The Milky Way Had A Catastrophic Encounter With The "Sausage Galaxy"

NGC 2207 getting hit by its companion IC 2163. ESO

Every galaxy in the universe will experience a certain number of collisions over its lifetime. Some will be minor and not change much. Some will be so big they transform a galaxy completely. Our own Milky Way has had its share of galactic encounters and now researchers have found evidence of another one.

Between 10 and 8 billion years ago a dwarf galaxy had a full frontal collision with the Milky Way. The small galaxy was destroyed but its stars began orbiting our galaxy. Their orbits remained peculiar though, and that’s how astronomers learned about the collision. By using the data from the European Space Agency’s Gaia telescope (which has studied over 1.7 billion stars) they tracked the motion of many stars and found that they used to belong to another galaxy.

“The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits that are long and narrow like needles," Vasily Belokurov, of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute in New York City, said in a statement.

The Gaia data shows the stars moving “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,” said Belokurov.

The peculiar orbits are also responsible for the nickname of the defunct dwarf galaxy. The team called it the Gaia Sausage because it definitely looks like a big sausage next to the galactic disk. The stars must have originated from the same object because they all turn around the center of the Milky Way in the same way.

“We plotted the velocities of the stars, and the sausage shape just jumped out at us," Wyn Evans of Cambridge University explained. "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.” 

The velocity distribution of the stars in the disk and in the Sausage. V. Belokurov (Cambridge, UK) and Gaia/ESACaption

The Sausage galaxy has been estimated to be quite big, around 10 billion times the mass of the Sun. That’s the same size as the Large Magellanic Cloud, one of the nearest satellites of the Milky Way, which is visible from the Southern Hemisphere.

The discovery is reported in a series of five papers, one is published in the Monthly Notices of the Royal Astronomical Society, one in the Astrophysical Journal Letters, and the remaining three are available on the arXiv.

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