and Cosmology

6. Clusters and Groups of Galaxies
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Fig. 6.36. Left: the tidal (or shear) field of the cluster
Cl 0024+17 is indicated by sticks whose length and direction
represent the strength and orientation of the tidal gravitational
field. Right: the surface mass density is shown, reconstructed
by means of the weak gravitational lens effect. The bright
galaxies in the cluster are seen to follow the (dark) matter distribution;
the orientation of the isodensity contours is the same
as the orientation of the light in the center of the cluster
Mass Reconstruction of Galaxy Clusters. By means
of this method, the reconstruction of the mass density of
a large number of clusters became possible. The most
important results of these investigations are as follows:
the center of the mass distribution corresponds to the
optical center of the cluster (see Fig. 6.36). If X-ray
information is available, the mass distribution is, in general,
found to be centered on the X-ray maximum. The
shape of the mass distribution – e.g., its ellipticity and
orientation – is in most cases very similar to the distribution
of bright cluster galaxies. The comparison of
the mass profile determined by this method and that determined
from X-ray data agree well, typically within
a factor of ∼ 1.5 (see Fig. 6.37 for an example). Through
the weak lensing effect, substructure in the mass distribution
is also detected in some clusters (Fig. 6.38)
which does not in all cases reflect the distribution of
cluster galaxies. However, in general a good correspondence
between light and mass exists (Fig. 6.39). From
these lensing studies, we obtain a mass-to-light ratio for
clusters that agrees with that found from X-ray analyses,
about M/L ∼ 250h in Solar units. Clusters of
galaxies that strongly deviate from this average value
do exist, however. Two independent analyses for the
cluster MS 1224+20 resulted in a mass-to-light ratio of
Fig. 6.37. Radial mass profile of the galaxy cluster Abell 2218.
The data points with error bars are mass estimates from the
weak lensing effect, the solid and dashed curves are isothermal
sphere models assuming different velocity dispersions. The
cross denotes the mass estimated from luminous arcs, and the
triangle depicts the mass obtained from the central cD galaxy