The Milky Way has absorbed other galaxies

A space telescope helped reconstruct the collisions that occurred six to eleven billion years ago.

According to astronomers, our Milky Way has swallowed up several large galaxies over billions of years and has grown significantly as a result. A German-British research team has now reconstructed this process using so-called globular clusters in a kind of family tree. As a result, our home galaxy merged with at least five large such star systems in the period from 6 to 11 billion years ago, as a team led by Diederik Kruijssen from the University of Heidelberg writes.

The researchers were also able to identify a previously unknown amalgamation with a mysterious galaxy, which they named "octopus" after the legendary sea creatures. The name is intended to suggest that the earlier galaxy was difficult to identify and emphasize the massive impact the collision had.

In the Milky Way there are around 150 globular clusters in which up to a million stars are arranged in a certain way. Some of these clusters were formed within the Milky Way. Others, according to Kruijssen, come from galaxies that the Milky Way has absorbed over time.

When two galaxies merge, their globular clusters, to put it simply, are whirled around. It is therefore relatively difficult for astronomers to determine which clusters used to form a galaxy of their own. So far, astronomers have identified four earlier galaxies that have merged with the Milky Way, Kruijssen explains.

The researcher and his team are now drawing an even more precise picture. Among other things, they made use of the Gaia space telescope from the European Space Agency (ESA), which measures the three-dimensional position and movement of stars and globular clusters. Although the clusters are scattered within the Milky Way, they can be assigned to a common precursor galaxy due to their specific movements within the Milky Way, their age of formation and their chemical composition.

The researchers analyzed this data with the help of artificial intelligence, which they had previously trained on the results of complex computer simulations on the formation of galaxies such as the Milky Way. This enabled them to determine relatively precisely what mass a precursor galaxy had and when it collided with the Milky Way. In addition to the four already known galaxies, they found evidence of the one that has now been identified.

Kraken merged with the then still young Milky Way about 11 billion years ago. At that time, our galaxy had, according to the researchers, only about a twentieth of its mass. As a result, the collision had a particularly strong effect. Overall, the Milky Way incorporated five large galaxies with more than 100 million stars each, plus around 10 smaller ones with at least 10 million stars each.