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Using gravitational lensing to weigh 70 galaxies
Posted: July 22, 2008

An international team of astronomers, including Dr. Adam S. Bolton of the University of Hawaii's Institute for Astronomy, has recently announced a finding that helps to settle a long-standing debate over the relationship between mass (the amount of matter) and luminosity (brightness) in galaxies.

The larger panel shows the Hubble Space Telescope image of one of the SLACS gravitational lenses, with the lensed background galaxy enhanced in blue. The smaller right-hand panels show the components of a model of this image: top, a model for how the more distant background galaxy would appear in the absence of the lensing effect; center, a smooth model for the brightness of the more nearby massive galaxy; bottom, the appearance of the background galaxy when its image is distorted by the gravity of the nearer galaxy. Credit: A. Bolton (UH/IfA) for SLACS and NASA/ESA
The team achieved this result by compiling the largest-ever single collection of "gravitational lens" galaxies-70 in all. A gravitational lens is a phenomenon similar to a terrestrial mirage, but it occurs on a scale of many thousands of light-years. When two galaxies happen to be precisely aligned with one another in the sky, the gravitational field of the nearer galaxy distorts the image of the more distant galaxy into multiple arc-shaped images or even into a complete ring, known as an "Einstein ring." These Einstein ring images can be up to 30 times brighter than the image of the distant galaxy would be in the absence of the lensing effect.

The discovery represents the culmination of the Sloan Lens ACS (or SLACS) Survey. The gravitational lenses were originally identified using data from the Sloan Digital Sky Survey, a major project that has used a dedicated 2.5-meter telescope in New Mexico to measure precise distances to nearly one million distant galaxies and quasars throughout one quarter of the sky. To observe and measure the details of the Einstein ring images, the SLACS astronomers then took advantage of the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope, which delivers pictures of unparalleled sharpness.

"The SLACS collection of lenses is especially powerful for science," said Bolton, lead author of two papers describing these latest results, which will be published in the Astrophysical Journal in August and September. "For each lens, we measured the apparent sizes of the Einstein rings on the sky using the Hubble images, and we measured the distances to the two galaxies of the aligned pair using Sloan data. By combining these measurements, we were able to deduce the mass of the nearer galaxy."

In other lens surveys of this scale, distances to the lens and background galaxies-and hence the lens galaxy masses-have not been measured precisely.

By considering these galaxy masses along with measurements of their sizes, brightnesses, and stellar velocities, the SLACS astronomers were able to infer the presence of "dark matter" in addition to the visible stars within the galaxies. Dark matter is the mysterious, unseeable material that is the majority of matter in the universe. And with such a large number of lens galaxies across a range of masses, they found that the fraction of dark matter relative to stars increases systematically when going from galaxies of average mass to galaxies of high mass.

The existence of gravitational lenses was first predicted by Albert Einstein in the 1930s, but the first example was not discovered until the late 1970s. In the 30 years since then, many more lenses have been discovered, but their scientific potential has been limited by the disparate assortment of known examples. The SLACS Survey has significantly changed this situation by discovering a single large and uniformly selected sample of strong lens galaxies. The SLACS collection promises to form the basis of many further scientific studies.

Other members of the SLACS collaboration are Dr. L.V.E. Koopmans (Kapteyn Instituut, Groningen, The Netherlands), Prof. Tommaso Treu (University of California, Santa Barbara), Dr. Raphael Gavazzi (Institut d'astrophysique de Paris), Dr. Leonidas A. Moustakas (JPL/Caltech), and Dr. Scott Burles (MIT).

Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.

Established in 1907 and fully accredited by the Western Association of Schools and Colleges, the University of Hawaii is the state's sole public system of higher education. The UH System provides an array of undergraduate, graduate, and professional degrees and community programs on 10 campuses and through educational, training, and research centers across the state. UH enrolls more than 50,000 students from Hawaii, the U.S. mainland, and around the world.

Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England.

This work was based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc. Support was provided by NASA through grants from STScI.