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Missing piece of cosmological puzzle found
Astronomers detect a part of long-searched baryonic matter in a filament
connecting two clusters of galaxies
The composition of the Universe still puzzles the astronomers: Over 90
percent consist of unknown matter - three quarters of the mysterious
Dark Energy which causes an accelerated expansion of the Universe and
about 21 percent of Dark Matter, the components of which the physicists
have not yet figured out. Just four percent are composed of the normal
material of which we ourselves are made, the so-called baryonic matter.
Even this minor part however has not yet been comprehended completely:
all discovered stars, galaxies and gases in the Universe amount to less
than a half of these four percent. Now a team of astrophysicists from the
Max Planck Institute for extraterrestrial Physics (MPE) and the ESO,
both in Garching, Germany, and of two institutes in the Netherlands has
found evidence of a part of the missing baryons in a bridge-like filament
connecting two clusters of galaxies (Astronomy & Astrophysics Letters,
Image credits: ESA/XMM-Newton/ EPIC/ ESO (J. Dietrich)/ SRON (N. Werner)/
MPE (A. Finoguenov)
A bridge of hot gas is connecting two clusters of galaxies. Composite
optical and X-ray image of the cluster pair Abell 222 and Abell 223.
The Universe is constructed like an oversized spider web: All visible
material is arranged along the filamentary structure of the Dark Matter.
On its threads and knots this web holds gigantic chunks of baryonic
matter which are made of quarks and leptons. Studies of the Big Bang and
the fluctuations of cosmic background radiation yield quite exact figures
on the existence of baryons in the Universe. So from the calculations
astronomers knew for a long time that the vanished pieces of the
cosmological puzzle must be hidden somewhere. To trace and apprehend
this solely observable component in the Universe is the precondition
to learn more about the web of Dark Material and to test the quality
of different cosmological models.
The missing part of the baryonic matter is imagined since nine years as
hot, ultra-thin gas haze of very low density between larger structures.
Due to its high temperature this gas is expected to emit primarily in the
far-ultraviolet and X-ray band. Scientists around Norbert Werner from
SRON Netherlands Institute for Space Research therefore used the X-ray
space observatory XMM-Newton to observe the two clusters of galaxies
Abell 222 and Abell 223 which are connected by a filament. This structure
was chosen because of its fortunate geometry: The astronomers could look
directly into the bridge instead of looking at it from the side.
The hot gas they found there is probably the hottest and densest part of
the diffuse gas which constitutes half of the missing baryons in the
Universe. The properties of the gas, for example its density and
temperature, are consistent with the results of simulations. "Having
discovered the hottest of the missing baryons is of particular importance
as various models, while all predicting the lost matter in some kind of
warm gas, tend to disagree about the extremes", explains Alexis
Finoguenov, Max Planck Institute for extraterrestrial Physics.
This is the first time that scientists see the bridge of gas connecting
two clusters of galaxies in X-rays. "So far we could only see the
clusters, the dense knots of the web. Now we are starting to see the
connecting wires of the immense cosmic spider web", says MPE
astrophysicist Aurora Simionescu, co-author of the publication.
The discovery of the gas is a significant step forward on the way to
a complete understanding of the cosmic evolution. The distribution
and composition of the baryonic matter gives information about what
happened after the Big Bang and which forces are dominating the
Universe today and in the future. The astronomers now want to trace
comparable galactic systems in further space missions. To this end it
will be necessary in the long term to launch a dedicated space observatory
to study the cosmos with a much higher sensitivity than possible with
the current satellites.
Astron. & Astrophys., 482, L29-L33, 2008
Dr. Mona Clerico
Max Planck Institute for Astrophysics
and Max Planck Institute for extraterrestrial Physics
Phone +49 89 30000-3980
Dr. Alexis Finoguenov
Max-Planck-Institut für extraterrestrische Physik
Phone: +49 89 30000-3644
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