The Milky Way swallowed a dwarf galaxy 11.8 billion years ago, a discovery revealed by Hubble and Gaia data, while Terzan 5 emerged as a rare fossil from the galaxy’s formation, according to new research.
The LKH Merger: A Fossil of the Milky Way’s Early History
This event, the earliest-known galactic merger in the Milky Way’s history, occurred when the universe was less than 15% of its current age. Researchers identified the merger by studying stellar clusters near the galaxy’s center, which contained stars with a mass equivalent to 500 million times that of the sun at the time of the merger, about a quarter the size of what the Milky Way was at the time.
The key finding of our research is the unambiguous discovery of the first merger event experienced by our galaxy in its infancy,
said astrophysicist Davide Massari of the National Institute for Astrophysics, lead author of the study. The merger infused the Milky Way with stars, interstellar gas, and dark matter, shaping its evolution. From the point of view of the gas, the merger was more 'explosive'—likely, the collision between the gas from LKH and that from the Milky Way triggered the formation of many stars,
Massari added, highlighting how the collision between LKH’s gas and the Milky Way’s own gas created a burst of new stars.
The research, published in Nature Astronomy, suggests the Milky Way has undergone at least two other massive galactic mergers. After LKH, it absorbed the Gaia-Sausage-Enceladus dwarf galaxy about 1.8 billion years after the LKH merger and over the past approximately 6 billion years has been combining with the Sagittarius dwarf galaxy. Every single one of these mergers had an influence on the events that have led to the formation of our own solar system, and ultimately of Earth,
Massari said, linking the galactic history to the origins of life.
Terzan 5: A Relic of Galactic Formation
Terzan 5, a crowded stellar system near the Milky Way’s center, has been reclassified as a bulge fossil fragment
after NASA’s James Webb and Hubble telescopes revealed its complex history. Initially mistaken for a globular cluster, Terzan 5 contains up to four separate generations of stars, suggesting it was a self-contained, self-enriching system that birthed new stars repeatedly. This challenges the traditional view of globular clusters as static, ancient groups of stars.
Michael Rich. The system’s ability to retain gas and dust allowed it to form stars over billions of years, unlike typical clusters that exhaust their resources early. Ferraro of the University of Bologna, who led the Webb observations.
Terzan 5’s discovery adds to the evidence that the Milky Way’s central bulge is not a uniform structure but a mosaic of ancient fragments. Only one other known object, Liller 1, has been reclassified in a similar way. Ferraro’s team plans to examine 40 to 50 additional globular clusters orbiting inside the bulge to see whether more hidden fossils are waiting there.
Implications for Understanding Galactic Evolution
The findings reshape the narrative of how galaxies form and evolve. The Milky Way’s mergers suggest that star formation was not solely driven by internal processes but also by external collisions. This is why I think reconstructing the pieces of the Milky Way’s past effectively contributes to answering the big question about, 'Where do we come from?'
Massari said.
The study also highlights the role of telescopes like Hubble and Webb in peering through the Milky Way’s dusty center. With Hubble we can get into enormous detail about what's happening on a holistic scale across the entire disk of the galaxy. You can't do that with any other large galaxy,
said principal investigator Ben Williams of the University of Washington, who led a Hubble survey of Andromeda. While Andromeda’s history differs from the Milky Way’s, its study provides a comparative framework for understanding spiral galaxies.
The Role of Hubble and Webb in Unveiling Cosmic Secrets
Hubble’s decade-long observations of Andromeda, the nearest galaxy, have provided a blueprint for studying the Milky Way. The telescope’s ability to resolve more than 200 million stars in the Andromeda galaxy—detecting only stars brighter than our Sun—offers insights into how galaxies grow. This is like trying to understand the layout of New York City by standing in the middle of Central Park,
Williams said, emphasizing the Milky Way’s embedded perspective.
Webb’s infrared vision have been critical in piercing the Milky Way’s dust clouds. By combining data from both telescopes, researchers identified Terzan 5’s multiple stellar generations and traced the Milky Way’s merger history.
The discoveries underscore the importance of multi-wavelength astronomy. While Hubble’s visible-light data revealed stellar movements, Webb’s infrared views exposed hidden structures. Together, they paint a richer picture of the Milky Way’s 13.8-billion-year journey, from its chaotic infancy to its current spiral form.
