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

1.2 Gravitational Lensing
increase at higher frequencies. The shape of the distorted spectrum of the
thermal component of the effect depends on the Compton y-parameter,
∫
y =
n e
k B T e
m e c 2 σ T dl, (1.17)
which is essentially a line of sight integral of the gas pressure.
The important features of the SZ effect are that its signal is independent
of redshift, unlike optical or X-ray surface brightnesses, and that it directly
probes the gas pressure, or thermal energy density, along the line of sight.
These properties make the SZ effect a unique and powerful observational tool
for detecting distant clusters.
SZ and X-ray observations are complementary to each other, specifically
in the outskirts of clusters where the X-ray surface-brightness is difficult to
observe, but the SZ signal remains substantial. The SZ distortion is linear
in the gas density, whereas X-ray emission is proportional to n 2 e. This means
X-ray measures the gas in the center of the cluster with better resolution, but
that the SZ effect is more suitable for tracing the gas across a much larger
region of the cluster.
1.2 Gravitational Lensing
1.2.1 Historical Overview
The deflection of light rays by gravity was predicted by Einstein’s General
Relativity (GR). Einstein’s calculation of the magnitude of deflection by the
gravitatioal field of the sun was confirmed in 1919 when angular shifts of
stars close to the Sun’s edge were measured during a total solar eclipse.
This served as a confirmation of GR, and light deflection and other related
phenomena are called “Gravitational Lensing”.
Observing multiple images of sources being lensed is another result of
the theory. Although Einstein had thought that multiple images due to
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