Ghostly Galaxy: Massive, dark cloud intrigues scientists

It looks like an empty patch of space, but astronomers say it holds a galaxy that contains no stars. If Robert Minchin of Cardiff University in Wales and his colleagues are right, they have found the first member of a population of galaxies that theorists have proposed but observers had never seen.

In 2000, Minchin’s team noticed two apparently isolated hydrogen clouds in a radio telescope survey of the Virgo Cluster of galaxies. Follow-up observations with visible-light telescopes showed that one of these clouds was associated with a faintly glowing galaxy. However, long exposures taken with the 2.5-meter, visible-light Isaac Newton Telescope in the Canary Islands offered up a surprise: The second cloud had no partner glowing galaxy.

“It’s a very intriguing object,” comments galaxy researcher Richard S. Ellis of the California Institute of Technology in Pasadena. “It’s puzzling how this ball of hydrogen hasn’t got any stars in it.”

It could be that several smaller knots of gas fall along the same telescopic line of sight and are masquerading as a single, much bigger cloud, Ellis cautions. Gravitational tugs-of-war between galaxies frequently pull small clouds of hydrogen out of galaxies. But Minchin’s team says that, in the case of VIRGOHI21, as their object is called, there are no suitable galaxies nearby to have donated the gas.

Assuming that the hydrogen is contained in one big cloud, its motion suggests that it’s a small part of a massive object weighing as much as a galaxy of 100 billion suns. And yet this object remains invisible.

“Seeing a dark galaxy—a galaxy without any stars—is like seeing a city without any people,” says Minchin. “We want to know why nobody lives there.”

Ordinary galaxies seem to be made of about 10 percent ordinary matter—the kind that forms stars that shine—and 90 percent dark matter, an invisible substance whose nature still eludes astronomers. In an upcoming Astrophysical Journal Letters, Minchin’s team reports that VIRGOHI21 has an ordinary-to-dark matter ratio of about 1 to 1,000.

Computer simulations of galaxy formation suggest that there should be many more small galaxies in the universe than observations indicate. Some theorists have suggested the missing galaxies elude observation because they’re rife with dark matter yet all but devoid of ordinary star-forming matter.

VIRGOHI21 just might be one of these elusive bastions of dark matter, says Gregory Bothun of the University of Oregon in Eugene.

If there are any ordinary-matter stars in VIRGOHI21, they’re few and extremely faint. Minchin’s group has requested use of the Hubble Space Telescope, which may be just powerful enough to detect individual stars in the object. The researchers plan to get higher-resolution radio observations with the Very Large Array radio telescopes near Socorro, N.M., to settle the question of whether the object is a single entity.

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