Record of the Milky Way’s galactic collision held by an ancient star

The ‘fossilised record’ of a collision between the Milky Way and a smaller galaxy 10-billion-years ago has been discovered locked within an ancient star.

This image of the galactic merger NGC 6052 — 230 million light-years away — shoes what a collision between two galaxies could look like. (ESA/Hubble/NASA/A. Adamo et al)

ν Indi is an unusual star labelled a ‘subgiant’ which lies in the constellation of Indus located 94 light-years from Earth. One may not expect such a star, ancient, bright and mostly isolated, to make an excellent witness to the Milky Way’s collision with a smaller galaxy — Gaia-Enceladus — in its distant past. But, that is exactly what an international team of scientists, led by the University of Birmingham, have uncovered.

By conducting a forensic examination of ν Indi the team were able to probe the history of the Milky Way, finding a ‘fossilised record’ of the environment in which the star formed. To conduct their investigation, the researchers used data from satellites and ground-based telescopes, publishing their findings in the journal Nature Astronomy.

Using data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS) and the application of a method known as asteroseismology, the team aged the star via its natural oscillations. When combined with data supplied by the European Space Agency’s (ESA) Gaia mission the team’s completed forensic report revealed that the star was born during the Milky Way’s earliest moments. They also found that the star’s motion through our galaxy was altered by the Milky Way’s collision with Gaia-Enceladus, something that then allows them to date said encounter.

“Since the motion of ν Indi was affected by the Gaia-Enceladus collision, the collision must have happened once the star had formed,” says Bill Chaplin, Professor of Astrophysics at the University of Birmingham and lead author of the study. “That is how we have been able to use the asteroseismically-determined age to place new limits on when the Gaia-Enceladus event occurred.”

Artist’s impression of the merger between the Gaia-Enceladus galaxy and our Milky Way, which took place during our Galaxy’s early formation stages, 10 billion years ago. The positions and motions of the stars in Gaia-Enceladus (represented with yellow arrows) in this early phase of the merger are based on a computer simulation that models a similar encounter to that uncovered by Gaia. ( ESA (artist’s impression and composition); Koppelman, Villalobos and Helmi (simulation); NASA/ESA/Hubble (galaxy image), CC BY-SA 3.0 IGO)

The collision between the Milky Way and Gaia-Enceladus — a galaxy estimated to be just slightly more massive than the Small Magellanic Cloud — was first proposed in 2018. Initial evidence of this collision was provided by a cluster of blue stars present in the Milky Way’s halo — a nearly spherical region of thinly scattered stars, globular clusters of stars, and nebulous gas that encircles our galaxy. Many of these stars within this halo region are moving in a different direction to the rest of the galaxy’s billions of stars. This is something that only makes sense in the context of a galactic collision.

The galaxy’s most ancient stars — remnants of the merger estimated to be between 10 to 13-billion-years old — tell astronomers that the collision between the Milky Way and Gaia-Enceladus also occurred roughly 10 billion years ago.

The Galactic disc itself is composed of two parts. There is the thin disc, which is a few hundred light-years deep and contains the pattern of spiral arms made by bright stars. And there is the thick disc, which is a few thousand light-years deep. It contains about 10–20 per cent of the Galaxy’s stars yet its origins have been difficult to determine. (ESA)

The researchers believe that the fact we see so many stars from Gaia-Enceladus means we can assume that the smaller galaxy had a significant impact on the evolution of the Milky Way. “Understanding that is now a very hot topic in astronomy, and this study is an important step in understanding when this collision occurred,” adds paper co-author Dr Ted Mackereth, also from the University of Birmingham.

But, there is another significant result from the study, the potential of TESS and the asteroseismology technique. The study demonstrates clearly the strong potential of the TESS programme to draw together rich new insights about the stars that are our closest neighbours in the Milky Way.

“This study demonstrates the potential of asteroseismology with TESS, and what is possible when one has a variety of cutting-edge data available on a single, bright star,” says Bill Chaplin.