'Dark Galaxy' continues to astronomers

The Hubble Space Telescope has failed to reveal the expected number of stars in the mysterious cloud of hydrogen on the size of a galaxy known as VIRGOHI21.

The research reinforces the idea that the gas cloud is the only known example of a "dark galaxy" that never initiates the birth of a star.

It is thought that a galaxy is formed of normal matter, or which has collected in clouds of hypothetical dark matter. But scans have shown fewer galaxies than expected, suggesting that, for unknown reasons, 

some galaxies are immature, and simply do not form stars.The discovery of VIRGOHI21 in 2005 seems to provide the first evidence that dark galaxies exist. However, several researchers suggested that VIRGOHI21 was expelled from the nearby galaxy NGC 4254 when another galaxy called NGC 4262 passed it to 900 kilometers per second.

Indeed, NGC 4254 has a unique extension arm of stars that curves VIRGOHI21, suggesting some kind of link between the two.

But Robert Minchin of Arecibo Observatory dismisses such models of "hit-and-run." "If the hydrogen VIRGOHI21 had been expelled from a nearby galaxy, the same interaction should also be expelled stars," says Minchin.

He and his colleagues used Hubble to observe a square of sky of 50,000 by 50,000 light-years, centered on the position of the hydrogen cloud. They found exactly 119 red giant stars. Is the amount found in a typical region of the same size of intergalactic space and three times lower than expected if the cloud were a big piece of celestial debris.