Galaxy formation meets Reionization in the THESAN simulations

October 01, 2021

Approximately 13 billion years ago, the radiation produced by the first galaxies completely transformed the Universe. The vast amount of hydrogen filling the space between galaxies was  ionized in a process called cosmic reionization. Despite their intimate connection, the formation of the first galaxies and the reionization process are typically studied separately. An international team led by and including MPA researchers has now produced the first suite of simulations designed to simultaneously investigate these two processes during the infancy of the Universe, unveiling features of their connection. This new numerical effort – soon to be released publicly – provides a unique platform for investigating the young Universe and to fully exploit the forthcoming James Webb Space Telescope. The first results from THESAN have already shown that its unique combination of physical accuracy and simulated scales allows to reproduce most of the available data, including some that escaped previous numerical efforts.

After the Big Bang, the Universe went through its Dark Ages, a period of time when no sources of light were present. This ended when the first stars and – shortly after – the first galaxies formed. Their intense UV radiation transformed the neutral hydrogen gas in the inter-galactic medium between galaxies into a highly-ionized plasma; a process called ‘Cosmic Reionization’ that took place 13 billion years ago. Despite this strong relationship, the details of the connection between the first galaxies and Cosmic Reionization are still poorly understood, as it is extremely difficult to observe this very remote time in the history of the Universe. Thanks to a plethora of forthcoming telescopes, however, this obstacle will soon be overcome. The first one of them, the James Webb Space Telescope, is going to be launched at the end of 2021.

In order to take full advantage of these future observations, an international team led by Dr. Enrico Garaldi at MPA, Dr. Rahul Kannan at Harvard, and Dr. Aaron Smith at MIT, and including other MPA researchers, has developed a new unique suite of simulations – named Thesan – that pushes beyond the state of the art. Simulations are an essential entry in the astrophysicist’s toolbox, since the number and complexity of physical processes relevant in the formation of galaxies renders pen-and-paper studies impossible. Using knowledge about the conditions left behind by the Big Bang and the physics governing the Universe, numerical astrophysicists simulate the formation and evolution of vast regions of space. They can then not only witness and unfold how structures grow but also use the detailed picture obtained from simulations to interpret cryptic observations.

What makes the new Thesan simulation suite unique is the combination of a long list of state-of-the-art numerical techniques that come together to create an exquisite and unprecedented view of the infancy of the Universe. In particular, the Thesan simulations combine an extremely successful galaxy formation model, an accurate and efficient algorithm that simulates the propagation of light, a model for the creation and destruction of cosmic dust, and a novel technique that ensures that the simulated structures are as statistically representative of the Universe as possible. The galaxy formation model is that of Illustris-TNG, which is able to reproduce many properties of galaxies found in the Universe, and includes the effects of energy and matter released from stars and black holes during their life, magnetic fields, and individual elements. Following the propagation of light is required to properly simulate Cosmic Reionization and cosmic dust needs to be included as well, since the molecules produced within the first galaxies give us a lot of information about their properties. Additionally, Thesan also explores different theories for the nature of dark matter and the sources of the photons powering Cosmic Reionization.

Reionization process in the Thesan simulations

3D view of the reionization process in the Thesan simulations, showing the evolution of the HI fraction (left) and density of ionizing photons (right). The 2D plots highlight the evolution of these quantities with time. The visualization starts at a redshift of about 16 (13.5 billion years ago) and runs to z=5.5 (about 1 billion years after the Big Bang), when the hydrogen in the simulation box is almost completely ionized.

It is the first time that all these different techniques are combined in a large cosmological simulation. In order to achieve this one-of-a-kind combination, researchers used one of the biggest supercomputers in the world, the SuperMUC-NG machine at the Leibniz-Rechenzentrum in Garching near Munich. There, the simulation was performed by simultaneously using approximately 60 000 computing cores, for a total of more than 50 million CPU-hours. If the same simulations were run on a normal computer, they would have required more than 5700 years to complete.

Unlike previous studies, the simulations in the Thesan suite were not tuned to match available observations of the reionization epoch. Rather, they build upon knowledge gathered over the years, in which MPA has played a pivotal role. Remarkably, the researchers have now demonstrated that the simulated galaxies and inter-galactic medium are in very good agreement with available data nevertheless.

The full analysis of the simulations will take many years, but bridging the gap between the formation of galaxies and Cosmic Reionization has already allowed researchers to reproduce for the first time the observed modulation of the radiation intensity around primeval galaxies. In order to allow the entire research community to benefit from this large effort, the researchers will make the simulation data freely available in the coming months. More information, visualization, and updates are available at

Thesan simulation fly-through

Flight through the main Thesan simulation, showing its different simulated properties. The animation then closes in on the largest galaxy in the simulated volume, reaching a final zoom factor of 420.

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