Hubble Solves a Mystery About the Milky Way's Early Years

The Hubble Space Telescope has provided definitive evidence that the Milky Way merged with a dwarf galaxy called LKH during its early evolution. The study, led by astronomers from the Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, found that this merger occurred about 11.8 billion years ago, just 2 billion years after the Big Bang. The research identified a third population of globular clusters in the Milky Way that are older than those formed in the Gaia-Sausage-Enceladus merger but younger than those formed in the Milky Way itself. These clusters came from a separate, earlier merger in which the Milky Way absorbed a dwarf galaxy of 500 solar masses.

The study used Hubble's sensitive observations to measure the age and metal content of globular clusters, allowing the team to distinguish a population of clusters that were born in LKH. These clusters tell researchers when and how massive the LKH galaxy was when it was devoured by the Milky Way. The discovery extends the known history of the Milky Way's merger events back to 11.8 billion years ago.

The findings challenge the previous assumption that the earliest stars in the Milky Way were only formed within the galaxy itself. Instead, they suggest that stars born in external galaxies also played a role in shaping the Milky Way's structure. The research team plans to continue studying globular clusters to characterize all the massive mergers the Milky Way has experienced. The Hubble observations of these clusters will help scientists better understand the early history of the galaxy.

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