Around 8 to 10 billion years ago, astronomers predict that our very own Milky Way galaxy had a head-on collision with a much smaller object, nicknamed the ‘Sausage’ galaxy. And, it’s this cosmic collision that scientists believe is responsible for reshaping the structure of the Milky Way, including its out halo and prominent inner bulge. 

Unfortunately, the dwarf galaxy did not survive the collision. It was quick to fall apart and disperse. According to Vasily Belokurov of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute, the dwarf was literally torn to shreds during the collision, leaving its stars spinning in orbits very close to the center of our own galaxy. “This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed,” says Belokurov.

Using data from the European Space Agency’s Gaia satellite, graduate student Gyu Chul Myeong and colleagues successfully outlined the details of this extraordinary event in several papers in the Monthly Notices of the Royal Astronomical Society, The Astrophysical Journal Letters, and

The Gaia satellite has been mapping out the contents of our galaxy for many years now and as a result of astronomers now know the location and trajectories of our celestial neighbors with great precision. It was the shape of the path of stairs leading from the merger that gave it the nickname “the Gaia Sausage”, says Wyn Evans of Cambridge. “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.”

While there have been other galactic collisions with our own Milky Way, none have been as significant as the Sausage Collison. Including all its gas, stars, and dark matter the total mass of the Sausage galaxy was more than 10 billion times that of our sun. When it crashed into the Milky Way the impact caused a lot of damage, causing the galaxy to have to regrow. 

In simulations of this event, stars from the Sausage galaxy enter elongated orbits. Evidence of this can be seen in the paths of those stars that have been inherited from the dwarf galaxy. “The Sausage stars are all turning around at about the same distance from the center of the galaxy,” says Alis Deason of Durham University. When the stars do this it causes the density in the Milky Way’s halo to decrease considerably. 

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