An international group of astronomers has released the first results from the largest-ever galaxy simulation. The project has an ambitious goal: to work out how spiral galaxies formed. The results are published in the Monthly Notices of the Royal Astronomical Society.
The Auriga Project ran 30 high-resolution simulations of galaxies in the universe and six extremely high-resolution simulations. The state-of-the-art computer models were able to reproduce the effect of gravity, star-formation, gas movement, supernova explosions, and, for the first time, the magnetic fields that stretch across the stars, creating the most comprehensive physical simulation to date.
"The outcome of the Auriga Project is that astronomers will now be able to use our work to access a wealth of information, such as the properties of the satellite galaxies and the very old stars found in the halo that surrounds the galaxy," lead author Dr Robert Grand, from the Heidelberg Institute for Theoretical Studies, said in a statement.
A spiral galaxy is formed by a disk of young stars surrounding a central region shaped either like a ball (bulge) or a cigar (bar) full of older stars. They are surrounded by a halo of gas and some odd stars, enveloped in dark matter and orbited by smaller satellite galaxies, so there’s a lot to consider when building a model.
Luckily, astronomers have never underestimated the complexity of understanding galaxies as a whole. Although governed by what we consider straightforward laws of physics, the interaction between hundreds of billions of stars, black holes, gas, and dark matter requires computational power that is hard to come by. The Auriga Project used two supercomputers, the Hornet and the SuperMUC, to achieve these results.
The end product is a simulation that shows black holes growing and interacting with their environments, stars being born and then dying, and gas being pushed around by extreme and less extreme events.
The simulation also provides clues to how galaxy mergers play a role in the formation of spiral galaxies. The researchers saw that when the collision between two galaxies is slow, the disk becomes enriched with new gas, increasing the spin of the dark matter halo around the galaxy. The largest disks form in this way.
"For a spiral galaxy to grow in size, it needs a substantial supply of fresh star-forming gas around its edges - smaller gas-rich galaxies that spiral gently into ours can provide exactly that." Dr Grand said. The Sagittarius stream, a flow of stars that ribbons around the Milky Way, is the leftover of such an event.
The next step for the team is to integrate the data from other observatories like the Gaia survey, to continue and expand our understanding of how mergers shaped our galaxy.