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The sources of reionization


The first sources of ionizing radiation formed just a few hundred million years after the Big Bang, when the Universe was only a small fraction of its present age. Their energy transformed the previously cold and neutral gas that filled intergalactic space into the hot and ionized cosmic plasma we observe today. This transformation is called reionization, a key epoch in the history of galaxy formation.

The European Union is making substantial investments into a new generation of telescopes that are, for the first time, powerful enough to see into and beyond the epoch of reionization. The European Extremely Large Telescope and the James Webb Space Telescope will hunt for the light from the hitherto undetected, most distant galaxies. The Low Frequency Array will detect the 21 cm signal from neutral hydrogen and map out the ionization history of the cosmic gas in space and time. The close advent of these and other telescopes creates an urgent need for improving the current theories of reionization.

One of the most pressing unsolved questions regards the nature of the ionizing sources. Past observational and theoretical efforts have identified the stars in the first galaxies as one of the most likely sources of reionization. However, a conclusive assessment stands still out, as do answers to many other open questions: Which galaxies were most important in driving reionization, and why? Were ionizing sources other than stars at play, such as the first black holes? What can we expect the upcoming observations to tell us about structure formation at these early times?

The proposed research will approach these and other essential issues by means of accurate radiation-hydrodynamical simulations of the Universe at large and small scales. It will provide the firm theoretical background needed for the interpretation of observations with the upcoming telescopes, and support and advance the theoretical and observational programmes of the European Union in this field.

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80539 Munchen

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Activity type
Research Organisations
Administrative Contact
Simon White (Prof.)
EU contribution
€ 167 390,40