Mass and Light distributions in Clusters of Galaxies - Henry A ...

1.2 Gravitational Lensing
Figure 1.2 – Illustration of the effects of convergence and shear on a circular source.
Convergence magnifies the image isotropically, and shear deforms it to an ellipse.
The magnification is then,
µ = 1
det A = 1
(1 − κ) 2 − |γ| 2 . (1.31)
Points in the lens plane where the determinant equals zero, i.e. locations
of infinite magnification, form closed curves, which are called critical curves.
Their equivalent curves in the source plane are called caustics. Images along
these curves will be magnified and distorted substantially.
Strong lensing occurs when the gravitational potential is extremely large,
called a supercritical lens, where Σ ≥ Σ cr , and the shear and convergence are
strong enough [see][](Hattori et al. 1999), often leading to multiple images of
a suitably positioned source. The effect causes great amplification and shear,
often visible in centers of massive clusters as large distorted arcs lying near
the critical curves. These have been observed with high quality and great
detail with the Advanced Camera for Surveys (ACS) aboard the Hubble Space
Telescope (HST ). Some examples of giant arcs can be seen in the clusters
Abell 370 and Abell 2218 (see Figure 1.3).
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