Milky Way family tree deciphered

Fusion tree of Milky Way galaxies deduced by applying the knowledge acquired from E-MOSAICS simulations to the population of galactic globular clusters. The main ancestor of the Milky Way is designated by the trunk of the tree, colored by its stellar mass. The black lines indicate the five identified satellites. The gray dashed lines illustrate other fusions that the Milky Way should have undergone, but which could not be linked to a specific ancestor. From left to right, the six images at the top of the figure indicate the identified progenitor galaxies: Sagittarius, Sequoia, Kraken, the main progenitor of the Milky Way, the ancestor of Helmi streams, and Gaia-Enceladus-Sausage. Credit: D. Kruijssen / Heidelberg University / License type: Attribution (CC BY 4.0)

Scientists have known for some time that galaxies can develop through the merging of smaller galaxies, but the ancestry of our own Milky Way galaxy is a long-standing mystery. Today, an international team of astrophysicists has successfully reconstructed the first complete family tree of our home galaxy by analyzing the properties of globular clusters orbiting the Milky Way with artificial intelligence. The work is published in Monthly notices from the Royal Astronomical Society.

Globular clusters are 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. Astronomers have suspected for decades that the ancient ages of globular clusters would mean they could be used as “fossils” to reconstruct the earliest assembly histories of galaxies. However, it was only with the latest models and observations that it became possible to fulfill this promise.

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 from John Moores University in Liverpool have now succeeded in deducing the history of the fusion of the Milky Way and to 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 galaxies. Their simulations, called E-MOSAICS, are unique in that they include a complete model for the formation, evolution and destruction of globular clusters.

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 this knowledge to groups of globular clusters in the Milky Way, they were able to not only determine the number of stars contained in these progenitor galaxies, but also when they merged into the Milky Way.

“The main challenge in relating the properties of globular clusters to the fusion history of their host galaxy has always been that galaxy assembly is an extremely complicated process, in which the orbits of globular clusters are completely altered,” explains Kruijssen.

“To make sense of the complex system that remains 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 fusion history of host galaxies. We have tested the algorithm tens of thousands of times on the simulations and were amazed at the accuracy with which it was able to reconstruct the fusion histories of the simulated galaxies, using only their globular cluster populations. “

Inspired by this success, the researchers set out to decipher the history of the Milky Way’s fusion. To do this, they used groups of globular clusters that would each have formed in the same progenitor 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 fusion times of the progenitor galaxies with great accuracy, but it also revealed a previously unknown collision between the Milky Way and an enigmatic galaxy, which the researchers named “Kraken”.

“The collision with Kraken must have been the most significant merger the Milky Way has ever seen,” adds Kruijssen. “Previously, it was believed that a collision with the Gaia-Enceladus-Sausage galaxy, which took place around 9 billion years ago, was the largest 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 really transformed what the Milky Way looked like at the time. “

Together, these results enabled the team of researchers to reconstruct the first complete fusion tree in our galaxy. During 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 stimulate future studies to search for the remains of these progenitor galaxies. “Debris from more than five progenitor galaxies has now been identified. With current and future telescopes, it should be possible to find them all,” concludes Kruijssen.