Astronomers using NASA’s Hubble Space Telescope have found evidence that the young Milky Way absorbed a dwarf galaxy nearly 12 billion years ago, providing new clues about how our galaxy was formed.
The findings, published Monday in Nature Astronomy, indicate that the merger occurred about 11.8 billion years ago, roughly 2 billion years after the Big Bang.
NASA said the discovery pushes evidence of a major event in the Milky Way’s growth about 1.8 billion years further back than previously documented.
The dwarf galaxy, identified as LKH, or Low-energy-Kraken-Heracles, had a stellar mass equivalent to about 500 million Suns when it was absorbed by the young Milky Way.
Researchers reconstructed the event by studying 39 ancient globular clusters located within 20,000 light-years of the Milky Way’s centre. They combined high-resolution Hubble observations with precision measurements from the European Space Agency’s Gaia mission to determine the clusters’ ages, chemical composition and origins.
The analysis identified three distinct groups of clusters. One originated in the early Milky Way, another was acquired during the Gaia-Sausage-Enceladus merger around 10 billion years ago, and a third group was formed in LKH before that galaxy was consumed by the Milky Way.
“Here, we have shown that stars born in external galaxies also need to be considered,” said Davide Massari, lead author of the study from the Astrophysics and Space Science Observatory of Bologna.
Scientists said the LKH merger was particularly significant because the Milky Way was much smaller at the time. The dwarf galaxy therefore contributed a substantial amount of stars, gas and dark matter to the developing galaxy.
The findings suggest that the Milky Way’s earliest growth was shaped not only by stars formed within it, but also by material brought in through mergers with other galaxies.