New data from NASA’s Hubble Space Telescope reveals definitive evidence that a dwarf galaxy merged with the Milky Way approximately 11.8 billion years ago, extending our understanding of galactic evolution to just two billion years after the Big Bang. According to researchers from the Osservatorio di Astrofisica e Scienza dello Spazio di Bologna (OAS), this ancient merger involved a dwarf galaxy of 500 solar masses, proving that stars born in external galaxies shaped the earliest phases of our galaxy’s growth.
How the Hubble Space Telescope Uncovered Ancient Galactic Mergers
Detecting the telltale signs of mergers billions of years in the past presents a major hurdle for astronomers. According to the research team, evidence of the earliest galactic collisions risks erasure over billions of years. To look back further in time than ever before, astronomers utilized Hubble to examine 39 globular clusters located 20,000 light-years from the center of the Milky Way, where ancient merger evidence is most likely preserved.
By leveraging Hubble’s sensitive observations, scientists determined each cluster’s precise age and heavy-element abundance, known as metallicity. This high-resolution data allowed researchers to isolate a third population of globular clusters in the inner regions of our galaxy. These clusters are older than the group collected during the later Gaia-Sausage-Enceladus merger, yet younger than those originally formed within the Milky Way itself.
Did you know? The Milky Way measures about 200,000 light-years in diameter today and contains roughly two hundred billion stars, growing over billions of years through a steady progression of star formation and galactic cannibalism.
Comparing Early Milky Way Mergers and Stellar Populations
Astronomers now track multiple distinct merger events that built the Milky Way over billions of years. Previous surveys, such as the European Space Agency’s Gaia mission, mapped more recent collisions, including the consumption of the Gaia-Sausage-Enceladus dwarf galaxy roughly 10 billion years ago. That event significantly altered the structure of the Milky Way’s stellar disk.
The newly identified LKH merger pushes the timeline back even further. While the Gaia-Sausage-Enceladus collision occurred 10 billion years ago, the LKH merger took place 11.8 billion years ago. “Some past studies have argued that the earliest phases of our galaxy’s evolution were defined by stars born only in our galaxy,” said Davide Massari, First Researcher at the OAS di Bologna and lead author of the study, in a published statement. “Here, we have shown that stars born in external galaxies also need to be considered.”
Future Astronomical Surveys and the Search for Lost Galaxies
Researchers plan to continue analyzing globular clusters across the Milky Way to fully characterize every massive merger in our galaxy’s history. According to co-author Fernando Aguado-Agelet, a researcher from the University of Vigo and the University of La Laguna in Spain, future discoveries rely heavily on untapped observational data.
“Hubble is observing globular clusters that have never been studied before, and this will help us characterize the merger events that are far back in time in the Milky Way galaxy’s history,” Aguado-Agelet stated.
Frequently Asked Questions
When did the Milky Way merge with the LKH dwarf galaxy?
According to researchers, the merger with the Low-energy-Kraken-Heracles (LKH) dwarf galaxy occurred approximately 11.8 billion years ago, which is just 2 billion years after the Big Bang.
How do scientists detect ancient galaxy mergers?
Astronomers use high-resolution data from the Hubble Space Telescope and the ESA Gaia mission to measure the precise ages and heavy-element abundances of ancient globular clusters located near the center of the Milky Way.
What institutions led the LKH galaxy research?
The research was led by astronomers from the Osservatorio di Astrofisica e Scienza dello Spazio di Bologna (OAS), alongside institutions including the Kapteyn Astronomical Institute, the Italian National Institute for Astrophysics (INAF), and Edinburgh University’s Astrophysics Research Institute.
Are there ongoing mergers happening to the Milky Way today?
Yes. The Milky Way is currently merging with the Sagittarius dwarf galaxy, a process that began over 6 billion years ago and continues today, alongside interactions with the Canis Major Dwarf Galaxy.
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