Milky Way's Spiral Arms Created By Collision With Another Galaxy
September 15, 2011

Milky Way’s Spiral Arms Created By Collision With Another Galaxy


The Milky Way's trademark features, its iconic spiral arms, were likely formed when the galaxy collided with the Sagittarius Dwarf galaxy, researchers announced on Thursday.

In their study, University of Pittsburgh postdoctoral research associate Christopher W. Purcell and colleagues were the first to use supercomputers to re-create repeated collisions between the two galaxies, which scientists previously believed had little impact on the Milky Way.

According to a University of Pittsburgh press release, experts have long believed that the spiral arms had formed through isolated evolution in the Milky Way. However, Purcell and colleagues have determined that those collisions, one of which happened approximately 1.9 billion years ago and the other which occurred 0.9 billion years ago, were profoundly affected.

"Researchers found that the collision set up instabilities--fluctuations in density of stars--in the flat disk of the rotating Milky Way," the school said. "Our galaxy rotates faster toward its center than toward its edges, so those instabilities were stretched and sheared, leading to the formation of spiral arms."

"The simulations also revealed that the impact gave rise to ring-like structures found at the edges of our galaxy," they added. "The second impact affected the Milky Way less, giving rise to only milder, less dense spiral-creating waves, because the Sagittarius Dwarf had by then lost most of its dark matter mass“¦ Without the dark matter to hold the dwarf galaxy together, its visible stars began to be pulled apart by the Milky Way's huge gravitational field and tidal forces."

The study was the first to simulate the collisions between the two galaxies, according to Purcell, and the results have been published in the journal Nature. Joining Purcell on the research team were University of California High-Performance AstroComputing Center (UC-HIPACC)-affiliates James S. Bullock, Erik J. Tollerud, and Miguel Rocha, all at the University of California - Irvine (UCI), as well as Sukanya Chakrabarti from Florida Atlantic University.

"It presents a new and somewhat unexpected way of thinking about why the galaxy we live in looks the way it does," Purcell said in a statement, adding that their work was the first to demonstrate the theory that Sagittarius might have been responsible for the spiral arm formation.

"Cosmologically speaking, it demonstrates the idea that relatively small impacts like this can have a dramatic impact on the structure of galaxies throughout the universe," he added. That first impact triggered instabilities that were amplified, and quickly formed spiral arms and associated ring-like structures in the outskirts of our galaxy."


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