Astronomers have identified evidence of the Milky Way's earliest known galactic merger, using Hubble Space Telescope data to trace ancient globular clusters and reveal a collision that shaped the galaxy's core nearly 12 billion years ago
New research has uncovered the earliest known evidence of a galactic merger in the Milky Way's history, revealing that our galaxy consumed a smaller system around 12 billion years ago. This ancient collision, detected through the study of globular star clusters, left a distinct imprint near the Milky Way's center and predates all previously identified merger events in our galaxy's timeline.
Tracing Ancient Collisions
The study, conducted by the ARMA (Cluster Ages to Reconstruct the Milky Way Assembly) project, focused on dense groups of old stars known as globular clusters. By analyzing the ages and chemical compositions of these clusters, researchers identified a population that could not be explained by any previously known merger. This group's properties pointed to a major accretion event that occurred roughly 1.8 billion years earlier than the previously established Gaia-Enceladus merger, which took place about 10 billion years ago.
Prior reconstructions of the Milky Way's assembly had only reached back to the Gaia-Enceladus event, leaving the galaxy's earliest growth phase largely uncharted. The new findings extend the timeline of known mergers, suggesting that the Milky Way's formative years were marked by even earlier and more complex interactions with smaller galaxies.
Hubble's Role in the Discovery
The breakthrough was made possible by high-precision data from the Hubble Space Telescope. Astronomers developed a method to determine the ages of globular clusters with unprecedented accuracy, even in the crowded and dust-obscured regions near the galactic center. This allowed them to distinguish three distinct groups of clusters: those originating from the Gaia-Enceladus merger, those native to the Milky Way, and a third, previously unidentified group.
The third group's unique age and metallicity profile indicated it was the remnant of a long-lost dwarf galaxy. Most of the material from this ancient system now resides within 20,000 light-years of the Milky Way's core, forming a "scar" that marks the site of the merger. The team estimates that the devoured galaxy had a mass of about 500 million solar masses, comparable to Gaia-Enceladus, while the present-day Milky Way contains roughly 1.5 trillion solar masses.
Reconstructing the Milky Way's Early Growth
The identification of this early merger provides new insight into the processes that shaped the Milky Way's structure. By mapping the distribution and properties of globular clusters, astronomers can reconstruct the sequence of accretion events that built up the galaxy's mass and influenced its evolution. The newly discovered merger, named Low-energy-Kraken-Heracles (LKH), honors the three studies that first proposed its existence.
These results, published in Nature Astronomy, highlight the importance of combining precise stellar ages with orbital data to unravel the Milky Way's complex history. The approach complements previous work that used Hubble data to identify ancient mergers, such as the study of the LKH event described in an earlier Science Report article.
Limits and Open Questions
While the evidence for the LKH merger is strong, significant uncertainties remain. The precise origin and fate of the devoured galaxy's stars are still being investigated, and the full impact of the event on the Milky Way's subsequent evolution is not yet fully understood. The study relies on models of stellar evolution and chemical enrichment, which carry their own assumptions and limitations.
Further observations, especially with next-generation telescopes and improved simulations, will be needed to refine the timeline of early mergers and clarify how these ancient events shaped the Milky Way's present-day structure. The discovery underscores the value of globular clusters as tracers of galactic history and the ongoing challenge of disentangling the galaxy's earliest assembly stages.
Globular clusters are tightly bound groups of tens of thousands to millions of stars, often among the oldest objects in a galaxy. Their ages and chemical compositions provide a fossil record of the conditions present during different phases of galactic assembly. By measuring the light from these clusters and analyzing their spectra, astronomers can estimate when and where they formed, and whether they originated in the Milky Way or were accreted from other galaxies. This makes globular clusters essential tools for reconstructing the merger history of large galaxies like our own.
