Galaxy crash sparks large spiral

Astronomers have spotted the largest known spiral galaxy - by accident.

A team was looking through data from the Galaxy Evolution Explorer (Galex) satellite for star-forming regions around a galaxy called NGC 6872.

But they were shocked to see a vast swathe of ultraviolet light from young stars, indicating that the galaxy is actually big enough to accommodate five of our Milky Way galaxies within it.

The find was reported at the American Astronomical Society meeting in the US.

NGC 6872, a galaxy about 212 million light-years away in the constellation Pavo, was already known to be among the largest spiral galaxies.

Near it sits a lens-shaped or lenticular galaxy called IC 4970, which appears to have crashed through the spiral in recent astronomical times.

Rafael Eufrasio of the Catholic University of America and Nasa's Goddard Space Flight Center and colleagues from the University of Sao Paulo in Brazil and the European Southern Observatory in Chile were interested in a number of regions away from the galaxy.

Besides being one for the record books, NGC 6872 updates the catalogue of known galaxy smash-ups, demonstrating how dramatically galaxies can be changed and added to by collisions.

"It shows the evolution of galaxies in the larger context of the Universe - how the large galaxies we had before were accreted from small clumps in the early Universe," Mr Eufrasio said.

"We're just seeing one example of two interacting galaxies but in the past that happened much more often - that's how the big spiral galaxy discs we have were probably formed. Putting that in a larger context, it's a very cool system."