Intergalactic pile-up, but no witnesses

The Andromeda Galaxy

The Andromeda Galaxy is the largest member of our Local Group of about 40 galaxies. Simulations suggest it formed during a merger of two galaxies.

  • Local Group of galaxies has around 40 members
  • Milky Way and Andromeda are biggest are the biggest of them
  • Andromeda and two smaller galaxies could have come from a cosmic collision

Did a major collision between two massive galaxies occur in the ‘Local Group’ of galaxies six billion years ago? Computer simulations suggest it could have.

The study—by a team of six researchers from Paris Observatory, the French National Centre for Scientific Research (CNRS), and the National Astronomical Observatories of Chinese Academy of Science (NAOC)—found that our biggest neighbour, the Andromeda Galaxy, as well as the smaller Magellanic Cloud galaxies, may well have been formed during a gigantic collision between galaxies.

The Magellanic Clouds are small, ‘irregular’ galaxies close to our Milky Way. They can be seen with the unaided eye under dark skies.

The Local Group includes nearly 40 galaxies and is dominated by two giant spiral galaxies—Andromeda (Messier 31) and our own galaxy, the Milky Way.

Many astronomers think Andromeda might have been formed through the merger of two galaxies of smaller mass. When could such a major event have occurred, and how would it have affected our neighbourhood?

The team, led by Francois Hammer from Paris Observatory, used computer simulations to model for the first time the detailed structural evolution of the Andromeda Galaxy.

They concluded that Andromeda might well have been the result of a collision between two galaxies, one slightly more massive than the Milky Way, and the other about one third as massive.

The first stage of the collision would have been about 9 billion years ago, with the final fusion slightly less than 5.5 billion years ago.

The following video shows how it might have happened.

Origin of the Magellanic Clouds

The simulations also predict that an amount of mass equivalent to one third of that of the Milky Way could have been expelled during the interaction, through the formation of gigantic tidal ‘tails’.

The Large Magellanic Cloud

The Large Magellanic Cloud, a small, irregular galaxy that orbits the Milky Way. Did it form from the wreckage of the Andromeda galaxy's birth?

Most of this matter would have been gas, and a large part of this matter would have been ejected in a particular direction…towards the Milky Way.

The researchers propose that the Magellanic Cloud galaxies formed within one of the tidal tails. They would have been ejected towards the Milky Way, at a very large velocity that has been recently re-evaluated to be one million kilometres per hour (350 km/s)!

This scenario could explain why the Magellanic Clouds are the only gas-rich and irregular galaxy companions of the Milky Way.

The researchers used the measured velocities of these galaxies to trace their positions back several billion years, and they found many solutions for which they could have originated from the Andromeda Galaxy.

If confirmed, these results may support both the hypothesis that most spiral galaxies have been formed by galactic mergers, and the prediction that many dwarf galaxies may originate from tidal tails during such events.

Adapted from information issued by the Observatoire de Paris / ESA / Hubble / NASA / R. Gendler.

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