Gaia-Enceladus: the galaxy that left its mark on the Milky Way

Some stars have been preserving the memory of a galactic collision for billions of years. They do not have to tell us about it: all we need to do is measure how they move and what elements they are made of. That is how astronomers reconstructed the story of Gaia-Enceladus, a galaxy absorbed by the Milky Way around 10 billion years ago, long before the Sun was born. A kind of cosmic archive, only without the dust and with quite a few more zeroes.

In 2018, astrophysicist Amina Helmi and her colleagues published evidence in the journal Nature that a substantial part of the inner stellar halo of our galaxy came from that ancient companion. The halo is the region surrounding the main part of the Milky Way. The galaxy that left its stars there was named Gaia-Enceladus: Gaia after the space mission, and Enceladus after the giant in Greek mythology.

The clue came with the second data release from Gaia, the European Space Agency’s mission, on 25 April 2018. Its measurements provided the positions, distances and movements of stars. Helmi’s team combined them with chemical information obtained through spectroscopic surveys. In a sample of seven million stars, the ESA describes a group of around 30,000 that moves differently and in the opposite direction to most of those analysed.

Direction and speed were only part of the story. Their chemistry also acted as a fingerprint of where they formed: Helmi and her team detected a composition different from that of stars born in the Milky Way. Taken together, these clues made it possible to reconstruct the group’s external origins. The collision does not appear in a cosmic film: its traces survive in stars that still travel through our galaxy.

The encounter also left its mark on the Milky Way itself. According to Helmi’s study, the merger heated and altered the disc that existed at the time and helped form what is known as the thick disc. The estimated mass ratio for the collision was approximately four to one: the Milky Way had around four times the mass of Gaia-Enceladus. It was no contest between equals, but the smaller galaxy left an enormous signature.

The merger took place around 10 billion years ago, according to the Nature paper. The 2026 Kavli Prize profile places the event between 8 and 11 billion years ago. To put that into perspective, the Sun formed around 4.6 billion years ago, according to NASA. By the time our star appeared, the ancient collision was already part of the distant past; its stars, however, were still carrying the evidence.

Fun fact: Gaia-Enceladus is also known as “Sausage” because, when certain velocities of its stars are plotted, the distribution takes on an elongated shape reminiscent of the sausage. Astronomy has its own cravings.

By Marie CurIA, virtual writer of La Gaceta de Guardamar’s science curiosities, an artificial intelligence: produced using publicly available sources. Sources consulted: , science.nasa.gov, , .