Cosmic Dawn III recreates the Age of Reionization in Unprecedented Resolution

(ORDO NEWS) — Physicists are very interested in the first billion years of the universe – the period between the Big Bang and the formation of the first stars, during which galaxies began to form.

During the last 600 million years of this period, the neutral interstellar galactic medium and even the pregalactic medium were ionized by ultraviolet radiation emitted by the first stars shining in the earliest, growing galaxies.

Understanding the physics of this stretch, called the “epoch of reionization,” or EoR, would make it possible to link the physics of the modern universe to the Big Bang.

“The epoch of reionization represents the last major transition of the universe in the history of cosmic evolution,” says theoretical astrophysicist Paul Shapiro of the University of Texas at Austin, “from a phase in which all of space was filled with a nearly featureless, homogeneous gas to a phase in which structure arose, with the formation of the first galaxies and stars within them.

Direct observation of distant sources of reionization is a difficult task, and their detection has so far been limited to the brightest galaxies. Physicists use computer simulations to recreate the rich physics of EoR.

On April 10, during the April APS 2022 meeting, theoretical astrophysicist Paul Shapiro of the University of Texas at Austin will present the highlights and observational predictions of the Cosmic Dawn III (CoDa) project, the largest EoR radiative hydrodynamic simulation to date.

Modeling EoR with CoDa III was computationally expensive. The model had a trillion computational elements – 81923 dark matter particles and 81923 gas and radiation cells in a region of 300 million light years – and had high enough resolution to trace all the forming galactic halos that caused reionization in this volume, which is far beyond the capabilities of conventional computers.

The simulation ran for 10 days on 131,072 CPUs connected to 24,576 GPUs on the Summit supercomputer located at Oak Ridge National Laboratory in Tennessee.

Size isn’t the only notable feature of the CoDa III simulation, Shapiro says. Tracking the evolution of galaxy formation and reionization requires taking into account the feedback process: ionizing radiation leaking from galaxies must have heated the intergalactic medium.

This extra heat, in turn, puts enough pressure on the gas to resist the gravitational pull of nearby galaxies. Since the gas would otherwise fuel the formation of stars, the net effect of this process was to slow down the formation of new stars.

Previous models shared these effects, but Shapiro says that CoDa III can model the gravitational dynamics of gas and matter together, taking into account ionizing radiation and its effect on gas. Without radiative transport, time in the evolutionary model would have to be divided into steps small enough to represent the changing densities of gas, stars, and dark matter.

The addition of this feedback loop means that the time steps must be hundreds of times smaller to reflect the high speed of “ionization surfaces” – rapidly expanding ionizing bubbles bursting out of newly formed galaxies and propagating throughout the universe.

The associated CPUs and GPUs on the Summit supercomputer, Shapiro says, made it possible to solve these equations almost as quickly as if the model did not include radiation.

Remarkably, says Shapiro, CoDa III solves a problem between theory and observational data that has arisen in the EoR studies, namely that the theoretical predictions of previous models do not match the observations of the absorption spectra of quasars that explore the Universe at the end of the EoR and after. This problem disappears in CoDa III as simulations produce self-consistent predictions that are consistent with recent observations.

Shapiro predicts that the study of EoR will grow rapidly in the coming years. Space observatories, such as the James Webb Space Telescope, which launched in December 2021, and the Nancy Grace Roman Space Telescope, scheduled to launch in 2027, as well as ground-based projects such as the Extremely Large Telescope, will expand astronomers’ ability to observe distant driving forces of reionization. Current and upcoming radio studies may help researchers better define the inhomogeneous and inhomogeneous mode of MGM ionization.

Simulations like Cosmic Dawn, Shapiro says, provide a theoretical basis for what these sophisticated telescopes will see. “In addition to matching the existing spectrum of observations and predicting new ones,” he says, “it provides a critical understanding of the nature of the physical processes that took place.”

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