The Milky Way appears as a calm band of stars from our view on Earth, but our home galaxy has a violent and active past. Astronomers have long known that the galaxy grew through star formation and mergers with other systems over the last 13 billion years. A new study published in Nature Astronomy provides specific details about one of the earliest and most formative events in our galaxy's history.
Led by Davide Massari from the Astrophysics and Space Science Observatory of Bologna, the research team used precise data from globular clusters to track the timeline of early galactic mergers. Globular clusters are dense groups of stars that form simultaneously, acting as reliable cosmic clocks. By examining these clusters, researchers can compare age and metallicity to map out the chemical history of the stars.
The findings reveal that the Milky Way experienced a major merger approximately 12 billion years ago. This event involved an incoming galaxy with a mass roughly equivalent to 500 million Suns. The team identified this progenitor as a combination of previously observed structures, now named Low-energy–Kraken–Heracles, or LKH.
This merger occurred about 1.8 billion years before the well-documented Gaia–Sausage–Enceladus event. By identifying these distinct age and metallicity sequences, astronomers are able to separate the chemical history of the early Milky Way from the galaxies it absorbed. Most of the material from this early LKH event remains located in the innermost parts of our galaxy.
This research allows for a transition from knowing a merger happened to reconstructing the conditions of the early universe. While telescopes like the James Webb Space Telescope show us snapshots of distant galaxies as they appeared billions of years ago, galactic archaeology allows us to study the surviving fossils of those processes here at home. These findings clarify the rapid growth stages that defined the first few billion years of our galaxy.