Ancient Galaxy Merger 12 Billion Years Ago Shaped Young Milky Way

Twelve billion years ago, when the Milky Way was still taking shape, a smaller galaxy drifted into its gravitational embrace and was pulled apart and absorbed. The young Milky Way at that time was nothing like the vast spiral we know today—it was smaller, still assembling itself from fragments of stars, gas, and dark matter. The collision that followed left no visible scar in the night sky, but it left something more durable: a chemical and kinematic fingerprint encoded in the ancient star clusters that survived the encounter.

Astronomers have now identified this long-hidden merger by studying globular clusters, those dense spheres of old stars that can hold onto information about their origins across billions of years. The dwarf galaxy involved, which researchers have named Low-energy-Kraken-Heracles, or LKH, carried a stellar mass roughly 500 million times that of the Sun. The merger occurred around 12.3 billion years ago, according to a study published in Nature—roughly 1.8 billion years before another well-documented collision called the Gaia-Sausage-Enceladus merger. This makes LKH one of the earliest substantial mergers in the Milky Way's history, a collision that happened when the galaxy was barely a billion years old.

The evidence emerged from careful detective work. Researchers examined 17 globular clusters in the inner Milky Way using observations from the Hubble Space Telescope, then combined their findings with existing data to create a homogeneous sample of 39 clusters. When they compared these clusters by age, chemical composition, and orbital properties, a pattern emerged: three distinct populations. Two of these populations were already known—one belonging to the original Milky Way and another to the Gaia-Sausage-Enceladus system. But the third population occupied an intermediate position, and twelve clusters in this group clustered tightly within about 6 kiloparsecs of the galactic center. This concentration suggested they came from somewhere else entirely, a separate system that had been absorbed long ago.

The merger was not a simple addition of one collection of stars to another. When LKH collided with the young Milky Way, it brought not just stars but also vast quantities of interstellar gas and dark matter. The collision between these gas clouds would have created turbulence and compression, likely triggering a burst of new star formation in the young galaxy. At the scale of individual stars, however, the encounter was fundamentally different from what the word "collision" might suggest. The distances between stars are so enormous that direct impacts between them are vanishingly rare. The real action happened at the scale of gas clouds and gravitational fields, reshaping the structure of the galaxy itself.

Some of the stars born in LKH may still exist somewhere in the Milky Way's crowded inner regions, but finding them is nearly impossible. Dust obscures that part of the galaxy, making individual stellar survivors invisible to current instruments. The globular clusters, by contrast, have remained detectable. Their collective properties—the ages of their stars, their chemical makeup, their paths through space—have survived as a readable record across more than twelve billion years.

This discovery fits into a larger picture that has emerged over the past decade: the Milky Way was built through repeated collisions and mergers. The Gaia-Sausage-Enceladus merger, which occurred around 10 billion years ago, left a profound mark on the galaxy's structure, shaping its thick disk. The Sagittarius dwarf galaxy represents a much more recent merger that is still happening today, its stars being torn apart by the Milky Way's gravity in real time. The LKH event pushes the known history of major mergers back further, into the galaxy's first billion years of existence.

Yet the story remains incomplete. The inner Milky Way contains material from several ancient building blocks, and some smaller progenitor galaxies may have left no surviving globular clusters to mark their presence. They could have been absorbed so completely that no trace remains visible. Researchers say that more precise measurements of the ages of large populations of stars in the inner galaxy will be needed to separate these different contributions and reconstruct the full, complicated history of how the Milky Way assembled itself from smaller pieces into the galaxy we inhabit today.