"Dark-Matter" Sagittarius Galaxy Smashed into the Milky Way Twice --Creating Outer Spiral Arms

A dwarf galaxy named Sagittarius loaded with dark matter has careened twice through our much larger home galaxy in the past two billion years, according to telescope data and detailed simulations, and is poised to strike the southern face of the Milky Way disk in another 10 million years or so.

Many astronomers believe that the large galaxies seen today were formed from smaller "dwarf" galaxies, which formed first after the Big Bang. Many of these dwarfs either clumped together to form larger galaxies or were gradually swallowed up by larger galaxies that continued to grow by "cannibalizing" 

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.

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

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 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."

Provided by The Daily Galaxy - University of California - Irvine