Hubble finds evidence of a dwarf galaxy merger 11.8 billion years ago

A dwarf galaxy named LKH crossed paths with the young Milky Way about 11.8 billion years ago. It did not survive the encounter: NASA’s Hubble Space Telescope has identified what the agency calls definitive evidence that the two galaxies merged, extending the known history of our galaxy 1.8 billion years farther into the past.

The clues were not found in a surviving galaxy, but in globular clusters — dense, roughly spherical groups containing tens of thousands to a few million stars. These clusters can preserve stars gathered during past mergers, acting as archaeological records of the Milky Way’s construction. Hubble measured their ages and metal content, the abundance of elements heavier than helium, while the European Space Agency’s Gaia mission supplied complementary measurements.

Researchers examined 39 globular clusters in the inner 20,000 light-years of the Milky Way, where traces of the oldest mergers should remain. The analysis separated clusters formed in the young Milky Way from those acquired during the Gaia-Sausage-Enceladus merger about 10 billion years ago. A third group was older than the Gaia-Sausage-Enceladus clusters but younger than the clusters born inside the Milky Way, pointing to a separate and earlier collision.

That population came from LKH, a dwarf galaxy containing roughly 500 million times the mass of the Sun in stars. For the young Milky Way, NASA says, that represented a significant fraction of the galaxy’s mass at the time. The discovery helps settle a question that observations and simulations had raised but not resolved: whether a major merger preceded the better-known events in the Milky Way’s history.

And concretely? Astronomers now have a more complete timeline for how our galaxy grew before it became the spiral galaxy home to hundreds of billions of stars. The result also shows why old globular clusters matter: even when billions of years have erased the visible shape of a collision, the ages and chemical fingerprints of their stars can still preserve the event. NASA says the study was published in Nature Astronomy.