Milky Way's nearest neighbour revealed

The nearest galaxy to our own Milky Way has been revealed. It is so close that the Milky Way is gradually consuming it by pulling in its stars. But it will be few billion years before it is entirely swallowed up.

The previously unknown galaxy lies about 25,000 light years from Earth and 42,000 light years from the centre of the Milky Way, beyond the stars in the constellation Canis Major. It is twice as close to the centre of our galaxy than the previous record holder, the Sagittarius Dwarf Galaxy, which was discovered in 1994.

Geraint Lewis, at the University of Sydney, Australia, was one of the international team that uncovered the galaxy. "It is an important discovery because it highlights that the Milky Way is not in its middle age - it is still forming."

The Canis Major dwarf galaxy had previously escaped detection because it is hidden behind dense clouds of dust. However, an international team of astronomers found it using infrared data from the recent 2 Micron All Sky Survey. (Graphics and animations of the Canis Major dwarf galaxy can be viewed here here.)

Raining down

Infrared's ability to "see" straight through the dusty regions of the Milky Way proved invaluable. The team were looking for a rare type of old red star but found many more than they had expected.

Another team member, Mike Irwin at the University of Cambridge, UK, told New Scientist: "We found a large excess of these stars just below the plane of the Milky Way, spread into an elongated shape covering about 10 times the apparent size of the full Moon."

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Coming from this main body, the astronomers also found trails of stars. Some of these are falling into the Milky Way and may even be passing through our own celestial neighbourhood. "In computer models we have run, some stars do seem to rain down close to the Solar System," says Irwin.

Which stars these are, if any, may remain unknown until next decade when the ESA spacecraft Gaia should launch. Gaia will measure stellar orbits to suggest which stars in the night sky have been cannibalised from this and other dwarf galaxies.

As for the discovery of more dwarf galaxies, Irwin is cautious: "Instrumentation is now so good that there are not too many hiding places left for dwarf galaxies. There may be a few more yet to be discovered, but not many."

A paper on the discovery of the new galaxy will be published in the Monthly Notices of the Royal Astronomical Society.

Stuart Clark