Cosmic Collision Creates Largest Spiral Galaxy, By Accident

The ultraviolet range that Galex can see in revealed a wealth of new stars at the galaxy's outer reaches

Astronomers have discovered the largest known spiral galaxy, by accident.

Scientists were routinely looking through cosmological data from the Galaxy Evolution Explorer (Galex) satellite for star-forming regions around a galaxy called NGC 6872.

But what they discovered instead was a huge tract of ultraviolet light that could only have come from young stars. This discovery means that the galaxy is vast enough to fit five of our Milky Way galaxies inside it.

The findings were reported at the American Astronomical Society meeting in the US.

Previously, NGC 6872 — a galaxy about 212 million light-years away in the constellation Pavo — was thought to be among the largest spiral galaxies. It is sited near a lens-shaped galaxy called IC 4970 which astronomers believe crashed through the spiral.

Rafael Eufrasio of the Catholic University of America and Nasa’s Goddard Space Flight Center, and teams from the University of Sao Paulo in Brazil and the European Southern Observatory in Chile are credited with the discovery.

“I was not looking for the largest spiral, it just came as a gift,” Eufrasio told BBC News.

Using Galex — a space telescope specifically designed to search for the ultraviolet light that new born stars radiate — scientists began to consider the possibility that NGC 6872 was made larger by the collision itself.

A combination of powerful telescopes — the Very Large Telescope, the Two Micron All-Sky Survey and the Spitzer space telescope — later revealed more stars of varying ages.

The youngest stars were found in the in the outer branches of galaxy’s enormous spiral arms, with the age of the stars getting progressively older toward the center.

From that scientists deduced that the star formation traveled down the arms of the spiral, and was in fact triggered by a collision with IC 4970.

“It’s been known to be among the largest for two decades, but it’s much larger than we thought. The galaxy that collided with the [central disc of NGC 6872] splashed stars all over the place – 500,000 light years away,” Eufrasio explains.

NGC 6872 now enters the record books of known galaxy crashes, and clearly shows how galaxies can be changed and added to by collisions, e!Science News notes.

“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,” says Eufrasio.

He added:

“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.”