Our Milky Way Galaxy and the Andromeda galaxy had a close encounter around 10 billion years ago, according to European astronomers led by Dr Hongsheng Zhao from the University of St Andrews.

Previous studies have suggested that our galaxy is set to crash into neighbor Andromeda Galaxy in 3 – 4 billion years, and that this will be the first time such a collision has taken place.

However, in the new study, accepted for publication in the journal Astronomy and Astrophysics (arXiv.org version), Dr Zhao’s team proposes a very different idea that the two galaxies collided once before, some 10 billion years ago. Remarkably, this would explain the observed structure of the galaxies and their satellites, something that has been difficult to account for until now.

The Milky Way is part of a group of galaxies called the Local Group. Cosmologists believe that most of the mass of the group is invisible, made of so-called dark matter. They suggest that across the whole Universe, this matter outweighs ‘normal’ matter by a factor of five. The dark matter in both Andromeda and the Milky Way then makes the gravitational pull between the galaxies strong enough to overcome the expansion of the cosmos, so that they are now moving towards each other at around 100 km per second, heading for a collision 3 billion years in the future. But this model is based on the conventional model of gravity and struggles to explain some properties of the galaxies we see around us.

Dr Zhao and his colleagues argue that at present the only way to successfully predict the total gravitational pull of any galaxy or small galaxy group, before measuring the motion of stars and gas in it, is to make use of a model first proposed by Prof Mordehai Milgrom of the Weizmann Institute in Israel in 1983. This theory, named Modified Newtonian Dynamics (MOND), describes how gravity behaves differently on the largest scales, diverging from the predictions made by Newton and Einstein.

The team has for the first time used MOND to calculate the motion of Local Group galaxies. Their work suggests that the Milky Way and Andromeda had a close encounter about 10 billion years ago. If gravity conforms to the conventional model on the largest scales then taking into account the supposed additional pull of dark matter, the two galaxies would have merged.

“Dark matter would work like honey: in a close encounter, the Milky Way and Andromeda would get stuck together, figuratively speaking”, explained senior author Prof Pavel Kroupa from Bonn University.

“But if Milgrom’s theory is right, then there are no dark particles and the two large galaxies could have simply passed each other thereby drawing matter from each other into long thin tidal arms,” added co-author Dr Benoit Famaey from the Observatoire Astronomique de Strasbourg.

In the new model, the Milky Way and Andromeda are still going to crash into each other again in the next few billion years, but it will feel like ‘deja vu‘.

“If we are right, the history of the cosmos will have to be rewritten from scratch,” Prof Kroupa concluded.

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Bibliographic information: HongSheng Zhao et al. 2013. Local Group timing in Milgromian dynamics. A past Milky Way-Andromeda encounter at z>0.8. Accepted for publication in Astronomy and Astrophysics