Andromeda 'born in a collision'
The nearest spiral galaxy to the Milky Way - Andromeda - was born when two smaller galaxies collided, say astronomers.
An international team conducted a computer simulation of how Andromeda evolved over time.
The results suggest that two galaxies collided some nine billion years ago and permanently fused about 5.5 billion years ago.
The study has been published in the Astrophysical Journal.
The lead author, Francois Hammer of the Paris Observatory, France, told BBC News that while scientists have been able to detect galaxies near the edge of the Universe, there many gaps in our knowledge of our immediate neighbourhood, known as the Local Group of galaxies.
The Local Group includes some 40 galaxies, of which the biggest are the Milky Way and Andromeda.
"Many astronomers, especially specialists in this field, thought that the Andromeda galaxy could be the result of a major merger," said Dr Hammer.
"However, this has never been tested and especially dated [until now]."
Dark matter insights
The researcher explained that his team's findings had "the potential to revise all our knowledge about the Local Group - and this may have also an impact regarding the amount of dark matter in galaxies".
Using a computer simulation, the astronomers were able to reproduce most of the peculiar properties of the Andromeda galaxy: the large thin disk including its giant ring of gas and dust, the massive central bulge, the gigantic thick galactic "disc", and the giant stream of old stars.
The simulations were run on high performance computers at the National Astronomical Observatory of China (NAOC) and at the Paris Observatory.
The scientists used up to eight million particles to simulate stars, gas, and dark matter.
Dr Hammer said that the study could also provide insights into the formation of our own galaxy.
"It doesn't mean that the Milky Way could not have been formed this way - [maybe it has been], but [it would have happened] much earlier," he said.