Astronomers simulate the universe with realistic galaxies

January 4, 2015

Part of a simulated galaxy, with magnified views (credit: Joop Schaye et al.)

An international team of astronomers has developed a simulation of the universe in which realistic galaxies are created — their mass, size, and age are similar to those of observed galaxies.

Previous computer simulations had limited success because their simulations were too old, too spherical, and either too massive or too small.

In the new study, by astronomers based at Durham University and Leiden University in the Netherlands, the galaxies formed in the EAGLE-simulation (Evolution and Assembly of GaLaxies and their Environments) are a much closer facsimile of real galaxies, thanks to modeling strong galactic winds.

Powered by stars, supernova explosions, and supermassive black holes, the winds blow away the gas supply needed for the formation of stars. As a result, EAGLE’s galaxies are also lighter and younger because fewer stars form and they form later. The sizes and shapes of the thousands of galaxies that form in the EAGLE simulation are also similar to those of galaxies that astronomers actually observe in the Universe.

The simulations took several months to run at the “Cosmology Machine” in Durham and at “Curie” in Paris, among the largest computers used for scientific research in the U.K. and France, respectively.

Astronomers can now use the results to study the evolution of individual galaxies in detail, from almost 14 billion years ago until now.

“The universe generated by the computer is just like the real thing,” says co-author Richard Bower from Durham University. “There are galaxies everywhere, with all the shapes, sizes and colors I’ve seen with the world’s largest telescopes. It is incredible. In the EAGLE universe I can even press a button to make time run backwards,”

The results were published in Monthly Notices of the Royal Astronomical Society on January 1, 2015.

Abstract of The EAGLE project: simulating the evolution and assembly of galaxies and their environments

We introduce the Virgo Consortium’s Evolution and Assembly of GaLaxies and their Environments (EAGLE) project, a suite of hydrodynamical simulations that follow the formation of galaxies and supermassive black holes in cosmologically representative volumes of a standard Λ cold dark matter universe. We discuss the limitations of such simulations in light of their finite resolution and poorly constrained subgrid physics, and how these affect their predictive power. One major improvement is our treatment of feedback from massive stars and active galactic nuclei (AGN) in which thermal energy is injected into the gas without the need to turn off cooling or decouple hydrodynamical forces, allowing winds to develop without predetermined speed or mass loading factors. Because the feedback efficiencies cannot be predicted from first principles, we calibrate them to the present-day galaxy stellar mass function and the amplitude of the galaxy-central black hole mass relation, also taking galaxy sizes into account. The observed galaxy stellar mass function is reproduced to ≲ 0.2 dex over the full resolved mass range, 108 < M*/M⊙ ≲ 1011, a level of agreement close to that attained by semi-analytic models, and unprecedented for hydrodynamical simulations. We compare our results to a representative set of low-redshift observables not considered in the calibration, and find good agreement with the observed galaxy specific star formation rates, passive fractions, Tully–Fisher relation, total stellar luminosities of galaxy clusters, and column density distributions of intergalactic C IV and O VI. While the mass–metallicity relations for gas and stars are consistent with observations for M* ≳ 109 M⊙ (M* ≳ 1010 M⊙ at intermediate resolution), they are insufficiently steep at lower masses. For the reference model, the gas fractions and temperatures are too high for clusters of galaxies, but for galaxy groups these discrepancies can be resolved by adopting a higher heating temperature in the subgrid prescription for AGN feedback. The EAGLE simulation suite, which also includes physics variations and higher resolution zoomed-in volumes described elsewhere, constitutes a valuable new resource for studies of galaxy formation.