Interacting Galaxies early in the universe -- Credit: Kirk Borne (Raytheon and NASA Goddard Space Flight Center, Greenbelt, Md.), Luis Colina (Instituto de Fisica de Cantabria, Spain), and Howard Bushouse and Ray Lucas (Space Telescope Science Institute, Baltimore, Md.) and NASA.

The large spiral galaxy called Andromeda is devouring a couple of small neighbouring dwarf galaxies, astronomers report in the July 5 issue of the journal "Nature". The finding was made by Dr Rodrigo Ibata (Observatoire de Strasbourg, France), Dr Michael Irwin (Institute of Astronomy, Cambridge, UK), Dr Geraint Lewis (Anglo-Australian Observatory, Australia), Dr Annette Ferguson (Kapteyn Institute, The Netherlands), and Dr Nial Tanvir (University of Hertfordshire, UK) who used the 2.5-m Isaac Newton Telescope on La Palma in the Canary Islands to make the first sensitive panoramic survey of Andromeda.

The evidence of galactic dismemberment is a stream of stars on the outskirts of Andromeda that appears to have been stripped from the dwarf galaxies M32 and NGC 205 by their larger companion.

Simulated collision of our galaxy with Andromeda. The sequence begins shortly before the collision to the time when the galaxies merge. Credit: John Dubinski

The finding supports the idea that big galaxies have been built up over time by smaller galaxies colliding and that this process is continuing today.

Andromeda is our Galaxy's "big sister", twice as large but otherwise very similar. It is the nearest large galaxy, lying only 2.2 million light-years away.

Astronomers have known for some years that our own Galaxy is a cannibal. Its outer parts are threaded through with tell-tale streams of stars from small galaxies it has engulfed.

Like a rat swallowed by a snake, stars of a swallowed galaxy stay as a recognisable lump for billions of years before eventually mingling with those of their conqueror.

"This has given the outskirts of our Galaxy, its 'halo', a rather lumpy structure," said team leader Dr Rodrigo Ibata. "We wanted to see if Andromeda's halo was the same."

The new survey was possible only because the digital devices that have replaced photography in astronomy have now been developed enough to cover fairly large areas of sky. Even so, more than fifty long exposures had to be pieced together to give a panorama of the halo on only one side of Andromeda.

The ripped-off stars can be seen as a distinct stream. They can also be distinguished from other stars in Andromeda's halo by their slightly different chemical composition.

"Andromeda is close enough for us to be able to see individual stars stripped from the satellite galaxies and measure their speeds. From this we will be able to map the distribution of 'dark matter' in the halo surrounding Andromeda," said team member Dr Geraint Lewis. "No-one has been able to do this definitively before."

In the ultimate corporate merger, our Galaxy will collide with Andromeda three billion years from now. Astronomers have known for almost a century that the two galaxies are falling together at 500 000 kilometres an hour.

"Colliding with a dwarf galaxy is only like having a cream pie hit your windscreen," Dr Lewis said. "When our Galaxy and Andromeda collide it'll look like a car crash -- very messy."