PIO Source
Dr Christian Veillet
Executive Director
Canada-France-Hawaii Telescope
veillet@cfht.hawaii.edu
1-808-885 7944 or 1-808-938 3905

Contacts in France:

Remi Cabanac – remi.cabanac@ast.obs-mip.fr - OMP
Jean-Paul Kneib – jean-paul.kneib@oamp.fr - LAM
Jean-Francois Sygnet - sygnet@iap.fr - IAP

Group of galaxies found to bend the light of remote galaxies

The discovery of a new class of gravitational lenses, the groups of
galaxies, by an international team of astronomers using the
Canada-France-Hawaii Legacy Survey (CFHTLS), comes 20 years after the
publication in January 1987 of the first image of a gravitational arc,
made also at CFHT with one of the first CCD cameras in operation at an
observatory. This discovery of gravitational arcs in the center of galaxy
groups is an important step in our understanding of the large scale
structures of the universe. These new results will allow a better
understanding of the distribution of the dark matter and the formation
mechanisms of the groups of galaxies, structures intermediate in mass
between galaxies and clusters of galaxies.

Twenty years ago at CFHT, French astronomers observed for the first time
galaxies distorted in giant arcs at the center of the most massive galaxy
clusters. These observations brought to light one of the most spectacular
effects of what is called "gravitational lensing". According to Einstein’s
theory of General Relativity, spacetime is curved by the presence of
matter. Therefore, the light passing close to an important concentration
of mass will be bent. When an observer, a galaxy cluster and a remote
galaxy are in nearly perfect alignment, the remote galaxy appears to the
observer as one or more luminous arcs resulting from the fusion of images
of the remote galaxy distorted and amplified by the galaxy cluster acting
as a complex gravitational lens. The shape, brightness and distribution of
these gravitational arcs bring invaluable information on the mass
distribution of the lensing cluster.

Up to recently, only the most massive galaxy clusters and the massive
galaxies were the object of gravitational lensing studies.

Intermediate-scale structures like the galaxy groups should however be
looked in order to better understand the evolution of the structures in
the Universe.

Since the arrival of the MegaCam camera in 2003 on Megaprime, the new CFHT
prime focus, astronomers have been able to observe at once a large area of
the sky (1 square degree or 4 Full Moon) in 340 MegaPixel digital images
with an unprecedented resolution for such a field of view. The Canadian
and French communities decided to pull their resources together and to
devote 500 nights of telescope time over five years to a large project,
the CFHT Legacy Survey, which will cover around 1% of the sky visible from
Hawaii.

Thanks to a careful inspection aimed at detecting gravitational arcs in
one fourth of the CFHTLS, the team has been able to detect for the first
time numerous arcs around galaxy groups. This unexpected discovery
provides for the first time direct information on the structure of galaxy
groups which are key environments in the formation of structures in the
Universe. Scientists will be able to understand the role of dark matter in
the evolution of these groups and of the mass concentrations that make the
large structures of the Universe.

Images of some of the newly discovered arc systems, as well as historical
pictures of the first observation of a gravitational arc, are available here:

www.cfht.hawaii.edu/News/StrongLensing/