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Why the Milky Way is spiral

Astronomers show how the Milky Way galaxy got its spectacular spiral arms. According to telescope data and detailed simulations, the arms are likely the result of two-billion-year-old collisions between the Milky Way and a dwarf galaxy called Sagittarius.

india Updated: Sep 15, 2011 13:49 IST

UC Irvine astronomers have shown how the Milky Way galaxy got its spectacular spiral arms. According to telescope data and detailed simulations, the arms are likely the result of two-billion-year-old collisions between the Milky Way and a dwarf galaxy called Sagittarius.

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.

However, the collision that created the arms also deprived Sagittarius of its huge amounts of its stars and dark matter.

“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 UCI 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,” he added.

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.

The study was recently published in the journal Nature.