A collision that occurred in the early days of the Milky Way left traces that are still recognizable today, nearly 12 billion years later.
Our Galaxy has not always been its current size. It grew by forming stars, but also by absorbing smaller galaxies. Astronomers have now clarified one of the oldest episodes in this history. It would have occurred about 11.8 billion years ago, or 1.8 billion years before another major merger that is already known.
Artist’s view of the collision between the young Milky Way and the dwarf galaxy Low-energy-Kraken-Heracles, about 12 billion years ago.
Credit: NASA, ESA, Joseph Olmsted (STScI)
The absorbed galaxy no longer exists as such. The researchers named it Low-energy-Kraken-Heracles, or LKH. It is thought to have contained stars totaling about 500 million solar masses. At that time, this amount represented a significant portion of the young Milky Way. The encounter therefore contributed to its formation at a very early stage.
How can such an ancient event be traced? The team studied 39 globular clusters located in the inner regions of our Galaxy. These very dense groups each contain tens of thousands to several million stars. Their great age makes them witnesses to periods whose other traces have largely disappeared.
Observations from the Hubble Space Telescope made it possible to measure the ages and chemical composition of these clusters precisely. In particular, the researchers compared their content of elements heavier than helium. Data from the Gaia mission also helped distinguish the different populations of clusters present at the heart of the Milky Way.
Three groups then emerged. One corresponds to clusters formed in the young Milky Way. Another is associated with Gaia-Sausage-Enceladus, a galaxy absorbed about 10 billion years ago. Between the two lies a third population whose ages and composition indicate a different origin: LKH.
Most of its stellar matter is thought to have ended up within the central 6 kiloparsecs of the Milky Way, or about 19,600 light-years around the galactic center.
The work continues with new globular clusters observed by Hubble. Dating them could make it possible to identify other ancient mergers and gradually refine the chronology of the earliest stages in the formation of the Milky Way.