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Galaxies such as the Milky Way formed by the merger of smaller progenitor galaxies. An international team of astrophysicists led by Dr. Diederik Kruijssen of the Center for Astronomy at the University of Heidelberg has managed to reconstruct the history of the merger of our home galaxy, creating a complete family tree. To achieve this, the researchers analyzed the properties of globular clusters orbiting the Milky Way with artificial intelligence. Their investigations revealed a previously unknown galactic collision that must have permanently altered the appearance of the Milky Way.
Globular clusters are dense groups of up to a million stars that are nearly the same age as the universe itself. The Milky Way is home to over 150 of these clusters. “Many of them came from smaller galaxies that later merged to form the Milky Way we live in today,” explains Dr. Kruijssen. To study the history of fusion, the Heidelberg researcher and his colleague Dr. Joel Pfeffer of the John Moores University of Liverpool (UK) and their research groups developed a suite of advanced computer simulations, called E-MOSAICS. These simulations include a complete model for the formation, evolution and destruction of globular clusters.
The German-British team used these simulations to relate the age, chemical compositions and orbital movements of globular clusters with the properties of the progenitor galaxies in which they formed, more than ten billion years ago. By applying these insights to groups of globular clusters in the Milky Way, they not only determined how massive these progenitor galaxies were, but also when they merged with our home galaxy.
“The main challenge was that the fusion process is extremely disordered, because the orbits of the globular clusters are completely reshuffled,” explains Dr. Kruijssen. “To overcome this complexity, we developed an artificial neural network and trained it on E-MOSAICS simulations. We were amazed at how precisely artificial intelligence allowed us to reconstruct the merger histories of simulated galaxies, using only the their globular clusters “. The researchers then applied the neural network to groups of globular clusters in the Milky Way and precisely determined the stellar masses and melting times of the progenitor galaxies. They also discovered a previously unknown collision between the Milky Way and an unknown galaxy, which the researchers called “Kraken”.
“The collision with Kraken must have been the most significant merger ever experienced by the Milky Way,” adds Dr. Kruijssen. Previously, a collision with the Gaia-Enceladus galaxy around nine billion years ago was thought to be the largest collision event. However, the merger with Kraken occurred eleven billion years ago, when the Milky Way was four times less massive than it is today. “Consequently, the collision with Kraken must have really transformed the appearance of the Milky Way at that time,” explains the Heidelberg scientist.
Taken together, these findings enabled the team of researchers to reconstruct the first complete family tree of our natal galaxy. Throughout its history, the Milky Way has cannibalized about five galaxies with more than 100 million stars and about ten others with at least ten million stars. The most massive progenitor galaxies collided with the Milky Way between six and eleven billion years ago. Dr. Kruijssen predicts that these predictions will help future search for the remains of the progenitor galaxies. “Debris from more than five progenitor galaxies have now been identified. With current and future telescopes, it should be possible to find them all,” concludes the Heidelberg researcher.
Video: https://www.youtube.com/watch?v=3ulDALI8wT4&feature=emb_logo
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Materials provided by Heidelberg University. Note: The content can be changed by style and length.
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