An international team of scientists, led by a NASA-
funded researcher, announced today, they observed a nearby
head-on collision of two galaxy clusters. The clusters
smashed together thousands of galaxies and trillions of
stars. It is one of the most powerful events ever witnessed.
Such collisions are second only to the Big Bang in total
energy output.

The event was captured with the European Space Agency’s XMM-
Newton observatory. Scientists are calling the event the
perfect cosmic storm: galaxy clusters that collided like two
high-pressure weather fronts and created hurricane-like
conditions, tossing galaxies far from their paths and
churning shock waves of 100-million-degree gas through
intergalactic space. Data from the observations were released
today.

This unprecedented view of merger in action crystallizes the
theory the universe built its magnificent hierarchal
structure from the “bottom up,” essentially through mergers
of smaller galaxies and galaxy clusters into bigger ones.

“Here before our eyes we see the making of one of the biggest
objects in the universe,” said team leader Dr. Patrick Henry
of the University of Hawaii. “What was once two distinct but
smaller galaxy clusters 300 million years ago is now one
massive cluster in turmoil. The AOL takeover of Time-Warner
was peanuts compared to this merger,” he added.

Henry and colleagues, Drs. Alexis Finoguenov and Ulrich Briel
of the Max-Planck Institute for Extraterrestrial Physics,
Germany, present these results in an upcoming issue of the
Astrophysical Journal. The forecast for the new super-cluster
is “clear and calm,” they said, now the worst of the storm
has passed.

Galaxy clusters are the largest gravitationally bound
structures in the universe, containing hundreds to thousands
of galaxies. Our Milky Way galaxy is part of a small group
moving toward the Virgo Cluster. We are destined for a
collision in a few billion years.

The cluster in today’s announcement, Abell 754 in the
constellation Hydra, has been known for decades. However, the
new observation reveals the merger may have occurred from the
opposite direction than was previously thought. The
researchers found evidence for this by tracing the wreckage
left in the merger’s wake spanning a distance of millions of
light years. While other large mergers are known, none have
been measured in such detail.

For the first time, scientists created a complete “weather
map” of Abell 754 and can determine a forecast. The map
contains information about the temperature, pressure and
density of the new cluster. As in all clusters, most of the
ordinary matter is in the form of gas between the galaxies.
It is not locked up in the galaxies or stars themselves. The
massive forces of the merging clusters accelerated
intergalactic gas to great speeds. This resulted in shock
waves that heated the gas to very high temperatures, which
radiated X-ray light, far more energetic than the visible
light our eyes can detect.

The dynamics of the merger revealed by XMM-Newton point to a
cluster in transition. “One cluster has apparently smashed
into the other from the ‘northwest’ and has since made one
pass through,” Finoguenov said. “Now, gravity will pull the
remnants of this first cluster back towards the core of the
second. Over the next few billion years, the remnants of the
clusters will settle and the merger will be complete,” he
added.

The observation shows the largest structures in the universe
are still forming. Abell 754 is relatively close to Earth,
about 800 million light years away. The construction boom may
be over in a few more billion years, though. A mysterious
dark energy appears to be accelerating the universe’s
expansion rate. This means objects are flying apart from each
other at ever-increasing speeds, and clusters may eventually
never have the opportunity to collide with each other.

X-ray observations of galaxy clusters such as Abell 754 will
help better define dark energy and also dark matter, an
“invisible” form of matter that appears to comprise over 80
percent of a galaxy cluster’s mass. For more information
about this discovery on the Internet, visit:

http://www.gsfc.nasa.gov/topstory/2004/0831galaxymerger.html