Astronomers have glimpsed the largest cluster of galaxies
ever seen in the distant, early universe.
The discovery of this far-off group, estimated to contain as
much mass as a thousand large galaxies, offers further proof of the existence
of the enigmatic force called dark
energy.
"This is the most luminous, and therefore probably the
most massive, cluster
of galaxies discovered at this epoch," said Georg Lamer of the
Astrophysikalisches Institut Potsdam in Germany, who led the team that
discovered it. "The light we observe started about 7.7 billion years ago.
This is about half of the age of the universe, so it is from quite long ago, and quite far away."
When astronomers look at distant objects, they are looking
back in time, in this case seeing objects that are 7.7 billion light-years
away.
Lamer and his team discovered the cluster, known by its
catalogue number, 2XMM J083026+524133, by chance while they were surveying a
portion of sky for a catalogue of X-ray sources. Using the European Space
Agency's orbiting X-ray observatory XMM-Newton, they spotted an extremely
bright object without any galaxy visible in optical light nearby.
After noticing the aberrant object, they took a deeper
exposure with the Large Binocular Telescope in Arizona and determined that the
source of the light was a far-off group of galaxies containing the mass of
about 1,000 Milky Ways.
The astronomers say the discovery offers further proof of
the mysterious force called dark energy that scientists think propels the
acceleration of the universe's expansion. Dark energy is believed to account
for about 70 percent of the universe, with the remaining portion made up of
normal matter and its enigmatic sister, dark matter.
"The existence of the cluster can only be explained
with dark energy," Lamer said.
Since dark energy is contributing to the stretching
of the universe, and speeding up the process of galaxies receding from each
other, it hampers the growth of massive galaxy clusters in more recent times.
To test dark energy, scientists compare frequency of these
massive clusters today with earlier times. If there were no dark energy, they
would expect clusters to grow relatively quickly, so the largest clusters we
see now would be very small at half the age of the universe, and there would be
no gigantic clusters.
"The fact that we do find these clusters is a clear
confirmation of dark energy," Lamer told SPACE.com. "This was
a very long time ago and it's actually about as massive as the most massive
ones we observe today. Without dark energy we would observe much more massive
clusters and many more of these massive clusters than we actually do."
The discovery will be detailed in a forthcoming issue of the
journal Astronomy & Astrophysics.
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