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

Periodic Reporting for period 4 - FirstDawn (Imaging the cosmic dawn and the first galaxies with 21cm and atomic line intensity mapping)

Periodo di rendicontazione: 2019-10-01 al 2020-03-31

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 has advanced the theoretical foundations for two novel probes of this period - 21 cm tomography and the 21 cm global signal- that would enable three dimensional maps of the epoch of reionization. If realized through upcoming radio-frequency observations, these techniques would transform our understanding of the first galaxies.

The FIRSTDAWN project has worked 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 developed 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. A core accomplishment of the project has been to develop the framework for measuring and interpreting non-Gaussian statistics of the 21cm signal, especially the bispectrum and phase based statistics. These provide a new tool for analysing the 21 cm signal and present a new path to extracting a clean signature of reionization and the first galaxies from upcoming observations. More broadly, our work has helped to clarify the theoretical and observational road map to exploit next generation radio telescopes and helped refine the design of the Square Kilometer Array to achieve this ambitious goal.
During the first phase of this project, the FIRSTDAWN team developed 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 built 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, and is now being redeveloped for work on REACH, a ground based global 21 cm experiment in South Africa.

Radio telescopes targeting the 21 cm signal from the epoch of reionization have tended 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. FIRSTDAWN developed machinery for efficiently calculating the bispectrum from mock data and building the theoretical framework to interpret this for different reionization scenarios. A preliminary attempt to measure this signal was made with MWA data and the results suggest significant promise for future data sets from SKA.

All of this work requires 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 adapted 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.
At the end of the project, we have developed a suite of new statistical tools to aid in the analysis of 21 cm data. As the SKA moves towards construction in 2021, the results from the FIRSTDAWN project have helped build a sturdy foundation for the theoretical interpretation of the 21 cm observations it will make. The project has developed a new perspective on these observations through the use of non-Gaussian statistics and the application of machine learning tools. We hope that in the not too distant future we will have the opportunity to apply these tools to assist in making the first robust detections of the 21cm signal.
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