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Cosmic collision with ‘Sausage’ galaxy reshaped Milky Way

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Astronomers have discovered an ancient cosmic collision with an object, dubbed the “Sausage” galaxy, that reshaped the structure of the Milky Way galaxy. The crash was a defining event in the early history of the Milky Way and helped fashion both its inner bulge and its outer halo, according to a series of studies published in the Monthly Notices of the Royal Astronomical Society and The Astrophysical Journal Letters.

Scientists propose that around 8 billion to 10 billion years ago, an unknown dwarf galaxy smashed into our own Milky Way. The dwarf did not survive the impact: It quickly fell apart, and the wreckage is now all around us.

Researchers including those from Cambridge University in the UK used data from the European Space Agency’s Gaia satellite.

The Milky Way continues to collide with other galaxies, such as the puny Sagittarius dwarf galaxy. However, the Sausage galaxy was much more massive. Its total mass in gas, stars and dark matter was more than 10 billion times the mass of our Sun. When the Sausage crashed into the young Milky Way, its piercing trajectory caused a lot of mayhem. The Milky Way’s disk was probably puffed up or even fractured following the impact and would have needed to regrow.

Sausage debris was scattered all around the inner parts of the Milky Way, creating the ‘bulge’ at the galaxy’s center and the surrounding ‘stellar halo.’ Numerical simulations of the galactic mashup can reproduce these features, said Denis Erkal of the University of Surrey in the UK. In simulations run by researchers, stars from the Sausage galaxy enter stretched-out orbits. The orbits are further elongated by the growing Milky Way disk, which swells and becomes thicker following the collision. Evidence of this galactic remodeling is seen in the paths of stars inherited from the dwarf galaxy, said Alis Deason of Durham University in the UK.

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,” said Sergey Koposov of Carnegie Mellon University in the US, who has studied the kinematics of the Sausage stars and globular clusters in detail.

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