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Imaging the cosmic dawn and the first galaxies with 21cm and atomic line intensity mapping

Periodic Reporting for period 3 - FirstDawn (Imaging the cosmic dawn and the first galaxies with 21cm and atomic line intensity mapping)

Reporting period: 2018-04-01 to 2019-09-30

"Modern astrophysics has pushed the observational frontier to a time a billion years after the Big Bang. Lying beyond this frontier is the period when the first stars and galaxies formed, whose light heated and ionized the Universe in the process known as reionization. Understanding this ""epoch of reionization"" would fill in a key missing period in our picture of the history of the Universe. Existing observational techniques have scratched the surface, but new observational techniques are required to truly understand this early period of galaxy formation. Our work aims to lay the theoretical foundations for three novel probes of this period - 21 cm tomography, the 21 cm global signal, and line intensity mapping - that would enable three dimensional maps of the epoch of reionization. If realized through challenging radio-frequency observations, these techniques would transform our understanding of the first galaxies.

This ERC project – FIRSTDAWN – is working to build the theoretical framework needed to predict and interpret observations of line emission from gas in and surrounding the first generation of galaxies. My team aims to develop models of the interplay between radiation from the first galaxies and the heating, ionization, and illumination of hydrogen gas that lies in the space between galaxies. At the same time, we are building models of the formation and properties of the atomic and molecular gas that fills the space inside galaxies. By combining probes of this ""inner"" and ""outer"" space a complete nature of galaxy formation during the first billion years might be achieved. Analysis of sky averaged 21 cm observations will complement this with a broad overview of galaxies back to a few hundred million years after the big bang. This work will provide a clear theoretical road map to guide the design of next generation radio telescopes, such as the Square Kilometer Array, to achieve this ambitious goal.
During the first phase of this project, the FIRSTDAWN team has been developing new tools to analyse and interpret observations from upcoming radio telescopes and inform the development of those experiments. These fall into three main areas: the all-sky 21 cm signal, which is being targeted by single dipole experiments, the non-Gaussianity of the 21 cm signal, which encodes information about the nature of reionization and cosmic heating, and numerical simulations, which provide a framework for interpreting upcoming observations.

For the all-sky 21 cm signal, we have been developing an analysis framework to cope with the challenge of separating the signal from galactic and extra-galactic foregrounds. Our Bayesian analysis pipeline, would exploit both frequency and sky location information to disentangle these components. These work was incorporated into a proposal for a lunar orbing radio dipole experiment - the Dark Ages Radio Explorer (DARE), which has been considered by NASA.

Radio telescopes targeting the 21 cm signal from the epoch of reionization tend to focus on the power spectrum of brightness temperature fluctuations. The power spectrum is a very valuable statistic, but is only complete if the 21 cm signal is a Gaussian random field. In practice, the 21 cm signal will contain many features from the percolation of ionized regions that require additional statistical tests to probe. Our team has developed machinery for efficiently calculating the bispectrum from mock data and building the theoretical framework to interpret this for different reionization scenarios.

All of these require numerical simulations to interpret, since the complex interplay of ionization, heating, and illumination of the intergalactic gas by light from the first galaxies is hard to capture in simple analytic models. Alongside work on improving the existing simulations, we have been adapting tools from the machine learning community to allow rapid emulation of numerical simulations. These allow a wide parameter space to be explored much more rapidly, making a complete analysis of the relevant models more tractable.
As we move into the later phase of the project, we will working with our collaborators in SKA-EoR to integrate these statistical tools into a comprehensive Bayesian pipeline for the analysis of 21cm data. Looking ahead to SKA, it is to be hoped that this will help distinguish different reionization scenarios. Alongside this we will be incorporating more techniques from the machine learning community to boost our efforts to speed up emulation of 21 cm maps. Finally, we will be incorporating more physics into our simulations to better model other line intensity signals, such as in CO or CII lines, and exploring the synergy between observations in these lines with those in the 21 cm line.