It might have taken Nature a few billion years to build the Milky Way, but Lisa Wright and her colleagues did it in three monthsIt might have taken Nature a few billion years to build the Milky Way, but Lisa Wright and her colleagues did it in three months. That's how long it took their new computer simulation to create our home galaxy, starting with nothing but the foggy matter of the newborn universe and some physics assumptions.
Wright, a postdoctoral researcher at Cambridge University, said other computer models have examined how an existing galaxy might develop its spiral arms and the dense central bar structure common to the Milky Way and other large spiral galaxies. But they don't go back to the very dawn of time.
"We start from the beginning of the universe and let the system evolve," she told SPACE.com.
The computer program uses 195,000 dots to represent gas particles. It also starts with a distribution of exotic dark matter (another 195,000 virtual particles). This dark matter is a form of material that scientists theorize to exist but have never seen. Other studies have shown that dark matter is needed to trigger galaxy formation and, still today, must exist in order to hold galaxies together.
As the model runs, regions of instability are created in the distribution of dark matter and gas particles. The instabilities cause the particles to collapse under gravity, forming a protogalaxy, Wright explained. The gas in then cools and stars are born. It takes just over a billion years for four spiral arms to develop.
"Within 2 billion years, more and more gas collapses onto the galaxy and the spiral arms become unstable," Wright said. "A dense bar of material is formed across the center of the galaxy."
Such bars are observed today in other spiral galaxies and one is thought to cross the middle of the Milky Way, though scientists have a dim view of our own galactic center because it is obscured by all the stars, gas and dust between there and here.
The result of the simulation is a galaxy about two-thirds the size of our own, larger than early simulations were able to produce, Wright said.
Importantly, the program was not fed formulas designed to ensure a spiral disk galaxy.
"We simply begin with gravitational instabilities at the beginning of the universe and let the material collapse under gravity," Wright said. "In order to form a galaxy we also have to include the physics of the gas dynamics and allow the gas to cool and form stars."
Wright said the program could be used to look at how other galaxies might have formed. Many of the collections of stars in the universe are not classic spirals, but rather elliptical clumps. Astronomers think many or most of these elliptical galaxies are the products of giant mergers, and in fact our own Milky Way might one day lose its arms and take on an elliptical shape after merging with the Andromeda Galaxy.
"Our simulations would certainly be able to look at these details, but the current problem is that this high resolution simulation that has shown us the spiral arms takes approximately three months to run," Wright said. "Therefore we have yet to construct a statistical sample with which we can test theories of merging at this resolution."
Wright and her colleagues, Vincent Eke and George Efstathiou, are crafting a paper on their work to submit to a scientific journal. An animation of the computer simulation is here.
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