Hubble solves merger mystery from Milky Way's early years

NASA's Hubble Space Telescope has uncovered definitive evidence of a dwarf galaxy, LKH, merging with the early Milky Way 11.8 billion years ago. The findings extend our knowledge of the galaxy's history back 1.8 billion years, revealing that the Milky Way consumed another dwarf galaxy called Gaia-Sausage-Enceladus 10 billion years ago. The study, published in Nature Astronomy, shows that the Milky Way absorbed a dwarf galaxy containing roughly 500 million solar masses, which is a significant fraction of its mass at the time. The research used Hubble's high-resolution imaging to measure the age and metal content of globular clusters, which are among the oldest stars in the galaxy.

The study found that there was a third population of globular clusters in the inner regions of the Milky Way, older than those formed in the Gaia-Sausage-Enceladus merger but younger than those born in the Milky Way. These clusters came from a separate and even earlier merger, in which the Milky Way absorbed a dwarf galaxy. The team named this dwarf galaxy LKH, in honor of three earlier research papers that supported the idea of a merger early in the galaxy's history. The discovery has significant implications for understanding the evolution of the Milky Way and how it has grown over time.

The findings suggest that the Milky Way's early history involved multiple mergers with smaller galaxies, and that the process of merging has shaped the galaxy's structure. The research highlights the importance of studying globular clusters as cosmic archaeological sites that preserve the remnants of other galaxies. The team plans to continue their work by studying these clusters to better understand the history of the Milky Way and the many mergers it has experienced over cosmic time.

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