A Dwarf Galaxy Torn Apart by the Milky Way Left Behind the Gaia Sausage
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Learn more about the Gaia Sausage, a trail of stars from a dwarf galaxy the Milky Way absorbed billions of years ago.
Anastasia Scott is an Assistant Editor at Discover Magazine. Her work focuses on bringing clarity and creativity to scientific ideas. The Milky Way once absorbed a dwarf galaxy, scattering its stars across our own.
Billions of years later, some of those stars are still moving through the Milky Way on long, narrow paths that helped astronomers trace the ancient collision. Astronomers first described this wreckage in 2018, using data from the European Space Agency’s Gaia mission. When they plotted the stars’ motions, the structure took on a sausage-like shape and became known as the Gaia Sausage.
According to a 2018 study published in Monthly Notices of the Royal Astronomical Society, the dwarf galaxy merged with the Milky Way between about 8 billion and 11 billion years ago, around the time the galactic disk was forming. The galaxy had a total mass more than 10 billion times that of the sun. “The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits that are long and narrow like needles,” said Vasily Belokurov of the University of Cambridge in a press release.
“The stars’ paths take them very close to the center of our galaxy. This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit, and its fate was sealed.” The Gaia Sausage was identified by tracking how stars move through the Milky Way. Many stars in the galaxy’s disk orbit the galactic center in a more orderly rotation.
The Sausage stars stood out because they moved in highly radial orbits. These stars fall toward the center of the Milky Way, pass close to it, and then move far back out again. According to the 2018 Monthly Notices of the Royal Astronomical Society study, that motion pointed to debris from one larger satellite galaxy, not the gradual buildup of many smaller ones.
The Milky Way has absorbed smaller galaxies throughout its history, including the Sagittarius dwarf galaxy, which is still being pulled apart today. But the Gaia Sausage collision scattered debris through the inner Milky Way and likely disturbed the young galaxy’s disk.
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