Astronomers using NASA’s Hubble Space Telescope, operated jointly by NASA and ESA, have found compelling evidence that the young Milky Way merged with a dwarf galaxy approximately 11.8 billion years ago—just 2 billion years after the Big Bang.
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.
Today, the Milky Way is a massive spiral galaxy containing hundreds of billions of stars. However, our galaxy was much smaller in the early Universe and grew over billions of years by forming new stars and merging with other galaxies.
These mergers supplied the Milky Way with stars, gas, and dark matter, helping shape its present-day structure.
The most recent major merger in the Milky Way’s history involves the Sagittarius dwarf galaxy. That interaction began more than 6 billion years ago and is still continuing today.
Looking further back, astronomers discovered that the Milky Way absorbed another dwarf galaxy approximately 10 billion years ago. This event, known as the Gaia-Sausage-Enceladus merger, significantly influenced the structure of the galaxy’s stellar disk.
Several smaller mergers also contributed to the early growth of the Milky Way.
“Our home is the Milky Way, but we don’t know exactly how our home was built,” said Dr. Davide Massari, an astronomer at the Bologna Institute for Astronomical Sciences.
“In this paper, we discover where one of the first significant building blocks came from: a dwarf galaxy we call Low-Energy Kraken-Hercules.”
Dr. Massari and his colleagues analyzed observations from the Hubble Space Telescope alongside advanced statistical models. Their study focused on 39 globular clusters in the Milky Way, allowing the researchers to determine the clusters’ ages and chemical compositions.
Using a method known as age-metallicity sequencing, the team identified three major events in the Milky Way’s early history.
One sequence is associated with the formation of the proto-Milky Way. A second corresponds to the Gaia-Sausage-Enceladus merger, while a third reveals an even earlier merger that occurred approximately 1.8 billion years before the Gaia-Sausage-Enceladus event.
The Low-Energy Kraken-Hercules system, or LKH, had a stellar mass similar to that of the Gaia-Sausage-Enceladus dwarf galaxy. Most of its material is thought to have been deposited in the inner regions of the Milky Way.
“Some previous studies argued that the early evolution of our galaxy was dominated by stars born within the Milky Way,” Dr. Massari said.
“Our findings show that we must also consider stars that formed in other galaxies and were later incorporated into the Milky Way.”
The findings are reported in a paper published in the journal Nature Astronomy.
_____
D. Massari et al. Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger. Nature Astronomy, published online August 17, 2026. doi: 10.1038/s41550-026-02931-5
Source: www.sci.news