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.
Author
- Sven Buder
Astrophysicist, Australian National University
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.
Galaxies grow through mergers
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 2026 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.
Globular clusters sharpen the picture
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.
Galaxies chemically enrich themselves over time. As generations of stars form and die, they produce new elements that become incorporated into later generations. By comparing the ages and metallicities of these globular clusters, they were able to see the histories of how they formed.
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.
Much of its material was deposited in the innermost parts of the Milky Way. Connecting several previously named structures, the authors call its progenitor Low-energy-Kraken-Heracles, or LKH.
From knowing it happened to reconstructing what happened
This is what makes the result particularly interesting.
The advance is not simply another branch on the Milky Way's family tree. Earlier studies had already revealed this early accretion event. The new precision allows us to put a much better date and description on that branch.
The three age-metallicity sequences begin to separate the chemical histories of the early Milky Way, LKH and GSE. We can therefore ask not only when did these galaxies collide, but also what were they like, and how had they evolved beforehand?
That is especially valuable at these very early times. The first few billion years of the Milky Way were remarkably eventful, with major stages in its growth occurring in rapid succession.
There are still limitations. Globular clusters provide an incomplete record: some early galaxies may have formed few clusters, while others may have lost them. Reconstructing merger properties also inevitably relies on models. But the approach offers something unique.
Telescopes like the James Webb Space Telescope can now observe distant galaxies as they appeared more than 12 billion years ago. Those observations show us galaxies directly during the same era in which the young Milky Way was being assembled - but usually without resolving their individual stars.
The Milky Way provides the complementary view. Here we can examine surviving stars and clusters from that same epoch in extraordinary detail. Distant astronomy gives us snapshots of young galaxies. Galactic archaeology gives us their fossils.
Together, they are turning the Milky Way's once-blurry childhood into an increasingly detailed history of how galaxies formed in the young universe.
Sven Buder receives funding from the Australian Research Council. He is a member of the Astronomical Society of Australia.
