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Final Report Summary - 21ALPHA (Detecting patchy reionization and the first galaxies using the 21 cm and Lyman alpha lines)

21ALPHA worked to improve our understanding of the statistical signatures of reionization building the theoretical tools needed to extract science from upcoming observations. The focus is on two key techniques: 21 cm observations with radio interferometers, such as LOFAR and SKA, which will map the distribution of neutral hydrogen during the first billion years of the Universe; Lyman alpha emitters, whose light is particularly affected by scattering by neutral hydrogen and whose clustering properties are expected to contain a signature of reionization.

By first understanding key statistical properties of semi-numerical simulations of reionization, 21ALPHA aims to connect theory to observation in order to point the way towards answering questions such as “what was the topology of reionization?” and “what were the properties of the first galaxies?”.

Working with my PhD student – Catherine Watkinson (graduated Oct 2015) – we conducted a systematic survey of 1-point statistics of the 21cm signal, such as the variance and skew, resulting in three main papers. The first demonstrated that one point statistics can be used to distinguish different reionization topologies and we constructed a series of numerical simulations to explore a broad range of reionization models characterized as inside-out versus outside-in. The second worked to study the impact of neutral hydrogen in dense systems inside large ionized bubbles on the 21cm signal. Numerical simulations of reionization typically have ignored the contribution of neutral hydrogen from within the ionized bubbles. In collaboration with Andrei Mesinger and Emmanuelle Sobachi, we explored the impact of Lyman limit systems and hydrogen within galaxies on the 21cm power spectrum, variance, and skewness. In the final paper, we explored the sensitivity of one-point statistics to spin-temperature fluctuations induced by X-ray heating demonstrating that measurements of the skew and variance would help break degeneracies in power spectrum measurements alone.

Leading on from this work, I have been working with Catherine Watkinson and Suman Majumdar (an ERC funded postdoc) to develop a fast code for measuring the bispectrum of the 21cm signal from simulations and developing analytical models to interpret this signal. A series of papers currently in preparation will describe the fast algorithm that we have developed and it’s testing on a simple reionization model of random ionized spheres, for which there is an analytic bispectrum, and follow up papers describing the 21cm bispectrum during reionization and the period of X-ray heating. Such non-Gaussian statistics complement the power spectrum, which has been the main focus of 21cm studies in the past and will provide greater information once observed.

In separate work with Geraint Harker and Jordan Mirocha, we developed a new parametrisation for the 21cm global signal and demonstrated that it would improve the ability of all sky 21cm signal experiments to extract astrophysics information. In parallel, work with Adrian Liu, I showed that 21cm observations during the epoch of reionization could potentially measure the optical depth to the CMB in an independent way, improving cosmological parameter constriants from Planck. We demonstrated that the 21cm signal probes exactly the combination needed to measure tau and explored likely constraints from both interferometers, such as HERA, and global signal experiments, such as EDGES. This work has helped support the Dark Ages Radio Explorer (DARE) team in developing a NASA explorer satellite proposal, which was submitted at the end of 2016.

I have also been working to complete a series of semi-numerical simulations of LAEs and the Lyman alpha forest. These will be used to explore synergies with the 21cm signal during both the final stages of reionization and the post-reionization phase. The latter phase has been identified as being of interest in calibrating reionization simulations and as a test bed for early SKA observations. This component of the original proposal has taken longer than anticipated due to increased involvement in the SKA design process, which has taken up a considerable amount of time. Over the next year I hope to bring this to a publishable conclusion.

The work from 21ALPHA has been widely disseminated through a range of scientific papers, but with a range of activities designed to communicate this work to the general public and other scientific stakeholders. I have given a number of popular talks on SKA and the Cosmic Dawn and written several popular level articles describing this relatively rarely discussed period of cosmic history. At ESOF2016 in Manchester, this work was discussed in front of an audience of science journalists, policy makers, and others placing it in context with other probes of the early universe and the human shape of how cosmology is currently being done.

Alongside research, I have taught extensively at Imperial. For four years, I taught the MSci Cosmology course as well as second year python programming and a variety of undergraduate physics tutorials. I have assisted in the teaching of two Bayesian statistics summer schools at Imperial, covering introductory Bayesian inference and common statistical techniques for sampling a posterior distribution. In the last two years, I have supervised 7 undergraduate projects on reionization and radio astronomy topics, with several of these students going on to study for a PhD elsewhere. I am currently supervising two PhD students in 21cm related topics.

Through all these activities, I have been able to integrate myself into the UK and European scientific landscape after nine years in the US. Thanks to this CIG funding, I have been able to establish myself as a leader in the SKA epoch of reionization effort and obtain a permanent position as a Senior Lecturer in Astrostatistics at Imperial College. Moreover, looking to develop the ideas generated by this work still further I have obtained an ERC Starting Grant to build a sizable research group to take these ideas forward.

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Life Sciences
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