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The Mass
Contained in Clusters
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Clusters contain contain 1014
to
1015 solar masses.
They must have masses of this size to explain how they are
held together by gravity, but only about 1/10 of this mass can be identified
with matter that is emitting visible light.
This is known
as the `missing mass problem'.
Hot X-ray Gases in Clusters
One source of this additional mass is gas between the galaxies in a cluster.
Superposed on
an optical picture of a group of galaxies in the adjacent figure is an
X-ray image taken by
ROSAT.
The image shows
confined hot gas
(which produces X-rays)
highlighted in
false red color. In order to produce X-rays, the gases must have temperatures
of 10-100 million degrees
(Ref).
The presence of this confined gas
indicates that the gravity
exerted in groups and clusters of galaxies is larger than that expected from
the
visible mass in the
galaxies, for otherwise the hot gas would have been expected to
disperse long ago. Here is a
discussion of such hot X-ray gases as observed
in the Virgo Cluster.
Dark Matter
The hot X-ray gas is estimated have a mass comparable to the visible matter
of the galaxies
in such clusters, but this is still far too little matter
(by a factor of 5 to 10) to account for the
gravitational forces that are holding the clusters together. The matter that
is seen neither in the visible spectrum nor the X-ray spectrum (but is "seen"
by its gravitational influence) is termed
dark matter. The nature of the dark
matter is one of the great current mysteries in astronomy.
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