Ancient galactic collision 12 billion years ago shaped Milky Way

The Milky Way did not arrive at its current form through quiet accumulation. It grew by colliding with other galaxies, absorbing them whole, over billions of years. Astronomers have long known this happened at least once—a massive merger with a galaxy now called Gaia-Sausage-Enceladus occurred a little more than 10 billion years ago. But evidence has been mounting for something older and equally dramatic: another galactic collision that took place 12 billion years in the past, when the universe itself was still young.

A team led by Davide Massari at the Italian National Institute for Astrophysics has now published findings in Nature Astronomy that confirm this ancient merger. The existence of this collision had been debated for years. To settle the question, the researchers turned to an unexpected source of evidence: tight clusters of ancient stars called globular clusters. These clusters are like archaeological artifacts. Their ages and chemical makeup reveal where they originated—which galaxy they were born in before being swept up into the Milky Way during a merger event. By studying the precise ages of these clusters using data from the Hubble Space Telescope, the team identified three distinct age groups. One belonged to the original Milky Way. Another matched the known Gaia-Sausage-Enceladus event. The third group, standing apart in time, pointed to a previously unconfirmed satellite galaxy that had been absorbed even earlier. The researchers named this ancient intruder Low-energy-Kraken-Heracles—a composite drawn from suggestions by different research teams, combining references to the low-energy orbits of its clusters, a Scandinavian sea monster, and a Greek mythological hero.

The discovery matters because it rewrites the timeline of how our galaxy assembled itself. The Milky Way formed roughly 800 million years after the Big Bang, 13.8 billion years ago, as a small disc of gas. Today it spans 100,000 light-years and contains up to 400 billion stars. Most of those stars were born between 11 and 7 billion years ago—a period that coincides with the galaxy's major growth spurts through mergers. Luca Casagrande, an astrophysicist at the Australian National University who was not part of the study, noted that younger galaxies observed closer to the Big Bang were invariably smaller than the Milky Way is now. "We always knew that the Milky Way had to go through several merger events to build up," he said. The Gaia-Sausage-Enceladus collision was relatively easy to detect. The others are harder to find, which is why uncertainty has lingered around the older events.

What made this discovery possible was precision. The Hubble data allowed researchers to determine the ages of globular clusters with extraordinary accuracy. With those precise measurements in hand, they could see the three distinct groups clearly. Duncan Forbes, an astrophysicist at Swinburne University who had previously suggested this third galaxy existed, said the new study carried "more statistical weight" than earlier work. "With more precise relative ages, Massari and colleagues were able to estimate when the satellite was accreted onto the infant Milky Way," Forbes said. He noted that the satellite galaxy was one of the most massive objects ever to merge with the Milky Way as it grew across cosmic time.

The naming of Low-energy-Kraken-Heracles reflects a quirk of modern astronomy: there is no official convention for naming these ancient, absorbed galaxies. The Gaia-Sausage-Enceladus name itself is a patchwork—derived from the Gaia satellite that detected it, the sausage-like shape of the data in early analyses, and Enceladus, a figure from Greek mythology. Forbes had previously suggested calling the satellite galaxy "Koala" because both it and the animal are associated with low energy. The names may sound whimsical, but they serve a purpose: they mark discoveries that fundamentally changed how we understand our cosmic home.

The Milky Way's merger history is not finished. It is currently in the slow process of absorbing the Sagittarius Dwarf Spheroidal Galaxy, a process that began six billion years ago and continues today. In the coming billions of years, it may collide with Andromeda, though astronomers remain uncertain whether that encounter will actually occur. Massari suggested it is "possible, but rather unlikely" that major merger events remain hidden in the Milky Way's distant past. "I do not think there is room for another event this massive, at least in the far away past of our galaxy," he said. Casagrande, however, offered a more open view. As telescopes improve and more data accumulates, he said, astronomers might yet discover a fourth or fifth ancient collision. The story of how the Milky Way became what it is may not yet be fully told.