August 29, 2011
Simulation Creates Spiral Galaxy
Researchers at the University of California, Santa Cruz have used a supercomputer to create a spiral galaxy that matches our own Milky Way galaxy.
"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, a graduate student in astronomy and astrophysics at UC Santa Cruz and first author of a paper on the new simulation, called "Eris", said in a press release.
The Eris galaxy is a massive spiral galaxy with a central "bar" of bright stars and other structural properties consistent with galaxies like the Milky Way.
Piero Madau, professor of astronomy and astrophysics at UC Santa Cruz, said the project required a large investment of supercomputer time, including 1.4 million processor-hours on NASA's state-of-the-art Pleiades supercomputer.
"We took some risk spending a huge amount of supercomputer time to simulate a single galaxy with extra-high resolution," Madau said.
The results support the prevailing "cold dark matter" theory, which is when the evolution of structure in the universe is driven by the gravitational interactions of dark matter. The ordinary matter that forms stars and planets has fallen into the "gravitational wells" created by large clumps of dark matter.
However, efforts to reproduce this process for the past 20 years have failed to generate massive disk galaxies that look like the Milky Way.
Madau said that a realistic simulation of star formation was the key to Eris's success.
"Star formation in real galaxies occurs in a clustered fashion, and to reproduce that out of a cosmological simulation is hard," Madau said in a press release. "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. It shows that the cold dark matter scenario, where dark matter provides the scaffolding for galaxy formation, is able to generate realistic disk-dominated galaxies."
The researchers began to perform this simulation with a low-resolution simulation of dark matter evolving to form the haloes that host present-day galaxies. They chose a halo with an appropriate mass and merger history to host a galaxy like the Milky Way and "rewound the tape" back to the initial conditions.
They added gas particles and greatly increase the resolution of simulation by zooming in on the small region that evolved into the chosen halo.
"The simulation follows the interactions of more than 60 million particles of dark matter and gas. A lot of physics goes into the code--gravity and hydrodynamics, star formation and supernova explosions--and this is the highest resolution cosmological simulation ever done this way," Guedes, who is currently a postdoctoral researcher at the Swiss Federal Institute of Technology in Zurich, said in a press release.
The star-formation threshold in the Eris simulation allowed stars to form only in high-density regions, resulting in a more realistic distribution of stars.
"Supernovae produce outflows of gas from the inner part of the galaxy where it would otherwise form more stars and make a large bulge," Madau said in a press release. "Clustered star formation and energy injection from supernovae are making the difference in this simulation."
The paper has been accepted for publication in the Astrophysical Journal.
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