How long does it take to simulate the Milky Way? The answer is about nine months, if you're using a powerful supercomputer. That's how long it took for researchers at the University of California at Santa Cruz and the Institute for Theoretical Physics in Zurich to produce the first simulation of galaxy formation that approximates the look of our own Milky Way spiral.
"Previous efforts to form a massive disk galaxy like the Milky Way had failed, because the simulated galaxies ended up with huge central bulges compared to the size of the disk," Javiera Guedes said today in a news release about the project.
Guedes worked on the project during her time at UC-Santa Cruz, and is now a postdoctoral researcher at the Swiss Federal Institute of Technology. She's the first author of a paper accepted for publication in the Astrophysical Journal that describes the simulation, known as the Eris galaxy.
For 20 years, astronomers have been trying to come up with a simulated galaxy that comes close to the look of the Milky Way and other spiral galaxies — but fell short of the mark. Guedes and her colleagues were more successful in part because of the computer firepower at their disposal: 1.4 million processor-hours on NASA's Pleiades supercomputer, plus additional supporting simulations at UC-Santa Cruz and the Swiss National Supercomputing Center.
"We took some risk spending a huge amount of supercomputer time to simulate a single galaxy with extra-high resolution," said UC-Santa Cruz astronomer Piero Madau, one of the paper's co-authors.
The effort used a software platform known as Gasoline to trace the motions of more than 60 million particles, representing galactic gas as well as dark matter, over the course of more than 13 billion years.
Madau said developing a realistic simulation of star formation was another key to Eris' success.
"Star formation in real galaxies occurs in a clustered fashion, and to reproduce that out of a cosmological simulation is hard," he said. "This is the first simulation that is able to resolve the high-density clouds of gas where star formation occurs, and the result is a Milky Way type of galaxy with a small bulge and a big disk."
The recipe for the Eris galaxy limited star formation to the high-density regions of the galactic disk, which resulted in a more realistic distribution of stars. Within the high-density regions, supernova explosions powered an outflow of gas from the inner part of the galaxy, keeping the central bulge from getting too big.
The point of the exercise wasn't merely to come up with a pretty animation. The virtual conditions for Eris' creation are consistent with the theory that galaxy-scale structures coalesced from cosmic webs that were dominated by cold dark matter. Gravity drew primordial clumps of dark matter together into bigger clumps, and the "ordinary" matter that makes up stars and galaxies fell into those dark-matter clumps — giving rise to visible galaxies embedded in halos of invisible dark matter.
Cosmologists contend that the universe consists of 4.6 percent ordinary matter, 23.3 percent dark matter and 72.1 percent dark energy. But the fact that astronomers found it difficult to produce galaxies like the Milky Way using that formula led some to question the prevailing cosmological model of the universe. The Eris galaxy simulation "shows that the cold dark matter scenario, where dark matter provides the scaffolding for galaxy formation, is able to generate realistic disk-dominated galaxies," Madau said.
The research team's effort may be a tour de force for supercomputing, but don't confuse the virtual Eris with the real-life Milky Way. Even though Eris is an incredibly high-resolution simulation, its 60 million particles of gas and dark matter pale in comparison with the Milky Way's hundreds of billions of stars.
Extra credit: Eris is named after the Greek goddess of discord, in recognition of the decades of discordant debate that have surrounded the scenarios for forming spiral galaxies, according to a description of the project on the HPC-CH weblog. Guedes' website includes a quote from the Iliad: "The soldiers fought like wolves while Eris, the Lady of Sorrow, watched with pleasure." The simulated galaxy isn't the first astronomy-related object to bear that discordant name: Eris is also the name given to the dwarf planet that caused so much trouble for Pluto.
More about dark matter and cosmology:
- Has dark matter finally been seen?
- Dark-matter stars could solve cosmic mystery
- Gallery: Dark matter revealed!
- Dark matter mapped in 3-D detail
In addition to Madau and Guedes, co-authors of the paper include Simone Callegari and Lucio Mayer of the Institute for Theoretical Physics in Zurich. This research was funded by NASA, the U.S. National Science Foundation, the Swiss National Science Foundation and an ARCS Foundation fellowship to Guedes.
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