New Discovery Reveals Ancient Galaxy Collision That Shaped the Milky Way
Astronomers reconstructing the evolutionary history of the Milky Way have identified a collision with an ancient ancestral galaxy that occurred roughly 12.3 billion years ago, a discovery that reshapes our understanding of the galaxy’s early growth. Published in Nature Astronomy, the research demonstrates that the Milky Way’s expansion was driven by frequent and complex galactic mergers long before the well-documented impact with the Gaia-Sausage-Enceladus system.
Ancient Globular Clusters Reveal a Hidden Collision
By mapping these clusters across age-metallicity distributions, investigators sorted the stellar groupings into three distinct populations. The analysis separated indigenous Milky Way clusters from the known debris of the Gaia-Sausage-Enceladus collision while uncovering a third, intermediate sequence. This new sequence indicates that approximately 18億年前 prior to the Gaia-Sausage-Enceladus event—about 123億年前—the early Milky Way absorbed another massive progenitor galaxy containing roughly 5億倍太陽質量 of stellar material, which settled primarily within 2萬光年 of the galactic center.
Re-evaluating Early Galactic Assembly and the LKH Progenitor
To honor the newly identified structure and prior academic discussions surrounding ancient galactic collisions, the researchers designated the newly discovered progenitor the Low-energy-Kraken-Heracles, or LKH system. Until recently, the Gaia-Sausage-Enceladus event—which occurred around 105 億年前 when a galaxy boasting several billion solar masses slammed into the early Milky Way at high velocity—was considered the sole major ancient merger of the system. The impact of Gaia-Sausage-Enceladus tore the incoming system apart, scattered stars into the galactic halo, triggered widespread gas collapse, and reshaped the thick disk. The discovery of the LKH collision demonstrates that the early assembly of the galaxy involved multiple massive encounters before the Gaia-Sausage-Enceladus event took place.