Nearest Milky Way companion found

By Dr David Whitehouse
BBC News Online science editor

The new galaxy is shown in red

An international team of astronomers has found a previously unknown galaxy colliding with our own Milky Way.

Called the Canis Major dwarf galaxy after the constellation in which it lies, the star grouping is about 25,000 light-years away from our Solar System.

Its distinctive red stars are slowly being pulled into the Milky Way and the dwarf will soon lose all its structure.

The discovery's full details will be published in the Monthly Notices of the Royal Astronomical Society shortly.

Hidden galaxy

The nearby galaxy is even closer than the Sagittarius dwarf galaxy, discovered in 1994, which is also colliding with the Milky Way.

The new discovery was made possible because of a recent survey of the sky in infrared light - the Two-Micron All Sky Survey or 2Mass.

The red stars gave it away

Until that survey, the dwarf galaxy lay undetected behind the Milky Way's dense disc of stars.

It was found by its M-giant stars which are cool, red stars that shine especially brightly in infrared light.

"We have used these rare M-giant stars as beacons to trace out the shape and location of the new galaxy because its numerous other stars are too faint for us to see," says Nicolas Martin of Strasbourg Observatory, France.

"They are particularly useful stars as we can measure their distances, and so map out the three-dimensional structure of distant regions of the Milky Way disc."

Cannibal galaxy

The three-dimensional view shows the gravitationally dismembered corpse of the new dwarf galaxy.

It seems that streams of stars pulled out of the cannibalised Canis Major dwarf galaxy have merged with the outer reaches of the Milky Way's disc. They may even pass close to the Sun.

Just the stars of the new galaxy

In recent years, astronomers have found that large galaxies like the Milky Way grow by consuming smaller galaxies.

But until now, they did not appreciate that even the discs of galaxies can grow in this fashion.

Computer simulations show that the Milky Way has been taking stars from the Canis Major dwarf and adding them to its own disc.

Middle-aged galaxy

"On galactic scales, the Canis Major dwarf galaxy is a lightweight of about only one billion Suns," says Dr Michele Bellazzini of Bologna Observatory, Italy.

"This small galaxy is unlikely to hold together much longer. It is being pushed and pulled by the colossal gravity of our Milky Way, which has been progressively stealing its stars and pulling it apart."

Some remnants of the Canis Major dwarf form a ring around the disc of the Milky Way.

"The Canis Major dwarf galaxy may have added up to 1% more mass to our galaxy," says Dr Geraint Lewis of the University of Sydney, Australia.

"This is also an important discovery because it highlights that the Milky Way is not in its middle age - it is still forming."