Astronomers make the Milky Way vanish

The survey shows galaxies are arranged in sheets and strings

By BBC News Online science editor Dr David Whitehouse

Astronomers have produced the first picture of the night sky without the Milky Way, so that they can see the Universe beyond more clearly.

We've now cleared away the dirt and cobwebs, revealing many hundreds of previously hidden galaxies

Dr Lister Staveley-Smith

It is hoped that the image will help scientists understand how much normal matter the nearby Universe contains, and how it is distributed.

So far, key findings include large numbers of small and faint galaxies, and giant clouds of gas that give off no visible light.

The survey used the Parkes radio telescope in Australia to map the distribution of hydrogen gas over the Southern Sky. Hydrogen is the raw material from which stars are made.

Radio waves from the gas pass through the Milky Way relatively unhindered, revealing unseen galaxies lurking behind.

Large volumes

"Pretty as it is, the Milky Way is a nuisance," said Dr Lister Staveley-Smith, Project Scientist at the Australia Telescope National Facility. "Like a band of grime on a window, it blocks our view of about 15% of the sky.

Clouds of hydrogen accompany our galaxy in space

"But we've now cleared away the dirt and cobwebs, revealing many hundreds of previously hidden galaxies."

"There's a lot we don't know about the local Universe," said Professor John Huchra of the Harvard-Smithsonian Center for Astrophysics.

"With a survey like this we can make an accurate census of at least those objects with enough neutral hydrogen gas to be detected and map their locations."

The survey, known as Hipass (H1 Parkes All-Sky Survey), is the first hydrogen survey to give both positions and distances of hydrogen gas over a large volume of space.

Raw material

"By looking for gas rather than stars, we get a very different view of the Universe," said Dr Staveley-Smith.

The Parkes radio telescope was used for the survey

"We've been trawling for galaxies everyone else has missed - the ones hidden behind the Milky Way, the tiny ones, and the very faint, ghostly galaxies.

"We've found objects that put out no light at all - completely black gas clouds with masses tens or even hundreds of millions of times that of our Sun," he said. "We think they could be 'protogalaxies' - 'building blocks' left over from when our galaxy and its neighbours were formed.

"We wanted to know how much matter out there was being overlooked."

Dr Ken Freeman of the Australian National University added: "These galaxies have lots of raw material for stars but for some reason failed to make them.

"And we don't know much about how galaxies formed in the first place. Looking at different kinds of galaxies might help us to understand that process. And to start we need to know how many there are and where they are."