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The Milky Way collided and merged with many other galaxies over the course of its long life, but reconstructing its history was a tall order.
A new analysis of dense star clusters orbiting the Milky Way has now given us the story of the most complete galactic merger ever. And in that data is a previously unknown merger event that took place 11 billion years ago and completely altered the shape of our galaxy. Astronomers have called that galaxy – summed up by the Milky Way – the Kraken.
The star clusters in question are called globular clusters, often considered “fossils” of the early Universe. They are very dense spherical clusters of about 100,000 to 1 million very old stars, some nearly as old as the Universe itself. In each globular cluster, all of its stars formed at the same time, from the same gas cloud, which means we can use their chemical composition to calculate their origins.
But that’s only part of the puzzle. The Milky Way has around 150 globular clusters, and even processing and reconstructing their orbital motion and current shape (some are torn apart in long stellar streams), can also help reconstruct where they came from.
And that’s what a team of astronomers did, using an artificial neural network to simulate globular clusters orbiting galaxies similar to the Milky Way. These simulations, called E-MOSAICS, model the complete lifespan of globular clusters, from formation to evolution to destruction.
“The main challenge in linking the properties of globular clusters to the merging history of their host galaxy has always been that galaxy assembly is an extremely messy process, during which the orbits of globular clusters are completely reshuffled,” said L astronomer Diederik Kruijssen of the University of Heidelberg, Germany.
“We tested the algorithm tens of thousands of times in simulations and were amazed at the accuracy with which it was able to reconstruct the merger histories of simulated galaxies using only their globular cluster populations.”
The next step was to feed the real software data. In recent years, the Gaia satellite has worked diligently to map the Milky Way with high precision and detail not only in space, but also in time. This has given us the most accurate data to date on the positions and movements of galactic objects, resulting in some fascinating discoveries about our home galaxy.
Using data from Gaia, the team grouped globular clusters based on their orbital motion. This is because the clusters that have similar orbits around the Milky Way are thought to come from the same place; that is, a galaxy that was engulfed by the Milky Way at some point in its past.
(Kruijssen et al., MNRAS, 2020)
When this data was processed by the team’s software, the results agreed closely with five galactic collisions.
Four of them were known: the Gaia-Enceladus galaxy, also known as the Gaia sausage, devoured by the Milky Way about 9 billion years ago; the Helmi torrents, from a merger that took place about 10 billion years ago; the Sequoia galaxy, which merged with the Milky Way about 9 billion years ago; and the dwarf galaxy of Sagittarius, which has repeatedly pierced the Milky Way for billions of years.
The fifth event was pieced together by a recently discovered group of “low-energy” globular clusters. According to the software analysis, these all closely match the properties of a previously unknown – and very important – collision with what the team dubbed the Kraken galaxy.
“The collision with Kraken must have been the most significant merger the Milky Way has ever experienced,” Kruijssen said.
“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. “
Using this information, the team put together a “ family tree ” describing the Milky Way’s most up-to-date collision history: the five major collisions with galaxies containing over 100 million stars, which occurred between 6 and 11 billion years ago. ; and about 15 smaller merger events with galaxies containing over 10 million stars.
This tree should form the basis for ongoing and future efforts to understand the epic events that changed the history of our galaxy.
“Debris from more than five progenitor galaxies has now been identified,” Kruijssen said. “With current and future telescopes, it should be possible to find them all.”
The research was published in Royal Astronomical Society Monthly Notices.
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