Astronomers using the NASA/ESA Hubble Space Telescope have found definitive evidence that the young Milky Way swallowed a dwarf galaxy just two billion years after the Big Bang, revealing one of the earliest major mergers in the history of our Galaxy.
About 11.8 billion years ago, a dwarf galaxy known as LKH merged with the early Milky Way. Image credit: NASA / ESA / Joseph Olmsted, STScI.
The Milky Way today is a massive spiral galaxy home to hundreds of billions of stars.
However, our Galaxy wasn’t always so large; it has grown by forming new stars from its gas clouds as well as collecting stars, gas, and dark matter from other galaxies through mergers.
The most recent massive merger in the Milky Way’s history took place with the Sagittarius dwarf galaxy, beginning over six billion years ago and still ongoing today.
Looking back into the even more distant past, researchers learned that the Milky Way consumed another dwarf galaxy called Gaia-Sausage-Enceladus 10 billion years ago.
This ancient merger greatly affected the structure of our Galaxy’s disk of stars. Other, smaller mergers occurred between these two.
“Our home is the Milky Way Galaxy, but we do not know how our house was built,” said Dr. Davide Massari, an astronomer at the Osservatorio di Astrofisica e Scienza dello Spazio di Bologna.
“In this paper we discover where the first significant batch of bricks came from: a dwarf galaxy that we call Low-energy-Kraken-Heracles.”
Dr. Massari and his colleagues analyzed 39 globular clusters in the Milky Way using Hubble observations and advanced statistical modeling to determine their ages and chemical compositions.
They identified three events in the Milky Way’s history using a process known as age-metallicity sequencing.
One sequence traces the proto-Milky Way itself, another corresponds to the Gaia-Sausage-Enceladus merger, and a third reveals a merger that occurred approximately 1.8 billion years before the Gaia-Sausage-Enceladus merger.
This system, Low-energy-Kraken-Heracles (LKH for short), is estimated to have had a stellar mass similar to the Gaia-Sausage-Enceladus dwarf galaxy and deposited most of its material into the inner regions of the Galaxy.
“Some past studies have argued that the earliest phases of our Galaxy’s evolution were defined by stars born only in our Galaxy,” Dr. Massari said.
“Here, we have shown that stars born in external galaxies also need to be considered.”
The discovery is reported in a paper in the journal Nature Astronomy.
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D. Massari et al. Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger. Nat Astron, published online August 17, 2026; doi: 10.1038/s41550-026-02931-5
