Hubble Telescope Uncovers Ancient Merger in Milky Way's Early History

NASA's Hubble Space Telescope has provided definitive evidence of a significant merger in the Milky Way's early history. About 11.8 billion years ago, a dwarf galaxy named LKH collided with the young Milky Way, contributing to its growth. This discovery extends our understanding of the galaxy's formation by 1.8 billion years further back than previously known.

The study, published in Nature Astronomy, utilized Hubble's observations of globular star clusters, which are dense collections of ancient stars. These clusters serve as cosmic archaeological sites, preserving evidence of past mergers. Researchers, including lead author Davide Massari from the Astrophysics and Space Science Observatory of Bologna, analyzed 39 globular clusters within 20,000 light-years of the Milky Way's center.

The team identified a distinct group of globular clusters that originated from the LKH dwarf galaxy. These clusters are older than those from the Gaia-Sausage-Enceladus merger but younger than those formed in the Milky Way. This indicates that the Milky Way absorbed LKH, which contained about 500 million solar masses in stars, a significant contribution to the galaxy's mass at the time.

The findings challenge previous assumptions that the Milky Way's early evolution was solely due to stars formed within it. The study suggests that stars from external galaxies played a crucial role as well. Co-author Chiara Zerbinati from the University of Bologna highlighted the precision of Hubble's imaging in determining the age and metal content of these clusters.

The research team plans to continue exploring the Milky Way's history by studying its globular clusters. Hubble's ongoing observations aim to characterize all major mergers in the galaxy's past. The Hubble Space Telescope, a collaboration between NASA and the European Space Agency, remains a vital tool for understanding the universe.