Milky Way absorbed smaller galaxy 11.8 billion years ago, study finds

Our Milky Way galaxy consumed a smaller galactic neighbor approximately 11.8 billion years ago, a pivotal event that shaped its early development. This ancient merger occurred when the universe was just 2 billion years old, representing the oldest known fusion event in our galaxy's history.

Researchers analyzed star clusters near the Milky Way's center, using data from the Hubble and Gaia space telescopes. They examined the age, chemical composition, and movement of hundreds of thousands of stars. These stars are believed to have become part of the Milky Way as a result of this ancient merger, which involved a dwarf galaxy scientists have named Low-energy-Kraken-Heracles (LKH).

The dwarf galaxy contributed stars with a total mass of about 500 million times that of our Sun. At the time of the merger, LKH was roughly one-quarter the size of the Milky Way. This finding demonstrates that the Milky Way's growth was not solely due to internal star formation but also through the assimilation of smaller galaxies drawn in by its gravity.

"The most important result of our research is the unambiguous discovery of the first merger event that our galaxy experienced in its early period," said astrophysicist Davide Massari, lead author of the study published in Nature Astronomy. He noted that this event significantly influenced the galaxy's subsequent evolution.

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The most important result of our research is the unambiguous discovery of the first merger event that our galaxy experienced in its early period.