The possibility that the path of light could be deviated, curved by
the gravity of a large object was predicted by Einstein. This effect
was observed soon after the theory of General Relativity was
published. Each of us has probably seen that a magnifying glass, or a
glass lens, "bends" light rays. By analogy, astronomers call any massive
object that "bends" light rays from a background object a
"gravitational lens". By measuring the amount of bending, we can
determine the mass of the "gravitational lens".
This week's image features Abell 370, a cluster of galaxies viewed
near the center of the image. Next to it, to the right, is a long blue
arc. This "giant" arc is the first ever detection of a gravitational
lensing event by a galaxy cluster. The first image was obtained in 1985,
fifteen years ago, by the CCD&Cosmology team from the Observatoire de Toulouse.
They observed again this cluster 3 years later (this image, a B-V-I
color-composite) using a larger and more sensitive CCD.
The lensing cluster Abell 370, located at redshift z=0.375, is the
first one for which a spectrum of a giant arc was obtained. Its
redshift was measured to be very different (0.724) from that of the
cluster and provided the definitive demonstration that the arc was a
"mirage", that it was the image of a faint source located far behind
the cluster but seen here because of gravitational lensing by the
massive cluster. Not only is its light magnified but the original
shape of the object, an ellipse like most of the blue objects on
this image, is elongated into an giant arc.
On this very deep image we also see many objects which are slightly
elongated. The amplitude of the elongation depends on the position of
the sources with respect to the center of the gravitational lens and
their orientation is perpendicular to the gradient of the gravitational
potential. The use of all these elongated objects permits to reconstruct
the gravitational potential of the lensing cluster and provides a direct
mapping of its dark matter distribution.
This image was obtained in 1988 at the prime focus of the Canada-France-Hawaii
Telescope with the FOCAM camera equipped with an RCA 640x1024 thinned
CCD. The pixel size is 0.2 arc-second. The exposure time was 5 hours
in B, 2h in V and 2h in I. The joint use of the 3 filters shows the
colors of the galaxies. The yellow-orange are early type cluster
galaxies. The faint blue objects are distant background galaxies.
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