Our Milky Way ate at least six other galaxies

Our galaxy has swallowed a smaller congener at least six times, astronomers conclude. We didn’t know that one copy was on the menu yet.

The Milky Way is a cannibal. When a smaller galaxy comes close to our galaxy, it is grabbed, pulled apart and eaten. Traces of that gorge can still be seen hundreds of millions of years later – for those who know what to look for. Around the Milky Way, in the so-called halo, there are wisps and groups of stars that can be traced back to galaxies that were once swallowed by our own.

astronomer Khyati Malhan of the Max Planck Institute for Astronomy and colleagues have now deduced from such scraps that our Milky Way is has outdone another system six times before† Five of those amalgamations or mergers we already knew. But the sixth, the devouring of a system the researchers call Pontus, is new.

Similar jobs

To track down previous mergers with other galaxies, Malhan and his team looked at stellar streams, globular clusters and satellite galaxies that are now part of the Milky Way. “To my knowledge, it is the first time that all that kind of data has been combined,” says astronomer Emma Dodd of the University of Groningen, itself not involved in the study.

To track down previous mergers of our Milky Way with smaller galaxies, astronomers looked at stellar streams (the colored ‘smears’), globular clusters (stars) and satellite galaxies (boxes). Illustration: S. Payne-Wardenaar/K. Malhan, MPIA

Malhan and colleagues based this on the ‘actions’ of these objects; three physical entities that describe their orbits through the Milky Way. “These values ​​always stay the same no matter where an object is in its orbit,” explains Dodd. “We assume that objects from the same system have similar orbits – and therefore similar actions.”

Missing amalgamations

By searching the latest data from the Gaia satellite for objects with similar actions, Malhan and his team found five amalgamations that astronomers had already found in other ways. Beautiful in itself, of course, if you arrive at the same phenomena via different routes.

The list of mergers found is just not complete. The Argentine-Dutch professor of astronomy Amina Helmicalso from the University of Groningen, discovered in 1999 the helmi current† But the amalgamation that resulted in this flow was not found by Malhan and his associates. The same goes for two other known mergers: those with the Kraken and Thamnos systems.

New or not?

However, the astronomers did find traces of a new merger. One star stream and seven star clusters turned out to be traceable to a galaxy that the researchers named Pontus. (In Greek mythology, the . is sea ​​god Pontus a son of Gaia.)

The only question is whether this really concerns a new amalgamation. The stars that would come from Pontus were previously attributed to another amalgamation: Gaia Enceladus† In doing so, Malhan and his team emphasize the differences between the orbits of Pontus objects and Gaia Enceladus objects. Yet it is also quite conceivable that they ended up in our galaxy at the same merger.

Appetizer or dessert

All in all, as far as Dodd is concerned, the new study is “a very interesting study, looking at a new dataset with a new approach”. “I would have liked to have seen the researchers demonstrate a bit more how well their method works with fake data and simulations.”

Fortunately, astronomers can now expect even more measurement data in the near future, including from the aforementioned Gaia satellite. And I’m sure there’s more to it about the different galaxies that the Milky Way has consumed in the past — and about the sequence. Because which system formed the starter and which system formed the dessert in this hundreds of millions of years of dinner: the study by Malhan and his team says nothing about that.