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.
About the University of California, Irvine: Founded in 1965, UC Irvine is a top-ranked university dedicated to research, scholarship and community service. Led by Chancellor Michael Drake since 2005, UC Irvine is among the most dynamic campuses in the University of California system, with nearly 28,000 undergraduate and graduate students, 1,100 faculty and 9,000 staff. Orange County's largest employer, UC Irvine contributes an annual economic impact of $4.2 billion. For more news, visit www.today.uci.edu.
