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A new study shows that the early Milky Way’s stellar disc was largely destroyed by a violent collision with the Gaia-Sausage-Enceladus galaxy around 11 billion years ago, and only later re-formed.
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Image of a simulation of the Milky Way's accretion history whithin the Auriga project
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Matthew D. A. Orkney, Chervin F. P. Laporte

A new study led by researchers at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institut d’Estudis Espacials de Catalunya (IEEC) reveals how the discs of galaxies like the Milky Way are impacted by ancient galactic collisions. 

Published in Monthly Notices of the Royal Astronomical Society, the research investigates how simulated galaxy collisions can fully or partially destroy stellar discs. Together with observational data of star clusters, the authors use this research to improve predictions for the time of the last significant galactic collision in our own Milky Way galaxy.
 

When did the Milky Way disc spin up?
 

The Milky Way disc is a vast, rotating system of stars shaped like a pancake, with spiral arms winding out from its centre. This disc contains most of the Galaxy’s stars, including the Sun, and rotates at a speed of over 220 kilometers per second.

Astronomers have long tried to determine when this rotating disc first formed. One key clue comes from the motions and ages of stars: at some point in the Galaxy’s early history, stars began to move 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 suspected that a violent collision with a smaller galaxy played a major role in shaping the Milky Way we see 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 around 10 billion years ago. This event is now known as the Gaia-Sausage-Enceladus (GSE) merger.

In this new study, simulations of Milky Way-like galaxies (the Auriga simulations) are used to investigate how rotating discs form under a variety of different scenarios. These show how galaxies like our Milky Way react to ancient collisions.
 

Key findings
 

The study shows that rotating stellar discs often form much earlier than previously assumed, but can be partially or completely destroyed by major galactic collisions. As a result, the moment when the Milky Way’s disc appears to “spin up” may not mark the first time a disc formed, but rather the time when the Galaxy recovered from a destructive merger.

 

​​​​​Shown is the evolution of a galaxy, from initial building blocks to final spiral disc, taken from the Auriga simulation suite. The main image represents the combined density of gas and dark matter, where the gas is coloured corresponding to its ambient temperature (for which blue is cooler and red is hotter). Circular inset panels show the stars in the central galaxy, oriented both face-on and side-on to the angular momentum of the disc. These are made by combining the K,B & U stellar photometry filters. The soundtrack is a combination of music from artist Aaron Parks, and noises that are directly translated from the gas movement and collisions within the simulation data. Credit: Dr. Matthew D.A. Orkney and Dr. Chervin F.P. Laporte, using data from the Auriga simulation project and resources from the Virgo collaboration & the nyx supercomputer in the University of Barcelona.

 

Using insights from these simulations, the authors infer that the Gaia-Sausage-Enceladus merger likely occurred around 11 billion years ago, earlier than many previous estimates. Crucially, this timing coincides with a sharp increase in the birth of star clusters in the Milky Way. Such 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 from the impact, raising star formation and fostering the formation of globular clusters. This is the first time this link has been made.” says collaborating author Chervin F. P. Laporte, researcher at 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 lead author Matthew D. A. Orkney, researcher at ICCUB and IEEC.

Whilst scientists can never travel back in time to observe the Milky Way in its youth, they can observe the formation of similar galaxies in the distant Universe with new data from the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA), a powerful radio telescope.

 

Reference:

The full paper is available here, and the Auriga simulation data is publicly accessible for further research.

Orkney, Matthew; Laporte, Chervin. Build-up and survival of the disc: From numerical models of galaxy formation to the Milky Way. Monthly Notices of the Royal Astronomical Society. DOI: https://doi.org/10.1093/mnras/staf2154