A Mysterious Structure in the Milky Way Has Scientists Puzzled

The Milky Way contains evidence of a previously unknown major merger, hidden in the age and chemical makeup of its ancient globular clusters.

Globular clusters are dense groups containing hundreds of thousands of stars. Researchers treat them as fossils of galactic mergers because clusters brought in by a merged galaxy form coherent sequences in age and metallicity, the amount of elements heavier than helium. Each sequence can point to a different source system.

Using a method that allowed them to determine the clusters' relative ages with exceptional precision, the study's authors found three distinct age-metallicity sequences among the Milky Way's globular clusters. One belongs to the progenitor of the Milky Way itself. Another corresponds to the Gaia-Sausage-Enceladus merger, which occurred 10 billion years ago.

The third, intermediate sequence points to a merger that had not previously been identified. Researchers estimate that it occurred about 1.8 billion years before Gaia-Sausage-Enceladus. The incoming system was massive, with a total stellar mass similar to that of GSE, around 500 million solar masses.

Most of that system's mass ended up in the inner Milky Way, within roughly six kiloparsecs of the galactic center. The researchers named the system Low-Energy Kraken-Heracles, or LKH, in recognition of earlier work. They say identifying this third merger resolves earlier questions about the history of the galaxy's inner region.