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[cdn-nucl-l] Massive merger of galaxies
In these days of awe, do you ever wonder who the first nuclear engineer was?
(Some people believe nuclear engineering is the world's oldest profession.)
European Space Agency: Massive merger of galaxies is the most powerful on
23 September 2004
With ESA's XMM-Newton observatory, an international team of scientists has
observed a nearby head-on collision of two galaxy clusters that has smashed
together thousands of galaxies and millions upon millions 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 details what the scientists are calling 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.
This unprecedented view of a merger in action crystallises the theory that
the Universe built its magnificent hierarchal structure from the 'bottom up
- essentially through mergers of smaller galaxies and galaxy clusters into
"Here before our eyes we see the making of one of the biggest objects in the
Universe," said Dr Patrick Henry of the University of Hawaii, who led the
study. "What was once two distinct but smaller galaxy clusters 300 million
years ago is now one massive cluster in turmoil."
Henry and his colleagues, Alexis Finoguenov and Ulrich Briel of the
Max-Planck Institute for Extraterrestrial Physics in Germany, present these
results in an upcoming issue of the Astrophysical Journal. The forecast for
the new super-cluster, they said, is 'clear and calm' now that the worst of
the storm has passed.
Galaxy clusters are the largest gravitationally bound structures in
Universe, containing hundreds to thousands of galaxies. Our Milky Way galaxy
is part of a small group of galaxies but is not gravitationally bound to the
closest cluster, the Virgo Cluster. We are destined for a collision in a few
thousand million years, though.
The cluster named Abell 754 in the constellation Hydra has been known for
decades. However, to the scientists' surprise, the new observation reveals
that the merger may have occurred from the opposite direction than what was
thought. They found evidence for this by tracing the wreckage today left in
the merger's wake, spanning a distance of millions of light years. While
other large mergers are known, none has been measured in such detail as
For the first time, the scientists could create a complete 'weather map' of
Abell 754 and thus determine a forecast. This map contains information about
the temperature, pressure and density of the new cluster. As in all
clusters, most the ordinary matter is in the form of gas between the
galaxies and 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 heat the gas to very high
temperatures, which then radiated X-ray light, far more energetic than the
visible light our eyes can detect. XMM-Newton, in orbit, detects this type
of high-energy light.
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
'north-west' and has since made one pass through," said Finoguenov. "Now,
gravity will pull the remnants of this first cluster back towards the core
of the second. Over the next few thousand million of years, the remnants of
the clusters will settle and the merger will be complete."
The observation implies that the largest structures in the Universe are
essentially still forming in the modern era. Abell 754 is relatively close,
about 800 million light years away. The construction boom may soon be over
in a few more thousand million years though. A mysterious substance dubbed
'dark energy' appears to be accelerating the Universe's expansion rate. This
means that objects are flying apart from each other at an ever-increasing
speed and that clusters may eventually never have the opportunity to collide
with each other.
X-ray observations of galaxy clusters such as Abell 754 will help to better
define dark energy and also dark matter, an 'invisible' and mysterious
substance that appears to comprise over 80 percent of a galaxy cluster's
This observation was announced at a NASA Internet press conference today. A
paper describing these results, by Patrick Henry and his collaborators, will
be published in the Astrophysical Journal.
Images and other visual material are available at the NASA galaxy merger