Deciphered family tree of the Milky Way

Scientists have long known that galaxies can grow from the merger of smaller galaxies, but the ancestry of our Milky Way galaxy has been a long-standing mystery. Now, an international team of astrophysicists has managed to reconstruct the first complete family tree of our natal galaxy by analyzing the properties of globular clusters orbiting the Milky Way with artificial intelligence. The work is published in Royal Astronomical Society Monthly Notices.

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 which formed in the smaller galaxies that merged to form the galaxy we live in today. Astronomers have suspected for decades that the old age of globular clusters meant that they could be used as “fossils” to reconstruct the earliest assemblage histories of galaxies. However, it is only with the latest models and observations that this promise has become possible.

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 managed to infer the history of the Milky Way’s merger and reconstruct its family tree using only its globular clusters.

To achieve this, they developed a suite of advanced computer simulations of the formation of Milky Way-like galaxies. Their simulations, called E-MOSAICS, are unique because they include a complete model for the formation, evolution and destruction of globular clusters.

In the simulations, the researchers were able to relate the age, chemical compositions and orbital movements 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 not only determine how many stars these progenitor galaxies contained, but also when they merged into the Milky Way.

“The main challenge in linking the properties of globular clusters to the merging history of the host galaxy has always been that the assembly of the galaxy is an extremely messy process, during which the orbits of the globular clusters are completely reshuffled,” explains Kruijssen.

“To make sense of the complex system that has remained today, we therefore decided to use artificial intelligence. We trained an artificial neural network on E-MOSAICS simulations to relate the properties of the globular cluster to the history of the fusion of the host galaxy. We tested the algorithm tens of thousands of times during the simulations and were amazed at how accurately it was able to reconstruct the merger histories of simulated galaxies using only their populations of globular clusters. “

Inspired by this success, the researchers set out to decipher the story of the Milky Way’s merger. To achieve this, they used groups of globular clusters that are thought to have formed in the same parent galaxy based on their orbital motion. By applying the neural network to these groups of globular clusters, the researchers were not only able to predict the stellar masses and melting times of the progenitor galaxies with high precision, but also revealed a previously unknown collision between the Milky Way and an enigmatic galaxy, which the researchers called it “Kraken”.

“The collision with Kraken must have been the most significant merger ever experienced by the Milky Way,” adds Kruijssen. “Earlier it was thought that a collision with the Gaia-Enceladus-Sausage galaxy, which occurred about 9 billion years ago, was the largest collision event. However, the merger with Kraken occurred 11 billion years ago, when the Milky Way it was four times less massive. Consequently, the collision with Kraken must have really transformed the appearance of the Milky Way at that time. “

Taken together, these results allowed the team of researchers to reconstruct our galaxy’s first complete fusion tree. Throughout its history, the Milky Way has cannibalized about five galaxies with more than 100 million stars and about fifteen with at least 10 million stars. The most massive progenitor galaxies collided with the Milky Way between 6 and 11 billion years ago.

The researchers expect their predictions to spur future studies looking for the remains of these 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 Kruijssen.