Astronomers have uncovered definitive evidence that the Milky Way experienced a major collision and merger with a dwarf galaxy named LKH 11.8 billion years ago. The discovery, based on Hubble Space Telescope observations of globular clusters, extends our knowledge of galactic history 1.8 billion years farther back in time.
Our home galaxy grew to its massive current size in part by consuming smaller systems. While researchers previously mapped later cosmic crashes, an ancient merger that occurred just two billion years after the Big Bang has remained heavily debated.
Uncovering the LKH Dwarf Galaxy Collision
The Milky Way today is a sprawling spiral home to hundreds of billions of stars, but it built those proportions over billions of years through stellar birth and galactic cannibalism. Until recently, the most distant major chapter scientists could read with confidence involved a collision with the Gaia-Sausage-Enceladus galaxy roughly 10 billion years ago. A more recent, ongoing merger with the Sagittarius dwarf galaxy began over six billion years ago.
Now, peering deeper into the galaxy’s murky past, astronomers have pushed that timeline back. The Hubble Space Telescope detected definitive proof of an earlier crash that took place about 11.8 billion years ago when a dwarf galaxy known as LKH collided with a young Milky Way and merged with it.
Massari serves as the lead author of a paper describing the discovery. In this paper we discover where the first significant batch of bricks came from: a dwarf galaxy that we call LKH,
he explained. The name LKH stands for Low-energy–Kraken–Heracles, an acknowledgement of three earlier research papers that wrestled with the theory of very early galactic mergers.
Using Globular Clusters as Cosmic Archaeological Sites
Looking back across cosmic evolution presents immense challenges. When the Milky Way was young, it was much smaller and closer in size to the galaxies it encountered. The chaotic environment of the early universe often erased clear markers of ancient violence.
To bypass this obstacle, researchers turned to globular star clusters—dense, roughly spherical collections containing tens of thousands to millions of stars. These clusters house some of the oldest stars in existence, acting as cosmic archaeological sites that preserve stars from other galaxies collected by the Milky Way.
Zerbinati, a co-author on the study, noted that combining Hubble data with measurements from the European Space Agency mission Gaia made it possible to isolate this unique group of clusters.
Calculating the Mass and Impact of LKH
Because the Milky Way was much smaller 11.8 billion years ago, LKH delivered a substantial injection of stars, gas, and dark matter.
The findings challenge traditional assumptions about galactic formation. The research was published in the journal Nature Astronomy.
Connecting Ancient Impacts to Modern Galactic Structure
The revelation of the LKH merger arrives alongside separate research exploring how subsequent galactic crashes reshaped the Milky Way’s architecture. A head-on collision with the Gaia Sausage dwarf galaxy about 10 billion years ago likely flipped the entire disc of the Milky Way by more than 90 degrees into its current orientation.
Using advanced supercomputer simulations of galaxies similar to the Milky Way, researchers at Durham University investigated this event. The models demonstrated that Milky Way-like galaxies involved in head-on collisions with the Gaia Sausage were likely to experience a “disc flip”.
