Gravity's lens: Finding a dim cluster ...Relying solely on a gravitational mirage rather than visible images, astronomers have discovered a previously unknown cluster of galaxies cluster of galaxies Gravitationally bound grouping of galaxies, numbering from the hundreds to the tens of thousands. Large clusters of galaxies often exhibit extensive X-ray emission from intergalactic gas heated to tens of millions of degrees. and measured its distance from Earth. This first-time feat, which uses the properties of mass to infer the location of unseen objects, bodes well for searches of dark matter--the mysterious, invisible material that presumably pre·sum·a·ble adj. That can be presumed or taken for granted; reasonable as a supposition: presumable causes of the disaster. makes up more than 95 percent of the mass of the universe. All matter, whether it's visible or not, exerts a gravitational tug. The tug of a massive object not only attracts other bodies but also bends the path of light from more distant objects that lie directly behind it. The bending can elongate e·lon·gate tr. & intr.v. e·lon·gat·ed, e·lon·gat·ing, e·lon·gates To make or grow longer. adj. or elongated 1. Made longer; extended. 2. Having more length than width; slender. the images of distant galaxies, as seen from Earth, or create multiple images of the same object. This phenomenon, known as gravitational lensing, is a well-documented prediction of Albert Einstein's theory of general relativity. To search for lenses, J. Anthony Tyson of Lucent Technologies' Bell Laboratories in Murray Hill, N.J., and his colleagues scanned a relatively barren patch of sky using a 4-meter telescope at the Cerro Tololo Inter-American Observatory Cerro Tololo Inter-American Observatory (sā`rō tōlō`lō), astronomical observatory located on Cerro Tololo peak, Chile, with offices in La Serena, about 40 mi (64 km) to the west. Funded by the U.S. in La Serena, Chile La Serena ("the serene one") is the second oldest city in Chile. The city, located 471 km north of Santiago, has a population of 147,815, according to the 2002 census. There are also 12,333 inhabitants of the immediately surrounding countryside. . With the aid of a computer program, they then identified elongated e·lon·gate tr. & intr.v. e·lon·gat·ed, e·lon·gat·ing, e·lon·gates To make or grow longer. adj. or elongated 1. Made longer; extended. 2. Having more length than width; slender. images of background galaxies--a sign that the light had passed through an intervening galaxy cluster that had not been previously seen. The program also generated an estimate of the mass and distance of the cluster. The distance estimate precisely matched follow-up observations of the redshift redshift Displacement of the spectrum of an astronomical object toward longer wavelengths (visible light shifts toward the red end of the spectrum). In 1929 Edwin Hubble reported that distant galaxies had redshifts proportionate to their distances (see of the galaxy cluster, a direct indicator of its distance from Earth. The team reported its findings in the Aug. 20 ASTROPHYSICAL JOURNAL LETTERS. "Most of the `big action' in the universe is governed by the mass of objects, not by their visible light or other electromagnetic radiation," notes Tyson. "This fundamentally new approach lets us measure mass instead of light," providing a truer view of the distribution of matter in the universe. Ultimately, he says, "we will be able to do a blind search and effectively become intergalactic prospectors for extremely faint clumps of mass in any direction, out to about half the estimated age of the universe." Tyson adds that the prevalence of gravitational lenses provides another test of whether or not the universe has revved up the rate at which it is expanding, as recent studies of distant supernovas have indicated (SN: 4/7/01, p. 218). |
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