and Cosmology

6. Clusters and Groups of Galaxies
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Fig. 6.34. The cluster of galaxies Abell 1689
has the richest system of arcs and multiple
images found to date. In a deep ACS exposure
of this cluster more than a hundred
such lensed images were detected. Six sections
of this ACS image are shown in which
various arcs are visible, some with an extreme
length-to-width ratio, indicating very
high magnification factors
equilibrium was disturbed by a fairly recent merger process.
For such clusters, the X-ray method is not well
founded because the assumptions about symmetry and
equilibrium are not satisfied. The distribution of arcs in
the cluster A2218 (Fig. 6.33) clearly indicates a nonspherical
mass distribution. Indeed, this cluster seems
to consist of at least two massive components around
which the arcs are curved, indicating that the cluster
is currently undergoing a strong merging event. This
is further supported by measurements of the temperature
distribution of the intracluster gas, which shows
a strong peak in the center, where the temperature is
about a factor of 2 higher than in its surrounding region.
From lens models, we find that for clusters with a central
cD galaxy, the orientation of the mass distribution
follows that of the cD galaxy quite closely. We conclude
from this result that the evolution of the cD galaxy must
be closely linked to the evolution of the cluster, e.g., by
accretion of a cooling flow onto the cD galaxy. Often,
the shape of the mass distribution very well resembles
the galaxy distribution and the X-ray emission.
The investigation of galaxy clusters with the gravitational
lens method provides a third, completely
independent method of determining cluster masses.
It confirms that the mass of galaxy clusters significantly
exceeds that of the visible matter in stars and
in the intracluster gas. We conclude from this result
that clusters of galaxies are dominated by dark
matter.
6.5.2 The Weak Gravitational Lens Effect
The Principle of the Weak Lensing Effect. In Sect. 3.8
we saw that gravitational light deflection does not only
deflect light beams as a whole, but also that the size
and shape of light beams are distorted by differential
light deflection. This differential light deflection leads,
e.g., to sources appearing brighter than they would be
without the lens effect. The giant arcs discussed above
are a very good example of these distortions and the
corresponding magnifications.
If some background sources exist which are distorted
in such an extreme way as to become visible as giant
luminous arcs, then it appears plausible that many
more background galaxies should exist which are less
strongly distorted. Typically, these are located at larger
angular separations from the cluster center, where the
lens effect is weaker than at the location of the luminous
arcs. Their distortion then is so weak that it cannot be
identified in an individual galaxy image. The reason for
this is that the intrinsic light distribution of galaxies is
not circular; rather, the observed image shape is a superposition
of the intrinsic shape and the gravitational lens
distortion. The intrinsic ellipticity of galaxies is considerably
larger than the shear, in general, and acts as
a kind of noise in the measurement of the lensing effect.
However, the distortion of adjacent galaxy images
should be similar since the gravitational field their light
beams are traversing is similar. By averaging over many