Hubble Telescope Uncovers Evidence of Ancient Galaxy Merger in Milky Way’s Early History

• NASA’s Hubble Space Telescope has uncovered definitive evidence of a dwarf galaxy, dubbed 'LKH,' colliding with the young Milky Way about 11.8 billion years ago.
• This discovery extends our understanding of the Milky Way’s evolutionary history back an additional 1.8 billion years.

The Discovery of LKH

• Researchers analyzed 39 globular star clusters located within the inner 20,000 light-years of our galaxy.
• As ancient collections of stars, these clusters serve as 'cosmic archaeological sites' that preserve evidence of past galactic mergers.
• By measuring age and metallicity, the team identified a distinct population of clusters predating those from the Gaia-Sausage-Enceladus (GSE) merger.
• LKH is estimated to have contained roughly 500 million solar masses of stars, representing a significant portion of the Milky Way’s mass at that time.

Context of Galactic Growth

• The Milky Way has grown through a combination of internal star formation and the consumption of smaller galaxies, gas, and dark matter.
• Other known major events include the GSE merger 10 billion years ago and the ongoing merger with the Sagittarius dwarf galaxy, which began 6 billion years ago.
• This study refutes the argument that the earliest phases of our galaxy’s development were defined solely by internal star formation.

Future Implications

• The research team plans to continue studying previously unexamined globular clusters with Hubble to fully characterize the history of massive mergers in the Milky Way.
• This ongoing work is essential for piecing together how our galaxy was originally assembled.