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Title: Understanding the first stars and galaxies with observations of the 21-cm line of hydrogen
Author: Watkinson, Catherine Allin
ISNI:       0000 0004 5367 6194
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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We have precisely characterised our Universe with the elegant LCDM model. However, this model predicts that around 70% of the Universe's content is dominated by a 'dark energy' that opposes gravity, and about 30% of its content is in some form of non-baryonic 'dark' matter that interacts only via gravity. Even when we consider the less exotic contents of our Universe, such as stars and galaxies, we are again forced to acknowledge the huge gaps in our knowledge. Observations of quasar spectra inform us that our Universe is in a highly ionized state during its later stages. However, from the cosmic microwave background we know that it was neutral for much of its history. It is sensible to assume that high-energy radiation, produced in abundance once stars and galaxies formed, would have driven the process of reionizing the Universe. However, the exact nature of early generations of stars and galaxies, and this process of reionization, are poorly constrained. In this work, we describe efforts to constrain the epoch of reionization, concentrating on the potential of the high-redshift 21-cm line of the hydrogen atom. Observations of the 21-cm signal would provide 3D maps of neutral hydrogen, and could provide vital constraints on the nature of reionization. Observing the 21-cm signal is challenging due to very strong radio foregrounds, we therefore concentrate on efforts to constrain the 21-cm signal statistically. In particular, we characterise the sensitivity of one-point statistics (or moments) of the 21-cm signal. We study the sensitivity of 21-cm moments to a wide range of morphological properties that might be exhibited by ionized bubbles during reionization. We then investigate how 21-cm moments are impacted by the details of inhomogeneous recombinations, where ionized hydrogen captures electrons to become neutral again. We then consider the sensitivity of 21-cm moments to the properties of X-ray production. This work is essential, as without fully understanding a statistic and how it depends on the underlying physical processes, we cannot hope to gain meaningful constraints from its observation. This work also exhibits how information rich the 21-cm moments are, providing strong motivation to better understand them, and for more effort to be put into constraining them from observations.
Supervisor: Pritchard, Jonathan ; Jaffe, Andrew Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral