No galaxy is an island, and the Milky Way is no exception. After our galaxy began to form, it merged with about 15 other galaxies to build up what we see today.
A study of star clusters has identified three of them – two that are still in the process of being absorbed now, and a third bigger one that hadn’t been identified before, that has been nicknamed Kraken.
Globular clusters are dense groups of old stars that are found in most massive galaxies. The evolution of these clusters is highly dependent on their environments, so clusters formed in different galaxies have different proportions of elements. By measuring their age and metallicity – the ratio of heavy elements like iron to lighter ones like hydrogen – we can estimate the mass of the galaxy they come from.
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Diederik Kruijssen at the University of Heidelberg in Germany and his colleagues used a catalogue of 61 massive globular clusters in our galaxy to trace back its history. They compared these clusters to a set of 25 simulated galaxies that formed in a variety of ways but ended up similar to the Milky Way.
They inferred our galaxy’s history by matching the globular clusters that we see to similar ones in the simulated galaxies, and then rewinding the simulated galaxies back to their formation.
“They’re starting to dig into this real complexity and look at how our galaxy really formed in a lot more detail,” says Bill Harris at McMaster University in Canada.
They found that the Milky Way started forming metal-rich clusters early on, meaning that our galaxy must have become very massive relatively quickly. There is no huge population of clusters that violate the Milky Way’s original patterns of age and metallicity, so it has not merged with any galaxies a quarter its size or larger in more than 12 billion years.
But there are 35 globular clusters that aren’t so metal-rich, indicating that they formed in lower-mass galaxies. The researchers divided these clusters into three “branches” in the Milky Way representing three galaxies that it has devoured.
One of those branches has much lower metallicity than the other two, indicating a dwarf galaxy. The researchers say that it likely corresponds with the Sagittarius dwarf galaxy, which the Milky Way is still in the process of absorbing.
The other two are closer together. Some of these are associated with a galaxy called Canis Major. But some come from a new galaxy that we didn’t know about before, which the researchers dubbed Kraken. “Kraken is probably the most massive galaxy that the Milky Way ever accreted,” says Kruijssen. It was probably about one tenth the mass of the Milky Way and absorbed between six and nine billion years ago.
This is a step towards understanding the complex process of star formation and galactic mergers that built the Milky Way. It may be difficult to apply to other galaxies and learn about galaxy formation more generally, though. “In other galaxies it gets a lot more difficult to measure the ages of globular clusters accurately enough to make this method work,” says Harris.
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