Hubble finds evidence of another early collision involving the Milky Way

When the universe was just around two billion years old, the Milky Way received a visitor that did not stay: Using the Hubble Space Telescope, a research team has found evidence of a collision 11.8 billion years ago. The long-since-swallowed dwarf galaxy now bears the name Low-energy-Kraken-Heracles, or LKH for short.

Its traces are found in 39 globular clusters. These dense groups of stars contain some of the oldest stars in the Milky Way. Among other things, the research team compared the abundance of elements heavier than helium—the so-called metallicity—and used it to determine the age of the stars. A third population emerged, whose formation occurred between two already known groups.

According to the researchers, the pattern is consistent with another merger. The dwarf galaxy is thought to have had around 500 million solar masses. That represented a significant share of the Milky Way's mass at the time. The collision therefore likely changed its development noticeably. More than half a dozen such collisions are now known at varying levels of detail, including Gaia-Sausage-Enceladus, Sagittarius and Pontus.

The discovery shows why Hubble still matters scientifically despite its age. The telescope was launched in 1990 aboard the Space Shuttle Discovery and repaired after a flaw was found in its 2.4-meter primary mirror. Several servicing missions improved the instrument; even more than 35 years after its launch, it continues to provide data for new insights. At the same time, its technical problems are mounting, and its age-related decline is impossible to overlook.

So what does this change in concrete terms? For astronomers, LKH provides another piece of the puzzle in reconstructing the early history of the Milky Way. The research team wants to search for more such collisions and gain a more precise understanding of the mergers already known.