The Milky Way has not always been the enormous spiral galaxy we see today. Like many large galaxies, it grew partly by swallowing smaller neighbors.
Now, observations from NASA’s Hubble Space Telescope have revealed strong evidence of one of the earliest major mergers in our galaxy’s history.
The discovery pushes scientists’ detailed knowledge of the Milky Way’s merger history about 1.8 billion years further into the past.
The research, published in Nature Astronomy, suggests that the young Milky Way absorbed a substantial dwarf galaxy around 11.8 billion years ago—only about 2 billion years after the Big Bang.
Today, the Milky Way contains hundreds of billions of stars. It reached this enormous size both by forming stars from its own gas and by acquiring stars, gas and dark matter from other galaxies.
Astronomers already know about several important mergers. The Milky Way’s interaction with the Sagittarius dwarf galaxy began more than 6 billion years ago and continues today. Even earlier, about 10 billion years ago, our galaxy collided with and absorbed a dwarf galaxy known as Gaia-Sausage-Enceladus, an event that helped reshape the Milky Way.
Scientists suspected that another significant merger happened before this, but evidence from such an early period is difficult to find. The young Milky Way was smaller and more chaotic, and billions of years of evolution have blurred many of the original clues.
To look deeper into this history, researchers turned to some of the galaxy’s oldest surviving structures: globular clusters.
These enormous, roughly spherical groups can contain tens of thousands to millions of stars. Because many formed very early in cosmic history, they act like archaeological remains, preserving information about galaxies that disappeared billions of years ago.
Researchers used Hubble to examine 39 globular clusters within the inner 20,000 light-years of the Milky Way. Hubble’s detailed observations allowed them to determine the clusters’ ages and metallicity—the amount of elements heavier than helium in their stars—with unusually high precision. They combined these measurements with data from the European Space Agency’s Gaia mission.
The analysis revealed an unexpected third population of clusters. They were older than clusters associated with Gaia-Sausage-Enceladus but younger than clusters that formed within the early Milky Way itself.
The researchers concluded that these clusters were brought into the Milky Way by a previously debated dwarf galaxy merger.
They named the lost galaxy Low-energy-Kraken-Heracles, or LKH, reflecting earlier studies that had proposed evidence for such an ancient collision.
LKH appears to have contained stars with a combined mass roughly 500 million times that of the sun. At the time, that represented a substantial fraction of the young Milky Way’s stellar mass, meaning the merger may have played an important role in shaping our galaxy during its earliest stages.
The finding also changes the picture of how the early Milky Way developed. Its oldest populations were apparently not produced entirely within the galaxy itself. Stars born in other galaxies were already becoming part of the Milky Way billions of years ago.
Researchers now plan to use Hubble to investigate additional globular clusters. By identifying which clusters arrived through different ancient mergers, they hope to reconstruct, piece by piece, how the Milky Way became the galaxy we call home.
Source: KSR.
