Researchers decipher family tree of Milky Way

The Milky Way seen behind South Stack lighthouse on the Isle of Anglesey
The Milky Way seen behind South Stack lighthouse on the Isle of Anglesey (PA Wire)
14:24pm, Fri 13 Nov 2020
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Astrophysicists have reconstructed the first complete family tree of the Milky Way by analysing millions of stars.

Scientists have known for some time that galaxies can grow by the merging of smaller galaxies, but until now the ancestry of our own galaxy has been a mystery.

Using artificial intelligence (AI) they analysed 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 hosts more than 150 such clusters, many of which formed in the smaller galaxies that merged to form the galaxy we live in today.

For decades astronomers have suspected that the old ages of globular clusters would mean they could be used almost as fossils to reconstruct the early assembly histories of galaxies.

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

An international team of researchers led by Dr Diederik Kruijssen, at the Centre for Astronomy at the University of Heidelberg (ZAH), and Dr Joel Pfeffer, at Liverpool John Moores University, has now managed to infer the Milky Way’s merger history and reconstruct its family tree, 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 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.”

The research, published in Monthly Notices of the Royal Astronomical Society, also revealed a previously unknown collision between the Milky Way and an enigmatic galaxy, which the scientists named Kraken.

Dr Kruijssen added: “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 nine 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.”

Taken together, the findings allowed the team of researchers to reconstruct the first complete merger tree of the Milky Way.

Over the course of its history, the Milky Way cannibalised about 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, researchers say.

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