Is It a Dark-Matter Galaxy -- or Just a Big Ball of Hydrogen?

By John Johnson
Los Angeles Times

The discovery of a big ball of hydrogen in a far-flung region of the universe may help unravel one of the thorniest problems in modern cosmology: where is the missing dark matter in the universe?

Researchers at Cardiff University in Wales have measured a giant ball of hydrogen in the Virgo cluster 50 million light years away that appears to be part of a much larger invisible galaxy of whirling debris. If the finding stands up under the scrutiny of other cosmologists, it would be the best evidence yet that most of the matter in the universe is made up not of stars but of cold dark matter hidden from earthly telescopes.

"The universe has all sorts of secrets still to reveal to us, but this shows that we are beginning to understand how to look at it in the right way. It's a really exciting discovery," said Jon Davies, a member of the Cardiff team that led the research. Other nations participating in the effort were France, Italy and Australia.

American scientists cautioned against making sweeping assumptions about the building blocks of the universe based on the European findings. "It's probably premature to say what we're seeing here," said Roger Blandford, a Stanford physicist.

"What's more significant is here we have a whole lot of gas that appears not to have formed stars," he said.

At the very least, then, the discoveries -- reported online and to be published next month in the Astrophysical Journal -- are likely to provide new insights into how and why stars form out of the cosmic foam. And, just as important, why they don't.

Based on a number of factors, including the accelerating speed at which galaxies are traveling, physicists have predicted that most of the mass of the universe is made up of unseen dark matter. While various math models have proved that dark matter must exist, the challenge has been to find clumps of it, and to figure out what it is.

The team of researchers had been searching for a dark galaxy using radio waves, since by definition a galaxy without stars could not be seen with a visible light telescope. In 2000, the radio telescope at the University of Manchester in England detected a ball of invisible hydrogen with a mass equivalent to 100 million suns. The finding was later confirmed by the Arecibo radio telescope in Puerto Rico.

The material in the ball was spinning, much like an ordinary spiral galaxy. The high velocity of the spinning matter convinced scientists that the dark galaxy, which they named VIRGOH121, was a thousand times more massive than the gaseous ball of hydrogen.

Given its size, "If it were an ordinary galaxy, then it should be quite bright and would be visible with a good amateur telescope," said Robert Minchin of Cardiff University.

Similar supposedly dark galaxies have been discovered in the past, only to turn out to contain hidden stars when observed with high power telescopes. Others turned out to be the remains of colliding galaxies. The Cardiff team spent much of the five years since the discovery of VIRGOH121 eliminating other possibilities.

They found no stars and no traces of a galactic collision.

"As Sherlock Holmes said, `When you have eliminated the impossible, whatever is left -- however improbable -- must be the truth," said Mike Disney of the Cardiff team.

The deeper question is, why isn't VIRGOH121 behaving like a normal galaxy? What is keeping the spinning gases and dust particles from gradually coalescing into fire-breathing stars?

Some American researchers expressed skepticism that VIRGOH121 is really a galaxy at all. A galaxy has borders and is kept clumped together by the gravitational pull of the material in it. In the case of visible galaxies like our Milky Way, that means all the stars in it. If this dark body has no boundaries, it could just be a drifting cloud of hydrogen with dark matter clumps hitching a ride.

"It's not really a dark matter galaxy, but a hydrogen cloud," said Ned Wright of the University of California, Los Angeles. "It's not really a bound system."

Other cosmologists were impressed by the team's effort to screen out other possibilities before announcing their finding ."It looks like they've done their homework," said Lynn Cominsky, a professor of physics at Sonoma State University. "I think they've done a pretty careful job."

"It's good work," agreed Craig Hogan, vice provost for research at the University of Washington.

Even if VIRGOH121 turns out not to be the first dark matter galaxy, it is already clear that this is something unusual. That's enough to keep cosmologists interested for years to come.

"This has opened a new point of observation in space," Blandford said. "As we survey more of the sky, we will see if it's a fluke."