The idea that the Milky Way (MW) and Andromeda (M31) will collide emerged after decades of observations by a host of astronomers. The Hubble played a decisive role in the determination during the early 2000s. It was a triumph of precision astronomy and space telescopes. Now, the Hubble has played an equally important role in cancelling the collision.
A galaxy collision is a colossal event. The MW and M31 contain as many as 1.5 trillion stars combined. While it's not likely that any individual stars will collide with other individual stars, the collision is still powerful. Gas clouds will collide and compress, triggering abundant star birth and adding billions of new stars. There will be more supernovae, and some stars will likely be ejected from the new, merged galaxy. Since the collision was forecasted to occur billions of years from now, the Sun would be a red giant, Earth would likely be uninhabitable or even destroyed, and humanity would likely be long gone by then.
Now it looks like there may be no collision after all, according to new research. It's titled "No certainty of a Milky Way–Andromeda collision," and it's published in Nature Astronomy. The lead author is Till Sawala, an astronomer at the University of Helsinki in Finland.
The collision was never an absolute certainty. Predicting anything five billion years in the future is a dicey task. However, there was solid evidence in favour of the possibility. However, new evidence from Hubble and Gaia casts serious doubt on the event.
"Here we consider the latest and most accurate observations by the Gaia and Hubble space telescopes, along with recent consensus mass estimates, to derive possible future scenarios and identify the main sources of uncertainty in the evolution of the Local Group over the next 10 billion years," the authors write.
At issue is the nature of theLocal Groupof galaxies. The MW and M31 dominate the Local Group, but it has other members, too. The Triangulum Galaxy (M33) is the other large galaxy in the Local Group, and there are also satellite galaxies of all three large galaxies, like the Large Magellanic Cloud (LMC).
Prior to colliding, the MW and M31 will orbit one another. This orbit is unstable and is the source of much of the uncertainty around the merger. "We found that the next most massive Local Group member galaxies—namely, M33 and the Large Magellanic Cloud—distinctly and radically affect the Milky Way-Andromeda orbit," the authors write in their research article.
"We have the most comprehensive study of this problem today that actually folds in all the observational uncertainties," said lead author Sawala in apress release. "Even using the latest and most precise observational data available, the future of the Local Group of several dozen galaxies is uncertain. Intriguingly, we find an almost equal probability for the widely publicized merger scenario, or, conversely, and alternative one where the Milky Way and Andromeda survive unscacthed."
These results come from new observations and simulations. The simulations contained the MW, M31, M33, and the LMC, with updated data for their coordinates and galactic centers. They also included 20 other variables, including the galaxies' halo masses, line of sight velocities, and proper motions. The researchers ran 100,000 Monte Carlo simulations that extended 10 billion years into the future.
"Our aim was not to predict the precise time of the merger, (in fact, we argue that such a prediction is futile based on the current data), so we simply assumed that any system that passes below a threshold distance will eventually merge, and we identified this time as the lower limit for the time of the merger," the authors explain. They selected 20 kpc as the merger threshold.
These results are a significant change from 2012, when astronomers published adetailed analysisof several years of Hubble observations and concluded that the MW and M31 would merge within five billion years. "After nearly a century of speculation about the future destiny of Andromeda and our Milky Way, we at last have a clear picture of how events will unfold over the coming billions of years," said Sangmo Tony Sohn of STScI at the time. Sohn was one of the authors of the analysis.
"It's somewhat ironic that, despite the addition of more precise Hubble data taken in recent years, we are now less certain about the outcome of a potential collision. That's because of the more complex analysis and because we consider a more complete system. But the only way to get to a new prediction about the eventual fate of the Milky Way will be with even better data," said Sawala.
It's unusual for improved data to lead to a less certain result, but that's the case with the MW-Andromeda collision. Each data point in the simulation has some uncertainty, and the uncertainty compounds.
Because there are so many variables that each have their errors, that accumulates to rather large uncertainty about the outcome, leading to the conclusion that the chance of a direct collision is only 50% within the next 10 billion years," said Sawala. "The Milky Way and Andromeda alone would remain in the same plane as they orbit each other, but this doesn't mean they need to crash. They could still go past each other.
The outcome might be different if the MW and Andromeda were solitary objects. But both spiral galaxies have satellites, and the simulation included the most massive ones: the Large Magellanic Cloud for the Milky Way, and M33 (Triangulum) for Andromeda.
"The extra mass of Andromeda's satellite galaxy M33 pulls the Milky Way a little bit more towards it. However, we also show that the LMC pulls the Milky Way off the orbital plane and away from Andromeda. It doesn't mean that the LMC will save us from that merger, but it makes it a bit less likely," said Sawala.
The researchers also examined the fates of the satellites M33 and the LMC. They found that the LMC is almost certain to merge with the MW long before any merger with Andromeda. In fact, that merger is already underway and could be completed within 1.3 billion years. "For M33, with a merger threshold of 20 kpc, we found an ~86% chance of a merger with M31 and a median time of 3.3 Gyr," the authors write.
Half of the simulations show the MW and Andromeda moving past each other at a distance of half a million light-years or less, which is about five times larger than the MW's diameter. The dark matter haloes of both galaxies experience dynamical friction that eventually causes them to orbit one another and spiral inward. In the far distant future, they will eventually merge. A lot can happen between now and then, and the other 50% show that they don't come close enough for the dynamical friction to play a role. They would orbit one another for an extremely long time.
Only a very small number of the simulations, about 1% or 2%, show the pair of galaxies colliding head-on in about five billion years.
So, where does that leave our home galaxy, the Milky Way?
"Based on the best available data, the fate of our Galaxy is still completely open," the authors conclude.
Press Release:Hubble casts doubt on certainty of galactic collision
New Research:No certainty of a Milky Way–Andromeda collision