and Cosmology

6.5 Clusters of Galaxies as Gravitational Lenses
Fig. 6.33. Top image: the cluster of galaxies
A 2218 (z d = 0.175) contains one of the
most spectacular arc systems. The majority
of the galaxies visible in the image are associated
with the cluster and the redshifts
of many of the strongly distorted arcs have
now been measured. Bottom image: the
cluster of galaxies Cl 0024+17 (z = 0.39)
contains a rich system of arcs. The arcs appear
bluish, stretched in a direction which
is tangential to the cluster center. The three
arcs to the left of the cluster center, and the
arc to the right of it and closer to the center,
are images of the same background galaxy
which has a redshift of z = 1.62. Another
image of the same source was found close
to the cluster center. Also note the identical
(“pretzel”-shaped) morphology of the
images
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significantly from spherical symmetry. Overestimating
the core radius was the main reason why the discovery
of the arcs was a surprise because clusters with core
radii like the ones determined from the early X-ray measurements
would in fact not act as strong gravitational
lenses. Hence, the mere existence of arcs shows that the
core radius must be small.
A closer analysis of galaxy clusters with cooling
flows shows that, in these clusters, the mass profile estimated
from X-ray observations is compatible with the
observed arcs. Such clusters are considered dynamically
relaxed, so that for them the assumption of a hydrostatic
equilibrium is well justified. The X-ray analysis
has to account explicitly for the existence of a cooling
flow, though, and the accordingly modified X-ray
emission profile is more sophisticated than the simple
β-model. Clusters without cooling flows are distinctly
more complex dynamically. Besides the discrepancy in
mass determination, lensing and X-ray methods can lead
to different estimates of the center of mass in such unrelaxed
clusters, which may indicate that the gas has
not had enough time since the last strong interaction or
merging process to settle into an equilibrium state.
The mass distribution in clusters often shows significant
substructure. Clusters of galaxies in which arcs
are observed are often not relaxed. These clusters still
undergo dynamical evolution – they are young systems
with an age not much larger than t cross , or systems whose