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| The ultraviolet range that Galex can see in revealed a wealth of new stars at the galaxy's outer reaches |
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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.
"I was
not looking for the largest spiral - it just came as a gift," Mr Eufrasio
told BBC News.
Galex - a
space telescope designed to search for the ultraviolet light that newly born
stars put out - hinted that NGC 6872 was made much larger in size by the
collision.
The team
went on to use data from a range of other telescopes including the Very Large
Telescope, the Two Micron All-Sky Survey and the Spitzer space telescope - each
of which sees in a particular set of colours, in turn evidencing stars of
varying ages.
They found
the youngest stars in the outer reaches of the galaxy's enormous spiral arms,
getting progressively older toward the centre.
That
suggests a wave of star formation that travelled down the arms, set off by the
collision with IC 4970, with the newest stellar neighbourhoods pushing the
galaxy into the top spot in terms of size.
"It's
been known to be among the largest for two decades, but it's much larger than
we thought," explained Mr Eufrasio.
"The galaxy
that collided with the [central disc of NGC 6872] splashed stars all over the
place - 500,000 light years away."
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| A simulation of the galactic collision suggests it happened 130 million years ago |
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."
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