Galactic collision may have reset Milky Way disk 11 billion years ago

A new study led by researchers at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (IEEC) reveals how the disks of galaxies like the Milky Way are affected by ancient galactic collisions.

Published in the Monthly Notices of the Royal Astronomical Society, the study analyzes, using simulations, how galaxy collisions can completely or partially destroy stellar disks. Together with observational data on star clusters, the authors use this study to improve predictions about the timing of the last significant galactic collision in the Milky Way.

When did the Milky Way’s disk spin up?

The disk of the Milky Way is a vast, rotating, pancake-shaped system of stars, with spiral arms winding out from its center. This disk contains the majority of the galaxy’s stars, including the sun, and rotates at over 220 kilometers per second.

For a long time, astronomers have tried to determine when this rotating disk formed. A key clue lies in the motions and ages of the stars: at some point in the galaxy’s early history, the stars began moving in a coherent, rotating pattern, marking what scientists call the galaxy’s spin-up time.

However, the Milky Way did not form in isolation. For decades, scientists have suspected that a violent collision with a smaller galaxy played an important role in shaping the Milky Way as we observe it today. This suspicion was confirmed in 2018, when data from the Gaia mission revealed a large population of stars whose unusual motions could only be explained by a massive merger that occurred about ten billion years ago. This event is now known as the Gaia-Sausage-Enceladus (GSE) merger.

In this study, simulations of Milky Way–like galaxies (the Auriga simulations) are used to investigate how rotating disks form under different scenarios. These simulations show how galaxies such as the Milky Way react to ancient collisions.

Galactic fires and ancient collisions

The study shows that rotating stellar disks often formed much earlier than previously thought, but can be partially or completely destroyed by major galactic collisions. As a result, the moment when the Milky Way’s disk appears to spin up cannot mark when the first time the disk formed, but rather the moment when the galaxy recovered from a destructive merger.

Applying insights from these simulations, the authors infer that the Gaia–Sausage–Enceladus merger probably occurred about 11 billion years ago, earlier than many previous estimates had indicated. Crucially, this timing coincides with a sharp increase in the formation of star clusters in the Milky Way. These bursts of star formation are a natural consequence of galactic collisions, which compress gas and trigger intense star formation.

“Models of the Gaia–Sausage–Enceladus merger predict that a galactic firework should have followed the impact, raising star formation and fostering the formation of globular clusters. This is the first time this link has been made,” says co-author Chervin F. P. Laporte, a researcher at the French National Centre for Scientific Research (CNRS).

“This research highlights the important relationship between galactic structure and ancient collisions, which must be understood in unison in order to understand the history of our galaxy,” adds Matthew D. A. Orkney, the study’s lead author and a researcher at ICCUB and IEEC.

Scientists cannot travel back in time to observe the Milky Way in its youth, but they can observe the formation of similar galaxies in the distant universe using new data from the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA), a powerful radio telescope.

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