School of Physics - Theses
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ItemLyα emitters as a probe of galaxy formation and ionisation history( 2016)Current observations suggest that the reionisation of hydrogen in the intergalactic medium had begun by z ∼ 10 and was completed around z ∼ 6. Directly observing this epoch is not possible with existing instrumentation, making it difficult to infer how the increased ionising background during this period affected galaxy formation. This thesis aims to put constraints on the galaxy formation history of the Universe with existing instruments, by modelling and observing the number densities of observed Lyα emitters in the ionised environments around z ∼ 2 − 3 quasars to mimic conditions found during the epoch of reionisation. The main work presented is a model for the ionisation state of the intergalactic medium around star forming galaxies in the vicinity of a luminous quasar, tuned by empirical relationships from conditions at z ∼ 2 − 3. This model suggests that the intense ionising radiation from a quasar offsets the increased density of the intergalactic medium found around it, implying that the direct detection of star forming galaxies by their Lyα emission in the vicinity of z ∼ 2 − 3 quasars is less obstructed by the intergalactic medium than galaxies in the field. The accuracy of this model is compared to existing Lyα galaxy surveys and found to be in good agreement. Discrepancies exist between the expected number of Lyα emitting galaxies this model predicts and the surveyed region around the super-luminous quasar PKS 0424-131, in which no Lyα emission was detected. The modelling done suggests that in order to be consistent with this null detection at the 68% (90%) level, galaxies below 2.5×10^12 M⊙ (4.2×10^12 M⊙) must be omitted. These results suggest that considerable radiative suppression of galaxy formation by PKS 0424-131 is taking place. This hypothesis is tested using observations made on the Baade telescope at the Las Campanas Observatory with the Maryland Magellan Tunable Filter. The unique suitability of tunable filters for the detection of high-redshift galactic Lyα emission is described in detail, along with their idiosyncratic calibration and data reduction processes. The adverse seeing conditions make it impossible to put limits on the impact of ionising radiation of galaxy formation using these observations, and an analysis of the factors that prevented detection is provided. Finally, suggestions are made for ways to improve the chance of success for future observations of this effect using tunable filters, as well as ways to remove spurious ghost reflections in the data analysis that are unique to tunable filter observations.
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ItemStudying the epoch of hydrogen reionisation in redshifted 21-cm radiation( 2011)The measurement of the spatial distribution of neutral hydrogen in the high-redshift intergalactic medium, through its 21-cm hyperfine transition, will revolutionise our understanding of the period in the evolution of the early Universe known as the cosmic dark ages. This period began once hot ionised gas, formed in the Big Bang, combined to form neutral gas, and lasted until the first stars, galaxies and quasars reionised most of the neutral hydrogen sometime between 400 000 to 700 million years after the Big Bang. The epoch of reionisation, driven by these first sources of light, is arguably the least understood, but one of the most important, periods in the evolution of the Universe. This thesis is an attempt to explain and explore some of the techniques we may employ in order to advance our understanding of this period and prepare for the first observations to come from a new generation of low-frequency instruments. Using a new efficient semi-numerical ionisation model to create simulations of the ionisation state of the intergalactic medium during the epoch of reionisation, we analyse the impact a percolating intergalactic medium has on redshifted 21-cm observations of high-luminosity quasar-generated regions of ionised hydrogen. We also investigate how a population of quasars modifies the 21-cm power spectrum during this period. The study of the reionisation history of hydrogen and, indirectly, the first galaxies will be limited not by raw sensitivity to the signal, but rather, by bright foreground radiation from Galactic and extragalactic radio sources and the Galactic continuum. We analyse the effect of non-polarised foreground subtraction on redshifted 21-cm observations, and demonstrate a method for removing polarised foregrounds using Faraday tomography.