Milky Way and Andromeda galaxies may not collide after all

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In roughly 4 billion years, our home Milky Way galaxy may collide with the neighboring Andromeda galaxy. We are approaching Andromeda at roughly 250,000 miles per hour, and scientists have continually debated the likelihood and timing of this catastrophic event. Now, a new study published June 2 in the journal Nature Astronomy is casting new doubt on what would be a cataclysmic universe-changing event.

The team from universities in Finland, the United Kingdom, and France, ran 100,000 simulations using the latest data from the Hubble Space Telescope and the European Space Agency’s (ESA) Gaia space observatory. These variables included the effect of our galaxy’s largest satellite–the Large Magellanic Cloud or LMC–and some new uncertainties.

The team found only a 2 percent probability that the galaxies will collide in the next five billion years. In slightly over half of the simulated scenarios, Andromeda and the Milky Way experience at least one close encounter before they lose enough orbital energy to collide and merge. However, this happened in eight to 10 billion years and not four or five, as previous estimates found.

Three simulations showing Milky Way and Andromeda bypass at 1 million light year separation. At 500,000 light-years, dark matter provides friction that brings galaxies to a close encounter. A 100,000 light-year separation leads to a collision. CREDIT: NASA/ESA

On that earlier timescale, the sun will have already burnt itself out. Yet in most other cases, both galaxies pass at a large enough distance that they continue to evolve without playing cosmic bumper cars for years.

While this new research does challenge the previously accepted fate of our galaxy, the team stresses that it is still very difficult to make a precise prediction. The new study does not imply a mistake to these earlier calculations, but instead includes more variables into the simulations with newer data from both space telescopes.

“When we tried to start from the same assumptions as previous researchers, we recovered the same results,” Till Sawala, a study co-author and astrophysicist at the University of Helsinki in Finland, said in a statement. “We’ve simply been able to explore a much larger space of possibilities, taking advantage of new data. 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 LMC.”

While the LMC’s mass is less than 10 percent of the Milky Way’s mass, its gravitational pull is directed perpendicular to the orbit with Andromeda. This alters the Milky Way’s motion enough to significantly reduce the chances of a mega merge with Andromeda. While earlier studies considered the most most likely value for each variable, the team here ran many thousands of simulations to allow for many observational uncertainties.

“These results are significant for the fate of our Galaxy,” added Alis Deason, a study co-author and galactic astronomer from Durham University in the UK. “It used to appear destined to merge with Andromeda forming a colossal ‘Milkomeda’. Now, there is a chance that we could avoid this fate entirely.”

[ Related: A terrifying fate may lurk inside the Milky Way. ]

The team are already looking ahead to studying further scenarios when even more datasets become available, since this new uncertainty about the future of the Milky Way and Andromeda may not last and our understanding of these events is constantly changing. Gaia is expected to deliver more precise measurements of some of the most crucial variables within galaxies. These variables include Andromeda’s transverse motion–or when all points on a wave oscillate along paths at right angles to the direction of the wave’s advance–which is difficult to measure directly.

“The Universe is a dynamic place, constantly evolving,” said study co-author and Durham University cosmologist Carlos Frenk. “We see external galaxies often colliding and merging with other galaxies, sometimes producing the equivalent of cosmic fireworks when gas, driven to the centre of the merger remnant, feeds a central black hole emitting an enormous amount of radiation, before irrevocably falling into the hole. Until now we thought this was the fate that awaited our Milky Way galaxy. We now know that there is a very good chance that we may avoid that scary destiny.”