May 24, 2000 -- An Australian-led team of 30 astronomers from four countries has used CSIRO's Parkes radio telescope to make the first picture of the sky in which the Milky Way - the stars and dust of our own Galaxy - no longer blocks our view of the Universe beyond.
The picture will be presented today at an international astronomy meeting in Socorro, New Mexico.
The research helps astronomers understand how much normal (baryonic) matter the nearby Universe contains and how it is distributed. Key findings include large numbers of small and faint galaxies, and giant clouds of gas that give off no light.
"Pretty as it is, the Milky Way is a nuisance," says Dr Lister Staveley-Smith, Project Scientist at CSIRO's Australia Telescope National Facility. "Like a band of grime on a window, it blocks our view of about 15% of the sky."
"But we've now cleared away the dirt and cobwebs, revealing many hundreds of previously hidden galaxies."
"Astronomers hunt for galaxies to build up a three-dimensional picture of the Universe," says Dr Rachel Webster of the University of Melbourne, research team leader. "Most large surveys have looked for galaxies by their starlight they give out, or its effects."
"But in this survey we searched for signs of another key feature, the cold hydrogen gas from which stars are made. This gas gives off radio waves that can pierce through the murk of the Milky Way," she explains.
"By looking for gas rather than stars, we get a very different view of the Universe," she says. "We've been trawling for galaxies everyone else has missed. The ones hidden behind the Milky Way, the tiddlers, and the very faint, ghostly galaxies."
The new survey is called HIPASS - the HI Parkes All-Sky Survey. It is so sensitive and covers so much sky it can pick up 100 times more hydrogen than any previous survey of this kind.
"We've found objects that put out no light at all - completely black gas clouds with masses tens or even hundreds of millions times that of our Sun, " says Dr Staveley-Smith. "We think they could be 'protogalaxies' - 'building blocks' left over from when our Galaxy and its neighbours were formed."
Finding objects like these was a key motive for the survey.
"We wanted to know how much matter out there was being overlooked," says Dr Staveley-Smith. "If we have a handle on how much ordinary matter there is, we can put limits on the amount of 'dark matter'." 'Dark matter' does not give off light, radio waves or any other kind of radiation. Astronomers think it makes up about 90% of all the matter in the Universe.
The survey should also help astronomers understand how faint and dwarf galaxies work. "They are not like the bigger spiral galaxies, for instance," says Dr Webster. "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."
"We've found that there are many more faint and dwarf galaxies than similar surveys suggested before," says Dr Webster. "We expect to find five to ten thousand objects in total, of which up to a quarter will be new. And the new objects we're finding are all on the low-mass side."
The exciting results from the Parkes surveys have been made possible by asophisticated 13-beam receiving system for the telescope, designed by CSIRO. Like a wide-angle lens on a camera, this lets the telescope see more sky at once than normal.
"It's like fishing: you catch more if you trawl than if you use a single hook and line," says Lister Staveley-Smith. "If we'd had to use a single-beam telescope that could see only a tiny piece of sky at a time these surveys would have taken decades. But our multibeam receiver has slashed that time to just over three years."
From today, the first dataset from the HIPASS survey is available to all astronomers via the Web. "We are offering users 1.2 million independent spectra, with 1000 channels in each," said Dr Staveley-Smith. "No other available survey gives such 3D information across the whole sky."
More information: Julian.Cribb@nap.csiro.au
NOTE: Images of the HIPASS results are available from http://www.csiro.au
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