Group of galaxies found to bend the light of remote galaxies (Forwarded)

Canada-France-Hawaii Telescope
Kamuela, Hawaii

Contact at CFHT:
Christian Veillet
1 (808) 885 2143 or 1 (808) 938 3905

Contacts in France:
Remi Cabanac, Observatoire Midi-Pyrenees
remi.cabanac @ ast.obs-mip.fr

Jean-Paul Kneib, Laboratoire d'Astronomie de Marseille
jean-paul.kneib @ oamp.fr

Jean-Francois Sygnet, Institut d'Astrophysique de Paris
sygnet @ iap.fr

December 22, 2006

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
he
http://www.cfht.hawaii.edu/News/StrongLensing/PRIm.html

Related links,
http://www.cfht.hawaii.edu/News/Stro...g/PRLinks.html