The Universe may be teeming with starless galaxies inhabiting its most isolated regions, says an Australian scientist.
By Dr David Whitehouse
BBC News Online science editor
Graduate researcher Brad Warren, of the Australian National University, has identified galaxies in our local region of space that are mostly gas with very few stars.
An unusual gassy galaxy
"If you look for gas with a radio telescope you see an enormous blob of gas. If you look for stars through an optical telescope you only see a small smudge of stars on the sky," he told BBC News Online.
For some reason these galaxies have failed to form stars out of their hydrogen gas. The search is on to find out why.
Starless and bible black
In research presented to the meeting of the International Astronomical Union (IAU) in Sydney, Brad Warren outlined his discovery of 20 gassy galaxies.
He used optical and radio telescopes to follow up a survey of hydrogen clouds in the southern sky. Some of the clouds were galaxies that were almost starless.
"When you look for gas in these galaxies the signal just booms in," Mr Warren told BBC News Online. "But when you look for stars, all you see is a barely recognisable smudge."
Galaxies made mostly of gas that contain very few stars have been seen before. This new study shows that they are more common, and stranger, than had been thought.
They are found in the great intergalactic voids - vast spaces between groupings of normal galaxies. Few objects inhabit these regions, where lonely stars, torn from their parent galaxies, live isolated lives before they fade and die alone.
The gassy galaxies are vast discs of hydrogen, tens of thousands of light-years across - somewhat smaller than our own Milky Way galaxy. Although they weigh more than a billion suns, they have only a tiny number of barely visible stars at their centre.
Stars never formed
According to Brad Warren, the reason they are stillborn may be their remoteness: "These galaxies are isolated. They haven't had any stimulation from other galaxies that may trigger star formation," he told us.
"Alternatively, there may be something within the galaxies themselves that is holding back star formation by keeping their gas spread out. We don't know. We're looking into that."
Only a few stars live there
Understanding why these galaxies failed to form stars may also help explain why most galaxies do.
"Discovering why will give us important insights into how, when and why galaxies, such as our own, formed," says Mr Warren.
He says the next step in his research is to investigate the few stars inside the galaxies to see if they are peculiar in any way.