Hubble and Gaia Uncover the Milky Way's Earliest Major Collision

The Milky Way did not emerge fully formed; it grew across cosmic time by drawing in surrounding gas clouds and absorbing smaller neighboring galaxies. Combining precise measurements from the Hubble Space Telescope and the Gaia spacecraft, astronomers have now identified clear evidence of a major collision that took place roughly 11.8 billion years ago, extending our timeline of the galaxy's early assembly by nearly two billion years.

Reconstructing Galactic History

Today, the Milky Way is a vast spiral galaxy containing hundreds of billions of stars spanning over 100,000 light-years. To understand how such a structure formed, astronomers look for cosmic clues left behind by ancient galactic collisions. When a smaller dwarf galaxy falls into a massive host galaxy, the gravitational tug of war rips the smaller system apart, dispersing its stars, dust, gas, and dark matter across the host's disc and halo.

Astronomers have already documented several key mergers in our galaxy's past. The Sagittarius dwarf galaxy began interacting with the Milky Way more than 6 billion years ago and continues to be gradually pulled apart today. Looking further back, researchers identified the Gaia-Sausage-Enceladus merger, an event roughly 10 billion years ago that reshaped the structure of our galaxy's stellar disc. While theorists and computer simulations suggested that an even older collision occurred before these events, conclusive observational proof had remained elusive until now.

Globular Clusters as Ancient Time Capsules

To peer into the earliest epochs of the Milky Way, researchers turned to globular clusters—densely packed, spherical collections containing tens of thousands to millions of ancient stars. Because globular clusters are among the oldest surviving structures in the universe, they serve as cosmic archaeological sites. Clusters that formed inside a devoured dwarf galaxy often retain distinct chemical and physical signatures that set them apart from clusters born locally within the main galactic disc.

Unraveling this history requires high-precision measurements of two key cluster properties: age and metallicity. In astronomy, metallicity refers to the fraction of an object's mass made of elements heavier than hydrogen and helium, which are forged inside stellar cores and scattered through supernova explosions over successive generations of stars. Earlier generations of stars generally possess lower metallicity because they formed before the cosmos was enriched with heavy elements.

Uncovering the LKH Dwarf Galaxy

The research team, led by astronomer Davide Massari of the Astrophysics and Space Science Observatory of Bologna, examined Hubble observations of 39 globular clusters located within the inner 20,000 light-years of the Milky Way. This central region is where astronomers expect the remnants of the earliest mergers to be concentrated, though intense stellar crowding and dust have historically made it challenging to study.

By coupling Hubble's sharp imaging with orbital tracking data from the European Space Agency's Gaia mission, the scientists categorized the clusters by age and chemical abundance. Alongside clusters native to the young Milky Way and those left behind by the 10-billion-year-old Gaia-Sausage-Enceladus merger, they discovered a distinct third population. These specific clusters were older than the Gaia-Sausage-Enceladus group but younger than the native Milky Way clusters of similar metallicity.

This distinct chemical and temporal fingerprint demonstrated that the clusters originated within a separate dwarf galaxy absorbed by the Milky Way approximately 11.8 billion years ago—just 2 billion years after the Big Bang. The researchers named this lost system Low-energy-Kraken-Heracles, or LKH, honoring previous theoretical studies that had predicted an early merger event.

The Scale of the Early Collision

Calculations indicate that the LKH dwarf galaxy contributed roughly 500 million solar masses in stars to the growing Milky Way. While 500 million times the mass of the Sun represents a modest amount compared to the total mass of the modern Milky Way, it made up a significant fraction of the galaxy's total mass during that primordial era.

The discovery provides direct evidence that external galactic mergers played a crucial role during the earliest phases of our galaxy's evolution. Previous models frequently assumed that the infant Milky Way grew primarily through internal star formation from native gas reserves. The presence of LKH proves that major cannibalistic merger events began delivering fundamental building blocks to our galactic neighborhood far earlier than confirmed by prior observations.

Mapping the Unseen Past

Astronomers plan to expand this survey by targeting additional, previously unstudied globular clusters across the Milky Way. Analyzing these ancient star systems with modern space observatories allows researchers to build a comprehensive family tree of all the dwarf galaxies that sacrificed their identities to build our home galaxy.

As telescopes peer deeper into both the distant universe and the ancient relics in our cosmic backyard, the story of the Milky Way continues to shift from a static picture into an active, dynamic record of galactic collisions spanning nearly the entire age of the cosmos.

What to explore next

Learn more about galactic structure, stellar nurseries, and the spacecraft unraveling the universe across these guides:

• Discover how galaxies form and evolve in our guide to Stars and Galaxies.
• Read about the instruments mapping our cosmos on the Space Missions hub.
• Learn how astronomers study ancient stellar remnants across Deep Space.