Astronomy: Traces of the Milky Way's last collision much younger than thought
Until now, it has been assumed that the Milky Way has been peaceful for many billions of years. Now data suggests that this is not the case.
On the left many "folds" between groups of stars at the edge of a galaxy, on the right without these remnants of a collision.
(Image: ESA/Gaia/DPAC, T Donlon et al. 2024/Stefan Payne-Wardenaar)
The last major collision in our Milky Way occurred many billions of years later than previously assumed. This is suggested by an analysis of measurement data from ESA's Gaia space telescope, which is dedicated to distortions in the distribution of stars. This has now been published by the Rensselaer Polytechnic Institute in the USA. According to the study, these "wrinkles" occur after galaxies merge and then disappear over time when the stars become evenly distributed again. Gaia data has now shown that the distortions in the distribution of stars in the Milky Way indicate a collision that occurred only three billion years ago. That would be at least five billion years later than previously assumed.
The history of the Milky Way is emerging
As the research team explains, they have found the trace of the much more recent collision in Gaia's gigantic amounts of data. The space telescope has been mapping billions of objects in the night sky for years, including stars in our home galaxy. Around 100,000 comparatively close objects have been examined to see how many and how large "wrinkles" they create. By this, the team means distortions between groups of stars, which ensure a less uniform distribution than would be expected after billions of quiet years. As clearly as they can be seen in the Gaia data, they must have formed comparatively recently. This suggests a further and much more recent collision than the one we know about.
Until now, a collision known as Gaia-Sausage-Enceladus (GSE) was considered to be the last in the history of the Milky Way. It is thought to have occurred around five to eight billion years ago. Five other named collisions are thought to have taken place before that. The merger whose possible trace has now been discovered does not yet have a name. However, the research team assigns it to an event called the "Virgo Radial Merger", which took place only three billion years ago. The discovery underlines once again how much we still have to find out about the history of the Milky Way, and at the same time how enormously valuable Gaia is for this. The research is presented in the Monthly Notices of the Royal Astronomical Society.
(mho)