Astrophysicists report first simulation to create a Milky Way-like galaxy
August 30, 2011
After nine months of number-crunching on a powerful supercomputer, a beautiful spiral galaxy matching our own Milky Way has emerged from a computer simulation of the physics involved in galaxy formation and evolution by researchers at the University of California, Santa Cruz (UCSC) and the Institute for Theoretical Physics in Zurich.
The “Eris” simulation solves a longstanding problem that had led some to question the prevailing cosmological model of the universe, the researchers said.
“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,” said Javiera Guedes at UCSC.
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. Its brightness profile, bulge-to-disk ratio, stellar content, and other key features are all within the range of observations of the Milky Way and other galaxies of the same type. “We dissected the galaxy in many different ways to confirm that it fits with observations,” Guedes 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, plus additional supporting simulations on supercomputers at UCSC and the Swiss National Supercomputing Center.
The results support the prevailing “cold dark matter” theory, in which the evolution of structure in the universe is driven by the gravitational interactions of dark matter (“dark” because it can’t be seen, and “cold” because the particles are moving slowly).
Gravity acted initially on slight density fluctuations present shortly after the Big Bang, pulling together the first clumps of dark matter, which grew into larger and larger clumps through the hierarchical merging of smaller progenitors. The ordinary matter that forms stars and planets (less than 20 percent of the matter in the universe) has fallen into the “gravitational wells” created by large clumps of dark matter, giving rise to galaxies in the centers of dark matter halos.
For the past 20 years, however, efforts to reproduce this process in computer simulations have failed to generate massive disk galaxies that look anything like the Milky Way, with its spiral arms in a large flat disk around a small central bulge made up of old stars. A realistic simulation of star formation was the key to Eris’s success, the researchers said.
To perform the Eris simulation, the researchers began with a low-resolution simulation of dark matter evolving to form the haloes that host present-day galaxies. Then 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. Zooming in on the small region that evolved into the chosen halo, they added gas particles and greatly increased the resolution of the simulation.
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, the researchers said.
The high resolution allowed for a more precise recipe for star formation. In a low-resolution simulation, with gas densities averaged out over relatively large areas, the threshold density for star formation has to be set so low that stars tend to form in diffuse gas throughout the galaxy. In the Eris simulation, the star-formation threshold allowed stars to form only in high-density regions, resulting in a more realistic distribution of stars.
Their research is being published in the Astrophysical Journal [link].