CFHT, Current Image of the Week


February 21th, 2000

Gravitational Lensing: giant arc caused by the galaxy cluster Abell 370

Each week, discover a new spectacular image obtained at CFHT. Browse the archive

Gravitational Lensing: giant arc caused by the galaxy cluster Abell 370

Credit: Image courtesy of B. Fort and Y. Mellier (IAP).

(for more details see: Soucail et al., 1987, Astronomy & Astrophysics, Vol. 184, p. L7.)

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.

Technical description:

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.

next week: A Black Hole at the Center of Our Galaxy

editors: François Ménard & Jean-Charles Cuillandre
[] & []

Copyright © 2000 by CFHT. All rights Reserved.

CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.