Violent past: Stars in Milky Way came from another galaxy 12 bn years ago

Sven Buder

Look up on a dark night, and the Milky Way appears as a calm band of stars across the sky. But our galaxy has a violent past.

For more than 13 billion years, it has grown through star formation and mergers with other galaxies. We already know that several substantial galaxies helped build the young Milky Way.

A new study, published in Nature Astronomy, now brings one of the earliest of these events into much sharper focus. Led by astronomer Davide Massari from the Astrophysics and Space Science Observatory of Bologna, Italy, the team used exceptionally precise ages of very dense clusters of stars to better determine when this ancient merger happened, how massive the incoming galaxy was, and how it evolved before becoming part of the Milky Way.

This takes us into one of the most formative – and most difficult to reconstruct – periods of our galaxy’s history: its first few billion years.

In our current picture of galaxy formation, large galaxies grow gradually as smaller systems fall together under gravity and merge. Gravity pulls an incoming galaxy apart and spreads its stars through the larger system. Billions of years later, those stars can still preserve clues to where they came from. The Milky Way therefore contains a fossil record of galaxies it has absorbed.

The importance of this idea was highlighted this year when the Kavli Prize in Astrophysics was awarded to Vasily Belokurov, Amina Helmi and Rodrigo Ibata for pioneering work uncovering the fossil evidence of past mergers and showing how the Milky Way grew through this process.

One of the clearest chapters in that history is Gaia–Sausage–Enceladus, or GSE: a substantial galaxy that merged with the Milky Way around 10 billion years ago.

But going further back becomes much harder. At earlier times the Milky Way itself was still small, and the galaxies merging with it were substantial building blocks rather than minor satellites falling into a mature galaxy.

Previous studies had already established evidence for another major merger event before GSE, associated with star populations known as “Kraken”, “Heracles”, and a “low-energy” group of globular clusters.

The new study sharpens our view of this event considerably.

Massari and colleagues used globular clusters: dense groups containing hundreds of thousands of stars that formed at roughly the same time.

This makes them unusually precise cosmic clocks. Dating individual ancient stars is difficult, but globular clusters allow astronomers to compare many stars sharing the same age. Using exquisite Hubble Space Telescope observations and sophisticated modelling, the researchers determined very precise relative ages for globular clusters across the Milky Way.

They then compared these ages with each cluster’s metallicity – the abundance of elements heavier than the two lightest elements of hydrogen and helium. The striking result is that the Milky Way’s globular clusters trace three distinct age–metallicity sequences.

One is associated with the early Milky Way itself. Another belongs to Gaia–Sausage–Enceladus. Between them lies a third sequence associated with the earlier merger.

The authors estimate that this event occurred about 1.8 billion years before GSE and involved a galaxy containing roughly 500 million Suns’ worth of stars – similar in stellar mass to GSE.