‘Immense Concentration’ of Dark Matter Believed Found

Astronomers studying a thin cloud of intergalactic gas in a small and unremarkable galaxy cluster said Monday they think they found an “immense concentration” of dark matter, the mysterious invisible material believed to account for most of the mass in the universe.

If confirmed by observations of other clusters, the discovery may let scientists estimate more precisely how much mass the universe holds--and thus predict whether it will continue to expand forever or eventually collapse back in on itself.

Astronomers from NASA’s Goddard Space Flight Center, the University of Maryland and Arizona State University drew their conclusion after noticing that the NGC 2300 cluster has enough “visible,” or conventional, mass to account for only a small fraction of the gravity necessary to contain the very hot, diffuse gas cloud surrounding the galaxy cluster.

In a paper offered at the American Astronomical Society meeting in Phoenix, the scientists --Richard Mushotzky of National Aeronautics and Space Administration’s Goddard Center, David Burstein of Arizona State and John Mulchaey and David S. Davis of Maryland--asserted that dark matter makes up the rest of the necessary mass.

Theoretical astrophysicists began to suspect that small and ordinary galaxy clusters, such as NGC 2300, contained large quantities of dark matter when the mysterious matter was detected in perplexingly small amounts in larger and richer clusters.

But the discovery presented Monday is the first direct evidence that the theory is correct. If the amount of dark matter in this cluster is also found in other small clusters--and that is a very big if, astronomers acknowledge--the universe could contain enough mass to prevent it from expanding forever.

There could, indeed, be enough mass to cause the universe to eventually collapse back in on itself.

Although such an event would not happen for tens of billions of years if it happened at all, a few astronomers have begun to call it the “big crunch,” in relation to the “big bang” believed to have started the universe.

Jack Burns, an astronomer at New Mexico State University, said the new data is exciting and potentially important, but still must be confirmed by studying NGC 2300 in more detail and comparing it with other galaxy clusters.

“We have to be cautious,” said Burns. “This is one finding in one group of galaxies. However . . . I have a sneaking suspicion there is a lot more of this dark matter out there than we suspected.”

The discovery was made with the Roentgen Satellite, or ROSAT, the orbiting X-ray observatory that was launched in June, 1990, as a joint project of Germany, the United States and Great Britain.

NGC 2300, which consists of three galaxies, is about 150 million light-years from Earth, toward the northern constellation Cepheus.

Finding a large amount of dark matter in that small, somewhat ordinary galaxy cluster gives scientists a boost in the effort to map concentrations of the mysterious material.

Over the last decade, astronomers concluded that “rich” galaxy clusters, those containing hundreds or thousands of galaxies and abundant intergalactic gas clouds, contain an unexpectedly small amount of dark matter. This has sent them scrambling to find the “missing” dark matter predicted in models of the universe.

Small galaxy clusters were logical candidates because they are as numerous as they are unspectacular.

“The universe is like the pre-industrial United States,” said Mulchaey. “ . . . The most conspicuous population concentrations were in a few big cities, but . . . most people actually lived in small towns and rural America.”

The problem in taking the time to poke around these many small clusters, or “small towns,” of galaxies, is that dark matter does not emit radiation--no visible light, infrared, radio waves, gamma rays or X-rays--so it cannot be seen with current scientific instruments.

Scientists deduced the existence of dark matter from what is believed to be evidence of its gravity, such as the unexplained bending of light from distant quasars or, now, the containment of very hot gas clouds.

Indeed, accepted scientific wisdom is that the overwhelming majority of the mass in the universe consists of dark matter; only the remaining fraction--as much as one-third or as little as 4%--can be detected.

Still unclear is whether scientists will ever be able to discover what dark matter is made of. Current hypotheses range from enormous dead stars and black holes to massless subatomic particles, called neutrinos, to something wholly new and incomprehensible.

Burstein said he wanted to study NGC 2300 for many years because one of its galaxies, a spiral, appears flattened on one side. He suspected the galaxy was moving rapidly though a gas cloud, but the cloud was too faint to see.