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First simulation of a
Milky Way-like galaxy

by Phil Unsworth
Posted: 31 August 2011

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A team’s efforts to create a computer simulation of a Milky Way-like galaxy is reported as successful, and helps to solve a recurring problem of galaxy formation.

Over the past twenty years various attempts have been made to recreate our own Galaxy in computer simulations, but have failed due to the complexity of the Milky Way’s composition, including the central bulge of stars, and the surrounding characteristic spiral arms. A new simulation named “Eris”, conducted using NASA’s state-of-the-art Pleiades supercomputer (along with supporting simulations from University of California Santa Cruz, and the Institute For Theoretical Physics, Zurich) has changed that, by modeling characteristics including brightness, stellar content, and the bulge-to-disc ratio.

On the left is Eris, as shown after 13 billion years. On the right is M74; a spiral galaxy similar to the Milky Way. Both are coloured to show gas clouds in red and stars in blue. Image: University of Zurich & NASA.

“Previous efforts to form a massive disc galaxy like the Milky Way have failed because the simulated galaxies ended up with huge central bulges compared to the size of the disc,” says Javiera Guedes, lead author of the paper describing the new simulation, which took nine months to process. “The simulation follows the interactions of more than 60 million particles of dark matter and gas. A lot of physics goes into the code, and this is the highest resolution cosmological simulation ever done this way.”

The top images shows the simulated galaxy Eris, as seen in the galactic plane. The lower image shows a similar view for the Milky Way, as taken in the infra-red by 2MASS. Both show the smaller bulge in comparison to the extending spiral arms. Image: 2MASS, Callegari & Guedes.

Dark matter, so called because it is so-far unseen in observations, and thought to comprise at least 80 percent of the matter in the Universe, is the main drive behind the prevailing theory of how structure in the Universe arose. Fluctuations in the dark matter density shortly after the big bang resulted in small clumps being pulled together by gravity. These small clumps then merged into larger and larger clumps with ‘normal’ matter falling into the gravitational wells created by the dark matter, resulting in galaxies sat within dark matter haloes. Given that the dark matter is thought to outnumber the normal matter, this has led to the “cold dark matter theory”, where structure in the Universe is driven by dark matter’s gravitational interactions.

“The simulation shows that the cold dark matter scenario, where dark matter provides the scaffolding for galaxy formation, is able to generate realistic disc-dominated galaxies,” says co-author Piero Madau, also of UCSC.

Along with the high density regions distributing star formations more accurately, the simulations also better reflect the behaviour of supernovae. In the high density regions, the exploding supernovae energy blows gas out of the galaxy, where it would otherwise accumulate and add to the central bulge. “Clustered star formation and energy injection from supernovae are making the difference in this simulation,” says Madau.

The team’s paper has been accepted for publication in The Astrophysical Journal.

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