Milky Way’s Family Tree
From small to large: this is one of the possible ways in which structures are created in the universe. Galaxies like our Milky Way are formed when several small predecessor objects join together. But what exactly did the Milky Way form from? An international team of astrophysicists led by Dr. Diederik Kruijssen from the Center for Astronomy at the University of Heidelberg has succeeded in reconstructing the merging history of our home galaxy and creating its family tree. To this end, the researchers analyzed the properties of globular star clusters orbiting the Milky Way.
Globular clusters are dense groups of up to one million stars that are almost as old as the universe itself. The Milky Way is home to over 150 such clusters. “Many of them come from smaller galaxies that later merged to form the Milky Way,” explains Kruijssen. To study the history of fusion, the Heidelberg researcher and his colleague Dr. Joel Pfeffer from Liverpool John Moores University and their research groups developed a series of computer simulations called E-MOSAICS. These simulations comprise a complete model for the formation, development and destruction of globular star clusters.
The German-British team used these simulations to relate the age, chemical composition and orbital motion of globular clusters to the properties of the precursor galaxies in which they were formed more than ten billion years ago. They then applied these findings to groups of globular clusters in the Milky Way, finding out not only how heavy these precursor galaxies were, but also when they merged with our home galaxy.
“The main challenge was that the merging process is extremely confusing because the orbits of the globular clusters are completely rearranged in the process,” explains Kruijssen. “To overcome this complexity, we developed an artificial neural network and trained it with the E-MOSAICS simulations. We were amazed at how precisely the AI enabled us to reconstruct the fusion histories of the simulated galaxies, even though we only used 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 fusion times of the precursor galaxies. They also discovered a previously unknown collision between the Milky Way and an unknown galaxy, which the researchers called “octopuses”.
“The collision with “Kraken” must have been the most significant fusion the Milky Way has ever seen,” Kruijssen adds. It was once thought that a collision with the galaxy Gaia-Enceladus about nine billion years ago was the largest collision event. But the fusion with Octopus took place eleven billion years ago, when the Milky Way was four times smaller than it is today. “Consequently, the collision with Kraken must have drastically changed the appearance of the Milky Way at that time,” says the Heidelberg scientist.
Together, these findings enabled the research team to reconstruct the first complete family tree of our home galaxy. In the course of its history, the Milky Way has swallowed about five galaxies with more than 100 million stars and about ten more with at least ten million stars. The heaviest precursor galaxies collided with the Milky Way six to eleven billion years ago. Kruijssen expects that these predictions will facilitate the future search for the remains of the precursor galaxies: “The debris of more than five precursor galaxies has now been identified. With current and future telescopes it should be possible to find them all”.