough to track the impact of X-rays on the thermal state of the IGM. Taking advantage of the computational efficiency of the Meraxes SAM, the impact of the galaxy X-ray luminosity on the 21-cm statistics, i.e. the 21-cm global signal and 21-cm power spectra (21-cm PS), are explored. Exploiting the large dynamic range of the model, the thesis also shows that the magnitude of the non-Gaussian term in the sample variance of the 21-cm PS is more than twice the magnitude of the Gaussian term at scales relevant to the upcoming Square Kilometre Array (SKA). The thesis then explores the astrophysical constraints that will be achievable with a future detection of the 21-cm PS. Using the Fisher matrix formalism, the fractional uncertainties in the model parameters enabled by a 21-cm detection spanning z in [5, 24] from a 1000 h mock observation with the SKA are forecasted. This work focused on the X-ray luminosity, ionising UV photon escape fraction, star formation and supernova feedback of the first galaxies. It is shown that it is possible to recover 5 of the 8 parameters describing these properties with better than 50 per cent precision using just the 21-cm PS. By combining UV luminosity functions with the 21-cm PS, we can improve our forecast, with 5 of the 8 parameters constrained to better than 10 per cent (and all below 50 per cent).