A Dark View of the Universe

R. Cowen

PureInsight | December 22, 2000

Like glittering jewels on a blanket of black velvet, the starlit disks of galaxies are awash in a vast ocean of dark material. More than just a cosmic backdrop, this invisible material, called dark matter, accounts for at least 90 percent of the mass of the universe. Astronomers believe it provides the gravitational glue that keeps stars from flying apart and holds clusters of galaxies together. They speculate that dark matter prompted gas and dust to coalesce into galaxies in the first place.

Three new reports shed light on the distribution of dark matter in the universe. Two of the studies indicate that invisible halos of this material provide enormous breadth and bulk to galaxies. The halos extend 1.5 million light-years from each galaxy's center and contain at least as much mass as 5 trillion suns.

That's two to three times larger and more massive than previous estimates, notes Philippe Fischer of the University of Michigan in Ann Arbor, who led one of the studies. 'This is a real surprise,' he says.

Dwarfing the visible portion of a galaxy, which has only about one-tenth the diameter and one-thousandth the mass, a halo this big has an intriguing implication: Our galaxy might actually brush up against its nearest large neighbor, the Andromeda galaxy, notes Joshua A. Frieman of Fermi National Accelerator Laboratory in Batavia, Ill., who collaborated with Fischer.

The third study probes dark matter on the grandest scale possible. Astronomers have for the first time 'unambiguously detected' gravitational distortions in the arrangement of superclusters of galaxies stretching 10 million to 100 million light-years across the sky, says Matthew Colless of the Australian National University in Canberra. By measuring the distortion, his British colleagues have obtained a preliminary estimate of the overall density of matter in the universe.

In line with numerous other studies, the new findings suggest that the density is only about one-third the value required to keep the universe from expanding forever, notes John A. Peacock of the Royal Observatory of Edinburgh in Scotland. Combined with other observations, the results are consistent with the notion that the universe's rate of expansion may be accelerating (SN: 12/19&26/98, p. 392).

Because dark matter can't be seen, astronomers must detect it indirectly, through its influence on visible galaxies. Its effects can be subtle, requiring analysis of detailed data on thousands of galaxies. That's why the new studies rely on two huge surveys of the heavens.

The Sloan Digital Sky Survey, which began taking preliminary data a year ago and continues through 2005, will ultimately image and map the locations of millions of objects across a huge stretch of the northern celestial hemisphere (SN: 1/23/99, p. 57). The survey, based at Apache Point, N.M., features a specially designed telescope and the largest electronic camera ever built. On a clear night, the system can image 25,000 galaxies.

Closer to completion but smaller in scope, the 2dF (2-degree Field) Galaxy Redshift Survey aims to measure the distances to some 250,000 galaxies and 50,000 quasars that astronomers have already imaged. Using the Anglo-Australian Telescope on Siding Spring Mountain in Australia, the survey examines objects in two large wedges of sky at the north and south galactic poles. Scheduled to finish in 2001, it has already measured the distances to 75,000 galaxies.

Without the surveys, says theorist Nicholas Kaiser of the University of Hawaii at Manoa in Honolulu, none of these observations would have been possible. This mammoth census of the heavens offers a new way to do science, he notes.

Studying galaxies imaged by the Sloan survey, Fischer and his colleagues took advantage of a cosmic mirage called gravitational lensing. From it, they deduced the extent of a typical dark-matter halo.

Any massive body, such as a galaxy, deflects the path of light passing by, distorting and magnifying the images of objects behind it. If two galaxies happen to lie roughly on the same line of sight, the nearer one will alter the appearance of the farther one. Fischer's team analyzed the shapes of 1.5 million distant galaxies and used the observed distortion to infer the pattern of dark matter around foreground galaxies.

The team faced two notable problems. First, the halo of a nearby galaxy elongates the diameter of the distant galaxy by less than 1 percent. Second, the scientists don't know the galaxies' true shape. Some are spherical, but many are a football-shape.

To overcome these obstacles, the astronomers relied on the power of a large sample. The 1.5 million galaxies they analyzed were approximately aligned with some 30,000 foreground galaxies. By stacking all the distorted images, they intensified the lensing effect. This procedure also averaged out the confounding effect of galaxies that were naturally misshapen. In these cases, the direction of the stretching is random rather than aligned by the foreground lens.

The technique 'is a powerful probe of mass in galactic halos,' says Richard S. Ellis of the California Institute of Technology in Pasadena. Fischer and his colleagues recently posted their study on the Internet ( http://xxx.lanl.gov/abs/astro-ph/9912119).

The team's surprisingly large estimates of halo size and mass, Fischer notes, are merely lower limits based on two nights of observations. They failed to find the halo's edge. With many more nights of data, the researchers hope to pin down a more accurate size. He cautions that as astronomers probe farther and farther out from a galaxy's visible core, it may become difficult to ascribe dark matter to a particular galaxy.

Further observations may also allow the researchers to discern the composition of the dark matter¡ªwhether it's made of ordinary material that doesn't emit much light or it's truly exotic stuff, built from something other than protons, neutrons, and electrons.

Another group of scientists, analyzing the Sloan data for other signposts of lensing, have gathered evidence that appears to confirm the vastness of the halos, Science News has learned.

A team that includes B. Jain and Alexander S. Szalay of Johns Hopkins University in Baltimore and Andrew Connolly of the University of Pittsburgh examined the Sloan images for two indications of lensing: a brightening and a lowered density of background galaxies.

These search criteria, says Szalay, provide a more far-ranging probe of the dark matter around galaxies than looking for distortions in shape does. He asserts that some of the dark matter his team has detected lies so far out¡ªsome 15 million light-years from the center of the nearest galaxy¡ªthat it's more accurate to say that this material resides in the space between galaxies. The scientists plan to present some of their findings Jan. 12 at a meeting of the American Astronomical Society in Atlanta.

Taken together, the results from both teams strengthen the halo findings, notes Kaiser. He adds, however, that the method employed by Fischer's group is more sensitive than that used by Szalay's team.

The size of the halo may make it easier for galaxies to gobble up their small neighbors, suggests Kaiser.

A British-Australian team has focused on the largest structures known in the universe, superclusters of galaxies that stretch across the sky in long filaments. Using the 2dF survey, these researchers have detected the large-scale influence of gravity on the mammoth pieces of cosmic architecture. The rate at which gravity is inducing galaxies to form superclusters¡ªa process that's still going on¡ªprovides a measure of the density of matter in the universe today, Peacock told Science News.

The 2dF Survey measures the velocities of a large sample of galaxies. It has revealed that the superclusters appear to be undergoing gravitational contraction, he notes.

The mutual gravitational attraction of galaxies in a supercluster should draw them toward each other, Peacock explains. To an observer, it will seem that galaxies on the near side of the supercluster are moving away from Earth slightly faster than they should be, while those on the far side are a bit too slow.

That's exactly what Peacock has now found, enabling him and his colleagues to begin estimating the cosmic density of dark matter. 'Our result . . . measures the density of dark matter on scales of 10 million to 100 million light-years,' he says. The report is as yet unpublished.

The study 'is really very compelling,' says Kaiser, who in 1987 predicted the gravitational effect in a supercluster. 'It's telling you how dark matter clumps on very large scales.'

Together, the three sets of new findings mark a milestone in understanding the distribution¡ªand ultimately the nature¡ªof dark matter throughout the universe.

'To me, the galaxies are like lights on a Christmas tree,' Kaiser notes. 'They show you the outline, but it's really the dark tree that you're trying to look at.'

Stay tuned, he and other astronomers urge, as they begin to reveal the branches.

From Science News, Vol. 157, No. 2, January 8, 2000, p. 30.

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