Astronomers have uncovered an ancient event in space history which involved a head-on collision between the Milky Way and a smaller object, dubbed the "Sausage" galaxy.
Discovered by an international team of astronomers, the cosmic crash was apparently a "defining event" in the early history of the Milky Way and reshaped the structure of the galaxy, fashioning both its inner bulge and its outer halo.
In a new research study, the astronomers propose it happened around eight billion to ten billion years ago, with an unknown dwarf galaxy smashing into our own Milky Way but not surviving the impact. The astronomers said it quickly fell apart, and the wreckage is strewn all around us.
"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 and the Center for Computational Astrophysics at the Flatiron Institute in New York City.
"The stars' paths take them very close to the centre of our galaxy. This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed."
To uncover the event, Belokurov and his colleagues used data from the European Space Agency's Gaia satellite, a spacecraft which has been mapping the stellar content of our galaxy and recording the journeys of stars as they travel through the Milky Way.
And it's thanks to Gaia that the astronomers now know the positions and trajectories of our celestial neighbours with unprecedented accuracy.
Professor Wyn Evans of the Institute of Astronomy at the University of Cambridge explained that it's the paths of the stars from the galactic merger that earned the moniker "the Gaia sausage".
"We plotted the velocities of the stars, and the sausage shape just jumped out at us. As the smaller galaxy broke up, its stars were thrown onto very radial orbits. These 'sausage stars' are what's left of the last major merger of the Milky Way," he said.
The new research also identified at least eight large, spherical clumps of stars called globular clusters that were brought into the Milky Way by the Sausage galaxy. Small galaxies generally do not have globular clusters of their own, so the Sausage galaxy must have been big enough to host a collection of clusters.
"While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all," added Sergey Koposov of Carnegie Mellon University, who has been studying the kinematics of the Sausage stars and globular clusters for some time.
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