Hubble and Gaia reveal 39 globular clusters from 11.8-billion-year-old dwarf galaxy merger

August 31, 2026 at 12:48 PM3 min

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NASA's Hubble Space Telescope and ESA's Gaia have identified 39 globular clusters in the inner Milky Way that originated from a dwarf galaxy swallowed about 11.8 billion years ago. The discovery, reported on 17 August, pushes the documented merger history of the Galaxy 1.8 billion years farther back than the previously known Gaia-Sausage-Enceladus event. The team named the dwarf galaxy Low-energy-Kraken-Heracles, or LKH, and estimates it held roughly 500 million solar masses in stars.

Key Facts

The 39 globular clusters split into three age-and-metal sequences, with the middle sequence older than clusters from the Gaia-Sausage-Enceladus merger about 10 billion years ago and younger than clusters born in the Milky Way itself.
The dwarf galaxy LKH was swallowed about 11.8 billion years ago, just two billion years after the Big Bang, and held roughly 500 million solar masses in stars.
The result, published in Nature Astronomy, pushes the documented merger history of the Milky Way 1.8 billion years farther back than Gaia-Sausage-Enceladus.
Davide Massari of the Astrophysics and Space Science Observatory of Bologna is the lead author of the paper.

The Discovery

Thirty-nine globular clusters in the inner 20,000 light-years of the Milky Way, timed by NASA’s Hubble Space Telescope and placed by ESA’s Gaia, split into three age-and-metal sequences instead of two. The middle sequence is older than the clusters brought in when Gaia-Sausage-Enceladus was swallowed about 10 billion years ago, and younger than the clusters born in the Milky Way itself, no matter how metal-rich they are. NASA and ESA reported on 17 August that those clusters came from a dwarf galaxy swallowed about 11.8 billion years ago, just two billion years after the Big Bang. The team named it Low-energy-Kraken-Heracles, or LKH, after three earlier papers that had argued for a merger this early.

Galactic Merger History

The Milky Way is a massive spiral with hundreds of billions of stars, but it grew by making stars from its own gas and by taking stars, gas and dark matter from smaller galaxies. The most recent massive merger, with the Sagittarius dwarf, began more than 6 billion years ago and is still going. Before that, Gaia-Sausage-Enceladus arrived about 10 billion years ago and left a mark on the disk. Hubble’s sample of 39 inner clusters is the evidence NASA now calls definitive for an even earlier merger.

Cluster Analysis

Globular clusters are roughly spherical collections of tens of thousands to a few million stars, among the oldest in the Galaxy, and they keep the chemistry of the systems that made them. Davide Massari, of the Astrophysics and Space Science Observatory of Bologna and lead author of the paper, said: “Our home is the Milky Way galaxy, but we do not know how our house was built. In this paper we discover where the first significant batch of bricks came from: a dwarf galaxy that we call LKH.” The team looked where the oldest debris should still sit, inside 20,000 light-years of the centre, and expected two families: clusters born in the young Milky Way, and clusters collected from Gaia-Sausage-Enceladus. Hubble’s ages and metallicities, the abundances of elements heavier than helium, turned up a third family that is older than the Gaia-Sausage-Enceladus group and younger than the in-situ group at every metal content measured. Chiara Zerbinati, of the University of Bologna, said the ages and metals only became usable because of Hubble’s resolution and depth, and that coupled with measurements from Gaia, this made it possible to distinguish a population of globular clusters born in LKH.