Astronomers using the NASA/ESA Hubble Space Telescope have found conclusive evidence that the young Milky Way galaxy swallowed a dwarf galaxy just 2 billion years after the Big Bang, revealing one of the earliest large-scale mergers in galactic history.
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 current Milky Way galaxy is a huge spiral galaxy that is home to hundreds of billions of stars.
But our galaxy wasn’t always so big. It has grown by forming new stars from gas clouds and by collecting stars, gas, and dark matter from other galaxies through mergers.
The most recent major merger in the history of the Milky Way galaxy occurred with the Sagittarius dwarf galaxy, which began more than 6 billion years ago and continues today.
Looking back even further into the past, researchers learned that 10 billion years ago the Milky Way swallowed up another dwarf galaxy called Gaia Sausage Enceladus.
This ancient merger had a major impact on the structure of our galaxy’s stellar disk. There were also other small mergers between these two companies.
“Our home is the Milky Way, but we don’t know how our home was built,” says Dr. Davide Massari, an astronomer at the Bologna Institute for Astronomical Sciences.
“In this paper, we discover where the first significant brick came from: a dwarf galaxy we call the low-energy Kraken-Hercules.”
Dr. Massari and his colleagues used Hubble observations and advanced statistical modeling to analyze 39 globular clusters in the Milky Way and determine their age and chemical composition.
They identified three events in the Milky Way’s history using a process known as age metallicity sequencing.
One sequence tracks the proto-Milky Way galaxy itself, another corresponds to the Gaia-Sausage-Enceladus merger, and a third sequence reveals a merger that occurred about 1.8 billion years before the Gaia-Sausage-Enceladus merger.
The system, Low Energy Kraken Hercules (LKH), has a stellar mass similar to the Gaia Sausage Enceladus dwarf galaxy, and most of its material is estimated to have been deposited in the galaxy’s interior regions.
“Some past studies have argued that the early stages of our galaxy’s evolution were defined by stars that were born only in our galaxy,” Massari said.
“Here, we showed that we also need to consider stars born in outer galaxies.”
This finding is reported in the following article: paper in a diary natural astronomy.
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D. Massari others. Evidence of a massive accretion event 1.8 billion years ago when Gaia, Sausage, and Enceladus merged. Nat Astronpublished online on August 17, 2026. doi: 10.1038/s41550-026-02931-5
Source: www.sci.news
