Mystery matter: through a lens, darkly.

Mystery Matter: Through a Lens, Darkly

Using faint background light from some of the most distant reaches of the universe, researchers have created the first "map" depicting the distribution of huge clumps of dark matter hidden inside galaxy clusters. The imaging technique, which relies on newly developed computer software to analyze low levels of light and to detect that light's bending by gravity, may one day help astronomers map the concentration of dark matter over large regions of the sky.

The finding "has the potential to be a major breakthrough," says astrophysicist John N. Bahcall at Princeton (N.J.) University. "What's most exciting is that the observations may be a new handle on [estimates of] dark matter."

Researchers believe that some form of dark matter -- mass hidden from view because it does not radiate at any observed wavelength -- must exist to explain several puzzles, including why fast-moving galaxies remain clustered together even though their visible mass could not generate the needed gravitational force. Dark matter, theorists say, provides the extra tug. Astronomers estimate it may make up 90 to 99 percent of the universe; exactly how much may determine whether the universe will expand forever or will eventually collapse because of its great mass.

Dark matter, by its very nature, can be detected only indirectly, posing a challenge for astronomers. Imagine a house-guest you can't see, smell or feel, but who eats a daily portion of roast beef from your refrigerator. You might deduce the mystery guest's weight from his or her eating capacity. Astronomers have similarly begun to infer the presence of dark matter through its one observable property: Like all mass, it bends light.

In 1916, Albert Einstein predicted such gravitational bending in his General Theory of Relativity; two decades later, he proposed that a massive object could act as a distorting lens, bending light from a more distant star so that it would appear as a ring or arc of light. Such optical illusions, known as gravitational lensing, were first observed in 1979, and since then astronomers have used the phenomenon to gauge the amount, though not the distribution, of dark matter in a handful of galaxy clusters.

These measurements were limited because observations of the lensing effect required bright light sources precisely aligned with a massive foreground object -- a relatively rare occurrence. Now, using fainter, less-aligned light sources and more sensitive detectors, J.A. Tyson of AT&T Bell Laboratories in Murray Hill, N.J., and his colleagues have uncovered unprecedented details about dark matter in a galaxy cluster. They say the new technique, which takes advantage of the vast background of faint, blue light sources, could map the dark matter distribution in many regions of the universe. "We now have a 'palette' of hundreds of faint, blue galaxies" to make dark matter measurements, says Tyson, who describes the findings in the Jan. 20 ASTROPHYSICAL JOURNAL LETTERS with Richard A. Wenk of Bell Laboratories and Francisco Valdes of the National Optical Astronomy Observatories in Tucson, Ariz.

The researchers examined gravitational distortions in the faint, blue light of a plentiful array of galaxies located several billion light-years from Earth, near the limits of the observable universe. About midway through its journey to Earth, some of the faint light encounters a massive galaxy cluster called A1689, which acts as a gravitational lens, elongating and reorienting the appearance of the more distant galaxies as viewed from Earth, Tyson says. Using sensitive charge-coupled devices to record the light and software that amplifies and recognizes the characteristic light patterns produced by the lensing, the group mapped both the amount and distribution of dark matter in A1689. They estimate the hidden mass equals at least 10 times the visible mass of the cluster.

Because the distribution of dark matter in A1689 appears to coincide with red light emitted by the cluster -- a phenomenon Tyson and his colleagues found in six other clusters they recently mapped -- he clumps that match the overall cluster shape. Any indication of whether the dark matter they mapped represents "cold dark matter" -- a purely theoretical concept describing matter composed of material other than the elementary particles so far detected -- awaits further observations, Tyson says.