Researchers have used artificial intelligence to reconstruct the history of our Milky Way - and found evidence of an ancient collision with a mysterious galaxy.
The researchers say that the collision with the “Kraken” galaxy must have reshaped the Milky Way 11 billion years ago.
It was the biggest collision the Milky Way galaxy experienced - although far from the only one.
Over the course of its history, the Milky Way cannibalised about five galaxies with more than 100 million stars, and about fifteen with at least 10 million stars.
Dr. Diederik Kruijssen at the Center for Astronomy at the University of Heidelberg (ZAH) said, “The collision with Kraken must have been the most significant merger the Milky Way ever experienced.
“Before, it was thought that a collision with the Gaia-Enceladus-Sausage galaxy, which took place some 9 billion years ago, was the biggest collision event.”
“However, the merger with Kraken took place 11 billion years ago, when the Milky Way was four times less massive. As a result, the collision with Kraken must have truly transformed what the Milky Way looked like at the time.”
The researchers used globular clusters - dense groups of up to a million stars that are almost as old as the Universe itself - to work out the history of the Milky Way.
The work was published in Monthly Notices of the Royal Astronomical Society.
The Milky Way hosts over 150 globular clusters, many of which formed in the smaller galaxies that merged to form the galaxy that we live in today. A
Researchers used the globular clusters like “fossils” to reconstruct the history of the Milky Way.
Using simulations, the researchers were able to relate the ages, chemical compositions, and orbital motions of globular clusters to the properties of the progenitor galaxies in which they formed, more than 10 billion years ago.
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By applying these insights to groups of globular clusters in the Milky Way, they could not only determine how many stars these progenitor galaxies contained, but also when they merged into the Milky Way.
Kruijssen said, “The main challenge of connecting the properties of globular clusters to the merger history of their host galaxy has always been that galaxy assembly is an extremely messy process, during which the orbits of the globular clusters are completely reshuffled,”
“To make sense of the complex system that is left today, we therefore decided to use artificial intelligence.
“We trained an artificial neural network on the E-MOSAICS simulations to relate the globular cluster properties to the host galaxy merger history.
“We tested the algorithm tens of thousands of times on the simulations and were amazed at how accurately it was able to reconstruct the merger histories of the simulated galaxies, using only their globular cluster populations.”