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How the Milky Way got its spiral arms

computer model of the Milky Way

Credit: Tollerud, Purcell and Bullock/UC Irvine

Computer model of the Milky Way and its smaller neighbor, the Sagittarius dwarf galaxy. The flat disk is the Milky Way, and the looping stream of material is made of stars torn from Sagittarius as a result of the strong gravity of our galaxy. The spiral arms began to emerge about two billion years ago, when the Sagittarius galaxy first collided with the Milky Way disk.

IRVINE — UC Irvine astronomers have shown how the Milky Way galaxy's iconic spiral arms form, according to research published today in the journal Nature.

A dwarf galaxy named Sagittarius loaded with dark matter has careened twice through our much larger home galaxy in the past 2 billion years, according to telescope data and detailed simulations, and is lined up to do it again. As the galaxies collide, the force of the impact sends stars streaming from both in long loops. Those continue to swell with stars and are gradually tugged outward by the Milky Way's rotation into a familiar ringed arm.

It's the weighty dark matter from Sagittarius that provided the initial push, the researchers said.

"It's kind of like putting a fist into a bathtub of water as opposed to your little finger," said James Bullock, a theoretical cosmologist who studies galaxy formation.

The smaller galaxy pays a steep price, though — sucked inward repeatedly by the Milky Way's mightier gravity, it's being ripped apart by the blows, sending huge amounts of its stars and dark matter reeling into the new arms.

"When all that dark matter first smacked into the Milky Way, 80 percent to 90 percent of it was stripped off," explained lead author Chris Purcell, who did the work with Bullock at UC Irvine and is now at the University of Pittsburgh. "That first impact triggered instabilities that were amplified, and quickly formed spiral arms and associated ring-like structures in the outskirts of our galaxy."

The Sagittarius galaxy is due to strike the southern face of the Milky Way disk fairly soon, Purcell said — in another 10 million years or so.

Additional authors are UC Irvine doctoral students Erik Tollerud and Miguel Rocha, and Sukanya Chakrabarti of Florida Atlantic University in Boca Raton.

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