School of Physics - Theses
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ItemSimulating the Epoch of Reionisation( 2021)The first stars and quasars to form in our universe drove a universal phase transition known as epoch of reionisation, where the diffuse hydrogen gas between galaxies was ionised. The epoch of reionisation is the last period in the history of our universe to be studied in detail, and requires detailed theoretical models in order to interpret our observations. Cosmological simulations provide a way to create detailed approximations of physical processes on large scales that are impossible to solve analytically. This allows us to draw connections between the physical parameters we wish to know, and the observable data we measure. In this thesis, we use different types of cosmological simulations to study the epoch of reionisation. We use the semi-analytic galaxy evolution model Meraxes to place constraints on the epoch using measurements of the intergalactic medium temperature. We use the hydrodynamic simulation Bluetides to make predictions for a future observational strategy where radio images of bright galaxies are stacked together to measure their average signal. Finally, we make enhancements to a hydrodynamic simulation code, MP-Gadget intended for use in the Asterix simulation to improve its calculation of the epoch of reionisation topology.
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ItemThe diversity and environments of the rarest objects during cosmic dawn( 2021)The Epoch of Reionization marks the transitional period of our Universe where the emergent growth of luminous structure begin to ionize the surrounding medium of neutral hydrogen gas. The characterisation of these luminous sources; galaxies or accreting black holes (i.e. quasars) play an important role by tracing sites of early structure formation, thus forms an integral aspect to our overall understanding in galaxy formation and evolution. In this thesis, we investigate the key properties of the rarest, brightest objects that are easily accessible by contemporary means; its large-scale environment and its number density, in the specific context of how stochasticity can affect the measured statistics of these rare objects. In the first part of this thesis, we investigate the impact of luminosity scatter in measurements of galaxy clustering, under the premise that luminosity scatter facilitates the odds for a common halo to contain an over-luminous outlier. We construct a Monte Carlo simulation of mock Hubble Space Telescope field-of-views to collect statistics of visible galaxy neighbours. We find that galaxy luminosity scatter is important in the overall clustering statistics within a field-of-view. However, current observations of weak clustering is consistent with our modeling predictions due to Poisson noise. There is scope in using future clustering measurements as a method to constrain luminosity scatter. The impact of luminosity scatter has a profound impact on the number distribution of luminous objects by broadening the bright end of the luminosity function. We extend the semi-empirical model for the evolution of the galaxy luminosity function by accounting for stochasticity in galaxy luminosities. Our model shows that a non-zero scatter in galaxy luminosities results in a departure from the Schechter function, and the amount of scatter can be constrained by additional large area observations of very bright galaxies. In the second part of the thesis, we extend our analysis of galaxy luminosity stochasticity to quasars. We investigate how quasar luminosity evolves with halo mass under different assumptions of quasar luminosity scatter and quasar duty cycle. We find that quasar luminosity scatter is linked to AGN and star formation feedback in our modeling. With a duty cycle of unity, we find that feedback acts on black holes and galaxies independently. Alternatively, a lower duty cycle favours the interpretation of a common feedback source. We conduct a similar Monte Carlo analysis on the weak clustering around $z\sim6$ quasars. We find that a duty cycle of unity places constraints on galaxy luminosity scatter in order to maintain consistency with observations, while a lower duty cycle does not have any restrictions on galaxy luminosity scatter. The quasar luminosity scatter is not strongly constrained by the field-of-view clustering measurements. Finally, we develop a single parameter semi-empirical model of the evolution for the quasar luminosity function that accounts for quasar luminosity scatter. Our model shows excellent agreement with the $z=3-6$ quasar luminosity functions and is capable of reproducing the $z=0-2$ luminosity functions assuming a small evolution in quasar luminosity scatter. Future measurements of the $z>6$ quasar luminosity function will be essential in validating the robustness of this model.