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Astrophysicists have reconstructed the first complete family tree of the Milky Way by analyzing millions of stars.
Scientists have long known that galaxies can grow from the merger of smaller galaxies, but until now the ancestry of our galaxy has been a mystery.
Using artificial intelligence (AI) they analyzed the properties of globular clusters – dense groups of up to a million stars that are almost as old as the Universe itself.
The Milky Way is home to more than 150 of these clusters, many of which formed in the smaller galaxies that merged to form the galaxy we live in today.
For decades, astronomers suspected that the old eras of globular clusters would mean they could be used almost like fossils to reconstruct the earliest assemblage histories of galaxies.
An international team of researchers led by Dr Diederik Kruijssen, at the Center for Astronomy at the University of Heidelberg (ZAH), and Dr Joel Pfeffer, at Liverpool John Moores University, has now succeeded in inferring the history of the Via Lattea and rebuild its tree family, using only its globular clusters.
The researchers developed a suite of advanced computer simulations of the formation of Milky Way-like galaxies, called E-Mosaics.
In the 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.
By applying these insights to groups of globular clusters in the Milky Way, they could determine how many stars these progenitor galaxies contained and also when they merged into the Milky Way.
Dr Kruijssen said: “The main challenge in linking the properties of globular clusters to the merging history of their host galaxy has always been that the assembly of the galaxy is an extremely messy process, during which the orbits of globular clusters are completely remix “.
The research, published in the Royal Astronomical Society’s monthly notices, also revealed a previously unknown collision between the Milky Way and an enigmatic galaxy, which scientists called the Kraken.
Dr Kruijssen added: “The collision with Kraken must have been the most significant merger ever experienced by the Milky Way.
“Earlier it was thought that a collision with the Gaia-Enceladus-Sausage galaxy, which occurred about nine billion years ago, was the largest collision event.
“However, the merger with Kraken happened 11 billion years ago, when the Milky Way was four times less massive.
“Consequently, the collision with Kraken must have truly transformed the appearance of the Milky Way at that time.”
Taken together, the results allowed the team of researchers to reconstruct the Milky Way’s first complete fusion tree.
Throughout its history, the Milky Way has cannibalized around five galaxies with more than 100 million stars and around 15 with at least 10 million stars.
The most massive progenitor galaxies collided with the Milky Way between six and 11 billion years ago, the researchers say.
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