Dark matter gravitational lensing
Gravitational lensing by dark matter in the galaxy cluster CI 0024+17, Hubble Space Telescope image. The cluster galaxies are bright yellow, the far more distant lensed galaxies appear as elongated blue smears. Gravitational lensing is an effect whereby the path of light from distant objects is bent and magnified by the gravity of a sufficiently massive foreground object in the same line of sight. The amount of lensing can be used to determine the mass of the foreground object. By this method, the cluster was found to contain a vast ring of dark matter (see image R980/230), providing the best evidence yet of its existence. Image taken by the Advanced Camera for Surveys on the HST in November 2004.
© M J Jee, H Ford/Nasa/Esa/Stsci/Science Photo Library
A Cosmic Magnifying Glass
Scanning the heavens for the first time since the successful December 1999 servicing mission, NASA's Hubble Space Telescope imaged a giant, cosmic magnifying glass, a massive cluster of galaxies called Abell 2218. This 'hefty' cluster resides in the constellation Draco, some 2 billion light-years from Earth. The cluster is so massive that its enormous gravitational field deflects light rays passing through it, much as an optical lens bends light to form an image. This phenomenon, called gravitational lensing, magnifies, brightens, and distorts images from faraway objects. The cluster's magnifying powers provides a powerful "zoom lens" for viewing distant galaxies that could not normally be observed with the largest telescopes. The picture is dominated by spiral and elliptical galaxies. Resembling a string of tree lights, the biggest and brightest galaxies are members of the foreground cluster. Researchers are intrigued by a tiny red dot just left of top center. This dot may be an extremely remote object made visible by the cluster's magnifying powers. Further investigation is needed to confirm the object's identity. The color picture already reveals several arc-shaped features that are embedded in the cluster and cannot be easily seen in the black-and- white image. The colors in this picture yield clues to the ages, distances, and temperatures of stars, the stuff of galaxies. Blue pinpoints hot young stars. The yellow-white color of several of the galaxies represents the combined light of many stars. Red identifies cool stars, old stars, and the glow of stars in distant galaxies. This view is only possible by combining Hubble's unique image quality with the rare lensing effect provided by the magnifying cluster.
Stephan's quintet. Optical image of Stephan's quintet. This group of galaxies is 300 million light years from Earth in the constellation Pegasus. It consists of NGC 7320 (upper right) NGC7319 (lower right), NGC7318A and B (centre left) and NGC7317 (upper left). The galaxies are so close together they are tearing each other apart with gravitational tidal forces, leaving trails of dust and gas. The red areas are hydrogen clouds and sites of star birth. It is argued that NGC7320 is not actually part of the cluster, but lies in the foreground.
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