Astronomers using the Hubble Space Telescope have identified evidence of a major merger between the young Milky Way and a dwarf galaxy nearly 12 billion years ago, pushing the timeline of our galaxy's assembly further into the past
New analysis of star clusters observed by the Hubble Space Telescope has uncovered evidence that the Milky Way absorbed a substantial dwarf galaxy around 11.8 billion years ago, extending the known history of galactic mergers to an earlier epoch than previously confirmed. The finding, published in Nature Astronomy, suggests that the Milky Way's formative years were shaped not only by internal star formation but also by the accretion of external stellar populations.
Tracing Ancient Mergers
The Milky Way's present-day structure is the result of a complex sequence of mergers and accretion events. While the ongoing interaction with the Sagittarius dwarf galaxy and the 10-billion-year-old Gaia-Sausage-Enceladus merger have been well documented, the possibility of an even earlier major merger has remained uncertain. By examining the ages and chemical compositions of globular clusters-dense, ancient groups of stars-astronomers have now identified a distinct population that points to a previously unconfirmed event.
Globular clusters serve as fossil records of galactic history. The team focused on 39 clusters within the inner 20,000 light-years of the Milky Way, where remnants of the oldest mergers are most likely to be preserved. Using Hubble's high-resolution imaging and combining it with precise astrometric data from ESA's Gaia mission, researchers measured both the ages and metallicities (the abundance of elements heavier than helium) of these clusters. This approach allowed them to distinguish between clusters formed in the Milky Way, those accreted from Gaia-Sausage-Enceladus, and a third, previously unidentified group.
Evidence for the LKH Event
The newly identified population of globular clusters is older than those associated with Gaia-Sausage-Enceladus but younger than the oldest clusters formed within the Milky Way itself. Their properties indicate an origin in a separate dwarf galaxy, which the researchers have named Low-energy-Kraken-Heracles (LKH), referencing earlier theoretical work that predicted such an early merger. The analysis suggests that LKH contributed roughly 500 million solar masses in stars-an amount significant enough to influence the young Milky Way's structure and evolution.
This merger is estimated to have occurred about 11.8 billion years ago, just 2 billion years after the Big Bang. The timing and scale of the event imply that the Milky Way's early growth was shaped by both internal and external processes. The study's findings challenge the view that the galaxy's oldest stars all formed in situ, instead supporting a scenario in which external galaxies played a key role in building up the Milky Way's stellar population.
Implications for Galactic Evolution
The discovery of the LKH merger pushes the timeline of confirmed major accretion events in the Milky Way's history back by nearly 2 billion years. It also highlights the importance of globular clusters as tracers of ancient galactic interactions. The team plans to extend their analysis to additional clusters, aiming to reconstruct a more complete sequence of mergers that contributed to the Milky Way's assembly.
These results build on a growing body of work that uses stellar populations to unravel the galaxy's past. Similar approaches have been applied to other galaxies and to the study of black hole-driven outflows, as seen in recent observations of quasar winds stirring intergalactic gas reported by Science Report. Together, these studies are refining our understanding of how galaxies grow and evolve over cosmic time.
Hubble's Continuing Role
The Hubble Space Telescope, now in its fourth decade of operations, remains central to studies of galactic archaeology. Its ability to resolve individual stars in dense environments and to measure their properties with high precision has enabled discoveries that ground theoretical models in observational evidence. Hubble's partnership with missions like Gaia, which provides detailed positional and motion data for over a billion stars, is proving especially powerful for reconstructing the Milky Way's assembly history.
As new instruments come online and more globular clusters are studied in detail, astronomers expect to further clarify the sequence and impact of ancient mergers. Each new finding helps to refine models of galaxy formation and to place the Milky Way in a broader cosmological context.
Globular clusters are among the oldest stellar systems in the universe, often containing stars that formed within the first few billion years after the Big Bang. By measuring both the age and metallicity of these clusters, astronomers can infer their likely origin-whether they formed within the Milky Way or were accreted from another galaxy. Metallicity, in particular, serves as a chemical fingerprint, since stars born in smaller galaxies tend to have lower abundances of heavy elements. The combination of precise photometry, spectroscopy, and astrometric data allows researchers to reconstruct the sequence of mergers that built up the Milky Way's stellar halo and inner regions.
