An understanding of the physics of the “Epoch of Reionization” or EoR, will connect the physics of the modern universe to the Big Bang. Physicists have become keenly interested in the first billion years of the universe—the stretch between the Big Bang and the formation of the first stars during which galaxies began to form. During the last 600 million years or so of this period, the neutral interstellar galactic medium – and even the pre-galactic medium surrounding fledgling proto-galaxies – became ionized with ultraviolet radiation emitted by the first stars glowing in the earliest, growing galaxies. The ultraviolet radiation from the first stars ionized hydrogen atoms, separating the protons from electrons, for the first time since a few minutes after the Big Bang.
In the image of the Epoch of Reionization shown above, neutral hydrogen, in red, is gradually ionized by the first stars, shown in white. The image was made by the University of Melbourne’s Dark-ages Reionization And Galaxy Observables from Numerical Simulations (DRAGONS) program. (Credit: Paul Geil and Simon Mutch}
“The Epoch of Reionization represents the last major transition of the universe in the story of cosmic evolution,” says theoretical astrophysicist Paul Shapiro, Frank N. Edmonds, Jr Regents Professor in Astronomy at the University of Texas at Austin, “from the phase when all of space was filled with a nearly featureless, homogeneous gas to the phase in which structure emerged, with the first galaxies forming and inside them, stars.”
Observing the distant sources of reionization directly is challenging, and detections are so far limited to the brightest galaxies. Physicists use computer simulations to recreate the rich physics of the EoR. On April 10, during the APS April Meeting 2022, theoretical astrophysicist Paul Shapiro will present highlights and observational predictions from the Cosmic Dawn III (CoDa) Project, the largest radiation-hydrodynamics simulation of the EoR to date.
Heavy Computational Lifting
Simulating the EoR with CoDa III required heavy computational lifting. With a trillion computational elements—81923 dark matter particles and 81923 gas and radiation cells in a region 300 million light-years across today—the model had a resolution high enough to follow all the newly forming galactic haloes that sourced reionization in that volume, well beyond the reach of ordinary computers. The simulation ran for 10 days on 131,072 processors coupled to 24,576 graphic processing units at the massively parallel supercomputer, Summit, located at Oak Ridge National Laboratory in Tennessee.
Size isn’t the only remarkable feature of the CoDa III simulation, says Shapiro. Tracking the evolution of galaxy formation and reionization requires accounting for a mutual feedback process: ionizing radiation that leaked out of galaxies had to heat the intergalactic medium. That additional heat, in turn, pressurized gas enough to resist the gravitational pull of nearby galaxies. Since the gas would otherwise have fueled the formation of star formation, the net result of this process is to stymie new stars.
Previous models have separated these effects, but Shapiro says CoDa III can simulate the gravitational dynamics of gas and matter together while accounting for ionizing radiation and its effect on the gas. Without radiative transfer, time in the evolutionary model would have to be divided into steps small enough to represent the changing densities of gas and stars and dark matter. The addition of this feedback loop means the time steps must be hundreds of times smaller to capture the high speed of the “surfaces of ionization”—rapidly expanding ionizing bubbles racing outward from newly-formed galaxies and sweeping across the universe. The linked processors and GPUs at the Summit supercomputer, Shapiro says, made it possible to solve these equations almost as quickly as if the model did not include radiation.
Transition neither Instantaneous nor Homogeneous.
The transition between fully neutral to nearly complete ionization of atoms in the intergalactic medium (IGM) was neither instantaneous nor homogeneous. Pockets of intense star formation and dense groupings of galaxies were subject to more ultraviolet radiation and reionization. Previous simulations failed to explain the inferred timescale of this EoR transition measured from the numbers, densities, and temperatures of intervening gas clouds observed along the lines of sight toward extremely distant quasars.
Syncing Theory with Observation
Notably, Shapiro says, CoDa III solves a problem between theory and observational data that has emerged in EoR studies; namely, that the theoretical predictions of previous models don’t line up with observations of quasar absorption spectra that probe the universe at the end of the EoR and after. This problem vanishes in CoDa III, as the simulation produces self-consistent predictions that agree with the latest observations.
The Last Word– “The Tip of the Iceberg”
“Theoretical models like our CoDa III simulation of reionization and early galaxy formation tell us that surveys of high-redshift galaxies present during this epoch have so far detected only the brightest ones — the tip of the iceberg, wrote Paul Shapiro in an email to The Daily Galaxy.. The more abundant, lower-mass and lower-luminosity galaxies our theory says dominated reionization are a prime target for study by the recently- launched James Webb Space Telescope and others to follow, in space and on the ground, including the Nancy Grace Roman Space Telescope, Giant Magellan Telescope, and Extremely Large Telescope.
“We find,” continued Shapiro, “that reionization and galaxy formation exerted a mutual feedback on each other, because UV starlight that escaped from galaxies to ionize the intergalactic gas also heated it, causing pressure forces to rise and oppose gravity, choking off the gas supply to small galaxies overtaken by reionization and suppressing their star formation, thereby causing reionization to ‘self-regulate.’ The next generation telescopes should be able to confirm this.
“The CoDa III simulation allows us to predict what they will find in detail, not by modeling individual galaxies and their surrounding intergalactic H II regions, in isolation, one at a time, but by modeling millions of galaxies and the interdependence of galaxy formation and reionization, all at once. In so doing, we will be able to test, not only the theory of the epoch of reionization, but the basic Cold Dark Matter paradigm of structure formation in the universe, itself.
Shapiro predicts that the study of the EoR is poised to undergo its own rapid expansion in coming years. The new space-based observatories will improve astronomers’ ability to observe the far flung drivers of reionization. Present and upcoming radio investigations could help researchers better constrain the clumpy, inhomogeneous way that the Intergalactic Medium became ionized.
Simulations like Cosmic Dawn, says Shapiro, provide a theoretical foundation for what these sophisticated telescopes will see. “Apart from matching the existing spectrum of observations and predicting new ones,” he says, “it provides critical insight into the nature of the physical processes that took place.”
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