© NASAArtist's concept of the Milky Way galaxy, with the "galactic bar" visible in the center. Like a stone dropping into a pond, a galactic collision almost two billion years ago resulted in vibrations that caused stars to clump together.
Their findings, which appear on the pre-press physics Web site arXiv, explain why the Milky Way is vibrating, or "ringing," and why stars in it are clustering together.
Astronomers have known for almost a decade that the Milky Way is ringing, said Ken Freeman of the Research School of Astronomy and Astrophysics at the Australian National University.
They have suspected the ringing was caused by a bar-like structure in the center of the galaxy.
"In the center of our galaxy is a bar-like structure that is quite massive," he said. "Because it's not round its gravitational field is not round, so you get a certain kind of resonance between the bar and the stars that are moving."
But Freeman and colleagues have now shown, through mathematical modeling, that a major cause of the galactic ringing was a collision by a small galaxy that occurred 1.9 billion years ago.
Like a stone dropping into a pond, the collision resulted in vibrations that set up a resonance in the Milky Way and caused stars to group into clusters.
Freeman said clusters of stars can occur during star formation. They also can form when a small galaxy is sucked into our own, broken up into pieces and its debris spread around.
He said it is important to "weed out" the clusters that form as a result of the ringing, in order to determine the formation of the Milky Way.
"It's pretty important to work out which one of these is which, if we want to try and trace what has happened in the galaxy over the years," he said.
The new model shows a series of dense star streams, which are similar to the distribution of stars observed in our solar neighborhood. Freeman said the model allowed him and his colleagues to turn back the clock and determine when the galaxy that helped set off the ringing collided with the Milky Way.
Although it is difficult to determine the exact size of the smaller galaxy, Freeman estimates that it would have been one-hundredth the size of the Milky Way.
Freeman is eager to collect more data to further test the model and is awaiting the launch of a European satellite named Gaia, which is scheduled to launch in 2015.
The orbiting telescope is expected to produce a massive increase in data.
"Gaia is going to measure the motions of about a billion stars, and it won't be just stars nearby, but stars up to around 10 kiloparsecs [32,600 light-years] away," said Freeman. "We'll be able to measure motions on the other side of the center of the galaxy. It will be a revolution in collecting data."
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