CFHT, Current Image of the Week


February 14th, 2000

Observational Cosmology: A Cluster of Galaxies at High Redshift

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Observational Cosmology: A Cluster of Galaxies at High Redshift

Credit: Image courtesy of M. Gladders and H. Yee (University of Toronto)

The exact cosmological parameters describing the shape and evolution of the Universe we live in are not well known yet. A detailed understanding of these parameters requires the study of objects at large distances. Unfortunately, individual galaxies are difficult to study in detail at large distances or equivalently at high redshift.

In our Galaxy, clusters of stars are known, for example NGC 2244 (CFHT image of the week of January 31st). On a much larger scale, clusters of galaxies also exist. They are the most massive gravitationally bound entities known in the Universe. Although they are rare, they are fairly luminous and have turned out to be extremely useful to study our Universe in great detail. Analyzing the number and distribution (in richness, or equivalently mass) as a function of distance of these clusters sets precise constraints on several cosmological parameters.

Large areas of the sky have been covered in the past in the course of, for example, the CNOC2 survey (Canadian Network for Observational Cosmology) at CFHT. Yet even this survey is only 1/66th the size of a new survey now underway at CFH. This new effort, named the Toronto Red-Sequence Cluster Survey, is designed specifically to find galaxy clusters, and hence measure the cosmology. In this week's image the candidate cluster "CL 1620+2929" from this survey is featured. It is likely at a redshift of z=0.9. This survey is being completed using the new CFH12K camera; without such a powerful instrument, a survey of this size is simply not feasible.

Technical description: This image has been constructed from deep V, R and z' data acquired in July 1999 at the Canada-France-Hawaii Telescope with the CFH12K camera. These images are deep, with magnitude limits roughly 24, 25.5 and 26 in z', R and V respectively. Objects that are "blue"("red") (i.e., brighter in the V-filter (z' filter)) appear in blue (red) on the image. The seeing on the smoothed combined images is about 0".9. The image is centered on the cluster core, at full pixel sampling, with some smoothing to bring all 3 filter PSFs into rough alignment. The image is 800x800 pixels (a tiny fraction of CFH12K's field of view) - at 0".206 / pixel this translates to 165x165 arcsec, or 0.85x0.85 h-1 Mpc.

More on this survey program at the University of Toronto.

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

editors: François Ménard & Jean-Charles Cuillandre
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.