No certainty about the predicted Milky Way - Andromeda collision

Scientists from Helsinki, Durham and Toulouse universities used data from NASA’s Hubble and the European Space Agency’s Gaia space telescopes to simulate how the Milky Way and Andromeda will evolve over the next 10 billion years. The team ran 100,000 simulations of both galaxies based on the latest observational data.

Only slightly more than half of the simulations resulted in a merger of the Milky Way and Andromeda. In around half of the cases, the Milky Way and Andromeda experience at least one more or less close encounter, before losing enough orbital momentum to eventually merge. In most other cases, the two galaxies pass each other at such a large distance that they continue to evolve largely unperturbed for a very long time. A direct collision within 4-5 billion years, as earlier studies had predicted, is only a 2% probability. The results has been recently published in .

“Our starkly different result is mainly due to two factors”, explains  from the University of Helsinki. “Firstly, while some earlier works had focused on the interaction between the Milky Way, Andromeda, and the Triangulum galaxy, we also include the effect of the Milky Way’s most massive satellite, the Large Magellanic Cloud. Although its mass is only around 15% of the Milky Way’s, its gravitational pull directed perpendicular to the orbit with Andromeda perturbs the Milky Way’s motion enough to significantly reduce the chance of a merger with the Andromeda galaxy. Secondly, while earlier studies only considered the most likely value for each variable, we ran many thousands of simulations, which allowed us to account for all the observational uncertainties. Even though we benefit from the most precise measurements, we now find considerable uncertainty about the outcome.”

According to the new study, if the Milky Way and Andromeda are to merge, the most likely time would be 7 - 8 billion years in the future, significantly later than previously predicted. However, the authors emphasise that based on the best available data today, it is simply not possible to make a precise prediction.

“The impending collision of our Galaxy and Andromeda is such a famous result that it can be found anywhere from textbooks to children’s literature.”, comments Jehanne Delhomelle from the University of Toulouse, who worked on the project as an exchange student in Helsinki. “It is part of the beauty of the scientific process that even widely accepted results can always be challenged, and possibly overturned. We have to keep an open mind about new results, even when they are surprising at first glance”.

The new conclusions do not imply a mistake in the earlier calculations, emphasises Sawala. “When we tried to start from the same assumptions, we recovered the same results. We have simply been able to explore a much larger space of possibilities.”

The new uncertainty about the future of our galaxy may not last, as the team is already planning for the future, where the Gaia space telescope will soon deliver more precise measurements of some of the most crucial variables, including the proper motions of Andromeda. 

“We are currently preparing the next set of simulations with improved physical models”, adds  from the University of Helsinki. “By combining these with updated observational data, we aim to make the most accurate prediction yet of the Milky Way’s fate in the coming years.”

“Although the Milky Way has experienced tens of minor mergers over its lifetime, its last major merger occurred 10 billion years ago. The potential Milky Way-Andromeda merger would be a rare and transformative event for both galaxies and it is thus critically important to model.”, comments  from the University of Helsinki.