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The sources of reionization

Final Report Summary - PROFESSOR (The sources of reionization)

The first stars and galaxies in our universe transformed the cold and neutral cosmic gas that was present shortly after the Big Bang into the hot and ionized plasma that fills space between galaxies today. Research into this transformation, which took place in the first billion years, during the Epoch of Reionization, is crucial for understanding the formation and evolution of galaxies, including our own galaxy, the Milky Way. The project objective was to carry out cosmological radiation-hydrodynamical simulations of reionization to help address key open questions in the field.

The simulations followed the assembly of the gas in galaxies and the formation of stars inside them. The simulations further followed the propagation of ionizing radiation emitted by the stars, the explosion of the stars in supernovae and their death in black holes. Finally, the simulations also accounted for the emission of X-rays that accompanies the accretion of gas by the black holes.

Simulating reionization is an extreme computational challenge mostly because of the large expense of the transfer of the ionizing and X-ray radiation. Conventional radiative transfer codes that trace the radiation on a regular computational mesh have struggled to treat the large range in scales and the large number of sources that are typical of reionization simulations. The project took a new numerical approach by applying a novel radiative transfer code, Traphic, that was designed for use in this computationally demanding astrophysical regime. The Traphic code implements a spatially adaptive radiative transfer technique that is effective at tracing radiation in complex geometries with large dynamic range. A large number of sources poses no problem because the computational cost of simulations with Traphic does not show the linear scaling with the number of sources known from conventional radiative transfer codes but is independent of it.

The simulations were executed using two major compute time allocations on the Supermuc supercomputer at the Leibniz Rechenzentrum Garching of about 30M CPU hours each. The first allocation was awarded by the Gauss Center Germany as part of the 10th Gauss call for large scale computing. The second allocation was awarded by PRACE, the Partnership for Advanced Computing in Europe, in the 8th European call for compute time proposals, and corresponded to a significant fraction of about 10% of the total compute time available in that call. Additional simulations were run on supercomputers at the Texas Advanced Computing Center and at the Rechenzentrum Garching. The competitive compute time allocations enabled the execution of some of the largest and most detailed radiation-hydrodynamical simulations of reionization to date.

The simulations showed that thermal feedback from photoionization is a key process that strongly impacts the formation of galaxies and the history of reionization. The heating associated with the absorption of ionizing stellar and X-ray radiation boils the gas out of low mass galaxies and impedes the accretion of fresh gas onto them. This reduces the fuel for star formation and lowers the ionizing luminosities of these galaxies. Because low-mass galaxies are among the main drivers of reionization, photoionization heating makes it substantially more difficult to reionize the universe and thus provides a negative feedback on reionization. On the other hand, photoheating also increases the pressure in the intergalactic gas. This reduces gas density fluctuations in the universe, which results in a decrease in the cosmic recombination rate. Because a reduced recombination rate makes it easier to keep the intergalactic gas ionized, photoheating provides also a positive feedback on reionization. Intriguingly, the simulations showed that this positive feedback may be especially strong if X-rays from accreting black holes preheat the gas before it is reionized.

The project has fostered collaborations of the host institute with external institutes and contributed substantially to the training of students. Initial results were presented at international conferences and workshops and published in refereed journals. Several new and powerful telescopes are currently being built or are already underway to hunt for the first galaxies and reionization. The results of the project may be used to inform the design of the upcoming observational campaigns and help interpret the data once it arrives.