The Milky Way was mixed from the beginning: Ancient merger traces found

The Milky Way absorbed a dwarf galaxy less than two billion years after the Big Bang, according to an international research team led by Italy’s National Institute for Astrophysics. The merger occurred 12.3 billion years ago, when the Milky Way had existed for less than a billion years, and 1.8 billion years before the Gaia-Sausage-Enceladus event.

Researchers had long suspected that an even earlier major merger had shaped the galaxy, but its traces proved difficult to identify. The young Milky Way was much smaller and the merger would have produced a more chaotic structure. After such a long period, many of the original signs had also disappeared.

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This study has revealed where the first major building block of the Milky Way came from.

The team turned to globular clusters, dense spherical groups of tens of thousands to millions of stars that formed almost simultaneously during the early universe. Using the Hubble Space Telescope, researchers examined 39 globular clusters within 20,000 light-years of the Milky Way’s centre. They measured the clusters’ ages and metal content, meaning the proportion of elements heavier than helium.

The analysis revealed 12 clusters that formed a distinct third group. They were older than clusters associated with the Gaia-Sausage-Enceladus merger but younger than native Milky Way clusters. Researchers concluded that the group came from a separate dwarf galaxy absorbed by the Milky Way. Their calculations suggest that the dwarf galaxy contained stars with a combined mass of about 500 million Suns, a substantial share of the young Milky Way’s mass.

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By combining Hubble’s high-precision observations with data from the European Space Agency’s Gaia space telescope, we were able to perfectly separate the cluster group born in LKH.

The researchers named the galaxy LKH, short for Low-energy-Kraken-Heracles, in recognition of three earlier studies that proposed an early merger. Davide Massari, who led the research, said the study identified the source of the first major building block of the Milky Way. Chiara Garbari, a co-author at the University of Bologna, said combining Hubble’s high-precision observations with data from the European Space Agency’s Gaia telescope made it possible to separate the LKH-born cluster group.

The team plans to examine additional globular clusters with Hubble and reconstruct the major mergers the Milky Way experienced across its history.

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Some scientists had believed that the early stages of galactic evolution were defined entirely by stars born internally, but this study proves that the role of stars originating in external galaxies must also be taken into account.