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Friday, February 20, 2009

In a cosmic case of delayed development, dwarf galaxies have been found growing in a ring of gas that seems to be left over from the early universe. Strangely, the new galaxies appear to lack dark matter, suggesting a fraction of the universe's first galaxies might have been born from gas alone.

The new dwarf galaxies are forming in a giant stream of hydrogen and helium gas called the Leo Ring. The ring surrounds two older galaxies some 35 million light years away from Earth.

The Leo Ring was discovered more than 25 years ago, but until now, no one had seen evidence that it was forming stars - searches for star formation in the ring using optical telescopes have come up blank.

But David Thilker of Johns Hopkins University and colleagues found the ring is creating stars that release a lot of ultraviolet light. Using NASA's Galaxy Evolution Explorer (GALEX), the team discovered that the ring contains several clumps of hot young stars - infant dwarf galaxies.

The galaxies weigh more than 1 million times the mass of the Sun, rivalling the size of some dwarf galaxies in orbit around the Milky Way. But unlike other dwarf galaxies, which can boast 10,000 times more dark matter than ordinary matter, the Leo Ring dwarfs do not seem to contain much of the mysterious stuff. (No extra, invisible dark matter is needed to account for the observed speed of matter orbiting within the dwarfs.)
Little dark matter

"We don't see evidence for the clumps of dark matter that we typically associate with galaxy formation," Thilker told New Scientist.

This is unusual, because dark matter is thought to play an important part in galaxy formation. By gravitationally attracting ordinary matter, clumps of dark matter are thought to have "seeded" the development of galaxies. Dark matter is also thought to prevent material in galaxies from being blasted away by supernovae.

Other dwarf galaxies have been found that do not have much dark matter. But these galaxies are born from streams of material drawn out of colossal collisions between larger galaxies, like the Antennae galaxies.
Primordial gas?

The Leo Ring does not seem to be created from such violence. Instead, the team suspects the ring is made up of gas that has remained unaltered for some 13 billion years. "The ring looks pretty settled, like it's been there for a long time," says team member David Schiminovich of Columbia University in New York.

The ring spins just once every 4 billion years or so and the gas may have had to take several orbits to become so evenly spread, Schiminovich says.

The team also found that the dwarf galaxies seem to be deficient in heavy elements, or 'metal poor'. That suggests the ring's gas did not originate in a galaxy, where stars and supernovae create heavy elements through nuclear fusion.

"This is really the best example of a primordial ring that we know of," Thilker says. "But there should have been more of these in the early universe." That's because interstellar gas tends to become more metal-rich over time, as successive generations of stars enrich it with heavy elements.
Better spectrum

"It's a kind of laboratory to study galaxy formation," comments Frederic Bournaud of the Commission for Atomic Energy in Saclay, France.

But Bournaud says he is not convinced the Leo Ring is a relic of the early universe. Since GALEX has only studied certain wavelengths of light, he says the ring may actually be richer in heavy elements than the team believes, meaning it could simply be recycled gas jettisoned from one or more galaxies during a collision.

To clinch the case, the team is planning follow-up observations using the Great Canary Telescope on the island of La Palma and one of the Magellan telescopes in Chile to look for small pockets of energised gas surrounding massive stars.

By measuring the spectrum of light emitted from such regions, the team should be able to discern whether the ring really is primordial. "That would tell us for sure if the metallicity was really low," Thilker says.

It is not clear what is triggering the new star formation. The culprit could be gravitational tugs from a spiral galaxy that sits some 300,000 light years away from the star clusters. The team plans to use GALEX to see whether clumps of stars are forming in parts of the ring farther from the spiral galaxy.


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