How Hubble Revealed the Milky Way’s Ancient Merger With LKH

That third, internally coherent sequence is the central clue. Its ages and chemical composition do not fit the native population or the Gaia-Sausage-Enceladus event, pointing instead to a separate progenitor galaxy that was later incorporated into the Milky Way.

How age and metallicity identify a lost galaxy

A globular cluster’s age provides a timeline, while its metallicity records the chemical state of its birth environment. A galaxy that formed its own clusters would have followed a particular pattern as successive generations of stars enriched its gas. Clusters born in a different galaxy can therefore preserve a different evolutionary track.

In this case, the third sequence supplied both clues at once: its clusters shared a distinct chemical history and occupied a different position in the age sequence from the Milky Way’s native clusters and the Gaia-Sausage-Enceladus population. Hubble’s precise measurements made that separation possible.

The team linked 12 ancient clusters to LKH, treating them as the surviving fossil record of the absorbed galaxy. The conclusion is not based solely on the clusters’ current locations or motions; it combines their ages, chemistry and broader dynamical context to reconstruct a population whose parent system has disappeared.

LKH was a significant contributor to the young Milky Way

Researchers estimate that LKH contained stars with a combined stellar mass of about 500 million times the mass of the Sun when it merged with the Milky Way. That is similar to the estimated stellar mass of Gaia-Enceladus, making LKH a substantial accretion event rather than a minor encounter.

The merger occurred when the universe was less than 15% of its present age, approximately 2 billion years after the Big Bang according to the supplied reporting. The academic source supplied with the research describes the event as occurring about 1.5 billion years after the Big Bang; the difference reflects the precision of the reported timeline, while both sources place the merger in the Milky Way’s very early history.

What this changes about the Milky Way’s formation

The finding gives the Galaxy a more complex origin story. The early Milky Way grew through internal star formation, but it also gained a substantial population of externally formed stars when it absorbed LKH. Those imported stars became part of the inner Galaxy alongside stars born in the proto-Milky-Way system and material contributed by later mergers.

It also extends the known timeline of major Milky Way assembly. Before this result, the merger history had been firmly traced back to the Gaia-Sausage-Enceladus event at roughly 10 billion years ago in the supplied research. LKH now provides evidence for a significant merger earlier than that, showing that the Galaxy was already assembling through collisions when it was only a young cosmic system.

A case study in galactic archaeology

This reconstruction is an example of galactic archaeology: using long-lived stars and stellar clusters as records of events that can no longer be observed directly. Globular clusters survive the destruction of their parent galaxies, preserving information about their formation age and chemical environment. By grouping clusters with similar histories, astronomers can identify fragments of galaxies that were dismantled billions of years ago.

The supplied material connects this age–metallicity and dynamical-fossil approach with the broader field of Milky Way reconstruction recognized by the 2026 Kavli Prize in Astrophysics. However, the provided sources do not specify the prize citation or individual laureates, so a more detailed claim about that connection would go beyond the available evidence.

What Hubble may find next

The method offers a way to search for additional ancient mergers. Hubble observations of previously unstudied globular clusters—especially in the crowded and obscured inner Galaxy—could reveal more distinct age–metallicity sequences. Each sequence might represent another external system absorbed during the Milky Way’s early growth.

The supplied evidence does not identify specific future merger candidates. What it does establish is a promising strategy: read the ages and chemistry of surviving clusters, separate their formation histories, and use those patterns to recover more chapters of the Galaxy’s otherwise erased past.