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Synergies between 21cm & optical surveys for probing large scale cosmic structure

Student thesis: Doctoral Thesis

We are currently living through an era of precision cosmology where we have gathered a substantial amount of data with the aim of understanding our Universe. However, our current understanding is far from complete as our most successful cosmological model relies on the Universe’s energy-matter content being vastly dominated by components that are not yet detected and not currently compatible with our wider general model of physics. This leaves plenty more investigation to be done and new techniques for probing our Universe are highly sought after.
Mapping unresolved neutral hydrogen within galaxies is one of these novel techniques and has been gaining momentum over the last decade. By using the 21 cm signal from neutral hydrogen, which traces the underlying large scale cosmic structure, we can map and statistically analyse 3D density distributions and compare these to theoretical models. I provide a detailed
introduction to this novel HI intensity mapping technique in Chapter 2.
This thesis also explores what gains can be made by combining HI intensity mapping data with more conventional optical galaxy redshift surveys. There are many reasons why a cross correlation such as this will be beneficial. While the intensity mapping technique is developed and refined, optical data can boost the inherently weak HI signal allowing detection and a deeper understanding of the intensity mapping process. Also in the future, when we have dedicated intensity mapping instruments gathering data, cross-correlations will see reductions in the different systematics which could otherwise dominate the uncertainty in any auto-correlations. With the use of computer simulations, I look to forecast benefits to be gained from this synergy and in Chapter 3 I provide an example of how HI intensity maps can be used to constrain photometric redshifts on optical imaging surveys.
The largest problem preventing the success of HI intensity mapping comes from 21 cm foregrounds whose signals dominate by several orders of magnitude over the weak HI cosmological signal. While we have several methods for cleaning these foregrounds, understanding the impact these reconstructions have on the data is crucial and is the key theme in Chapters 4 and 5. Again using computer simulations of cosmological HI intensity mapping signals and their foreground contamination, I show how foregrounds can be removed and with some additional treatment, successfully used in cross-correlation with an optical photometric galaxy survey.
This indicates a promising future for cosmology and suggests the next-generation of optical telescopes such as LSST and Euclid, should benefit hugely from synergies with intensity mapping data provided by a next-generation radio telescope such as the SKA.
Original languageEnglish
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Award dateSep 2019

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