Does a major collision between two massive galaxies occur in the Local Group 6 billion years ago? Numerical simulations made by six researchers from Paris Observatory, CNRS and the National Astronomical Observatory, Chinese Academy of Sciences (NAOC) suggest this. They find that our big neighbor, the Andromeda galaxy, as well as the Magellanic Clouds, may well have been formed during such a gigantic collision between two galaxies. Results will be published on November 20th and December 10th in two articles of the Astrophysical Journal.

Our neighbourhood, 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 believe that Andromeda may have been formed through the merger of two galaxies of smaller mass. When did such a major event occur, and what are the consequences to our neighbourhood? The team led by François Hammer from Paris Observatory, which includes researchers from NAOC, modelled for the first time the detailed structural evolution of the Andromeda galaxy. They were able to reproduce most of the peculiar properties of this galaxy: the large thin disk including its giant ring of gas and dust, the massive central bulge, the gigantic thick disk, the giant stream of old stars, as well as many other stellar streams discovered in the galaxy halo. They concluded that Andromeda may well be the result of the collision between two galaxies, one slightly more massive than the Milky Way, the other about three times less massive. The epoch of the event can be estimated from the observed stellar ages of the Andromeda structures: the first passage and the final fusion had occurred slightly less than 9 and 5.5 billion years ago, respectively.

The birth of the Andromeda galaxy
Such a collision would be the most important event that ever occurred in the Local Group history. This is because the Andromeda Galaxy and its satellites gather the largest fraction of the baryonic matter of the Local Group, that includes gas and stars. The collision must be particularly violent to generate a large enough amount of angular momentum (rotation), which is required to form the giant galactic disk of Andromeda. 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. Most of this matter is expected to be dominated by gas. A large part of this matter is ejected through a privileged direction, included in a hyperplane oriented
along the disk of the Andromeda galaxy. Because the Andromeda disk is seen almost edge-on (inclination of 77°), this hyperplane may easily include the Milky Way.

The origin of the Magellanic Clouds
In the second part of their study, the researchers investigated the possible consequences of such a major event in the immediate neighbourhood of our Galaxy. The origin of the Magellanic Clouds is indeed still a mystery. They proposed that they formed within one of the tidal tail produced during the gigantic interaction nine billion years ago. As such, they would have been ejected towards the Milky Way, at a very large velocity that has been recently re-evaluated to be one million kilometers per hour (350 km/s)! This scenario may also explain why the Magellanic Clouds are the only gas-rich and irregular companions of the Milky Way. The researchers used the measured peculiar velocities of
these galaxies to trace back their position several billion years ago, and they found many solutions for which they could originate from the Andromeda galaxy.

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

Simulations
Astrophysical simulations were performed on high performance computers at the NAOC in China and his counterpart at the Computer Division of the Paris Observatory. To reproduce the formation of the Andromeda galaxy by a merger, from 300,000 to 8 million particles were used including stars, gas, and dark matter. Some of these calculations
conducted at the Observatoire de Paris on 8-CPU machines lasted 1-20 days and generated 7 to 23 gigabytes of data. In total, nearly one hundred of models were tested.

Collaboration
The work involved: the Laboratoire Galaxies, etoiles, physique et instrumentation GEPI (Observatoire de Paris / CNRS / Universite Paris Diderot), National Astronomical Observatories, Chinese Academy of Sciences (NAOC) with the support of the Laboratoire international associe « Origins » (CNRS, Observatoire de Paris, Universite Pierre et Marie Curie, Universite Paris Diderot, NAOC), founded the 22 October 2008
at Beijing.

The first close encounter between galaxies occurred 700 million years after the beginning of the simulation. New stars are formed (green), until the fusion of the galaxies at 4.5 billion years. Several tidal tails are formed during the encounter, including one (see arrow) containing many relatively young stars. Later, part of this material returns to the galaxy,
forming the stellar Giant Stream, which has been discovered in 2001. At the end of the simulation, the spiral galaxy resembles that of Andromeda.

Note: This article has also been released as a Press Release by Observatoire de Paris on 22, November 2010.