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New super-simulation of how galaxies are formed

Computer simulation of the formation of galaxies. Photo: The AGORA Collaboration.
Computer simulation of the formation of galaxies. Photo: The AGORA Collaboration.

Astronomers can use supercomputers to simulate the formation of galaxies from the Big Bang 13.8 billion years ago to the present day. But there are a number of sources of error. An international research team, led by Lund University and Seoul National University, has spent hundreds of millions of computer hours over eight years trying to correct these.

The last decade has seen major advances in computer simulations that can realistically calculate how galaxies form. These cosmological simulations are crucial to our understanding of where galaxies, stars and planets come from. However, the predictions from such models are affected by limitations in the resolution of the simulations, as well as assumptions about a number of factors, such as exploding stars, galactic outflows and stellar motions. To minimise the sources of error and produce more accurate simulations, 160 researchers from 60 institutions - led by Santi Roca-Fábrega at Lund University, Ji-hoon Kim at Seoul National University and Joel R. Primack at the University of California - have collaborated and now present the results of their first supersimulation.

Santi Roca-Fábrega. Photograph by Oscar Agertz.
Santi Roca-Fábrega. Photograph by Oscar Agertz.

"To make progress towards a theory of galaxy formation, it is crucial to compare results and codes from different simulations. We have now done this by bringing together competing code groups behind the world's best galaxy simulators in a kind of super simulator," says Santi Roca-Fábrega, a researcher in astrophysics.

Three papers from this collaboration, known as the CosmoRun simulations, are now published in the scientific journal The Astrophysical Journal. In these, the researchers have analysed the formation of a galaxy with the same mass as the Milky Way. The simulation is based on the same astrophysical assumptions about ultraviolet background radiation, gas cooling, heating and star formation. The new results allow the researchers to conclude that disc galaxies like the Milky Way formed very early in the history of the Universe, in line with observations from the James Webb Telescope. They have also found a way to make the number of satellite galaxies - galaxies orbiting larger galaxies - consistent with observations. In addition, the team has revealed how the gas surrounding galaxies is the key to realistic simulations, rather than the number and distribution of stars, which had previously been the standard.

"The work has been going on for the past eight years, running hundreds of simulations and using a hundred million hours of supercomputing facilities,' says Santi Roca-Fábrega.

Now the journey continues to further refine the simulations of galaxy formation. With each technological achievement, Santi Roca-Fáberga and his colleagues hope to add new pieces to the dizzying puzzle of the birth and evolution of the universe and galaxies.

"This is the start of more reliable simulations of galaxy formation, which in turn will help us to better understand our home galaxy, the Milky Way", says Santi Roca-Fábrega.

In addition to Lund University, some 60 universities and organisations have participated in the work.

The study is published as three articles in The Astrophysical Journal, here as e-print versions via ArXiv:

The AGORA High-resolution Galaxy Simulations Comparison Project IV: Halo and Galaxy Mass Assembly in a Cosmological Zoom-in Simulation at z≤2.

The AGORA High-resolution Galaxy Simulations Comparison Project. V: Satellite Galaxy Populations In A Cosmological Zoom-in Simulation of A Milky Way-mass Halo.

The AGORA High-resolution Galaxy Simulations Comparison Project. VI. Similarities and Differences in the Circumgalactic Medium.

Santi Roca-Fábrega's profile in the Lund University Research Portal.