Commonwealth Scientific and Industrial Research Organisation

Wednesday, 24 May 2000

Astronomers wipe clean their cosmic window

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 a sophisticated 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."


CSIRO's Parkes Radio Telescope.
Copyright CSIRO Australia, 2000

Parkes Multibeam Receiver. Copyright CSIRO Australia, 2000

HIPASS survey A
The picture presented today: bright galaxies and gas clouds detected by the HIPASS survey, out to a distance of approximately 150 million light-years. The area covered is the whole southern sky, from the south pole (bottom) to the celestial equator (top). Lying in strings and sheets, the galaxies do not fill space uniformly but leave large empty 'voids'. the colours indicate different velocity ranges, which in most cases indicate how far away the galaxies are. Image: B.S. Korabalski and the HIPASS team/CSIRO ATNF.
Copyright CSIRO Australia, 2000

HIPASS survey B
Our Galaxy and its near neighbours as seen by HIPASS. The picture is centred on the South Celestial Pole and shows about a third of the southern sky. Upper left, our two nearest neighbouring galaxies, the Large and Small Magellanic Clouds. Gas is being sucked out of the Clouds by the gravitational pull of our own Galaxy, which stretches across the bottom of the picture. The different colours represent different intensities of the radio emission from the hygdrogen gas. Image: B.S. Korabalski and the HIPASS team/CSIRO ATNF.
Copyright CSIRO Australia, 2000

HIPASS survey C
The same piece of sky, looking at objects in the velocity range 0-1000 km per second. The string of bright blobs on the right is know as the supergalactic plane: among the blobs is one of the new lightless gas clouds. Also visible is the 'leading arc', a stream of gas pulled from the Magellanic Clouds by the gravitational force of our own Galaxy. Image: B.S. Koribalski and the HIPASS team/CSIRO ATNF.
Copyright CSIRO Australia, 2000

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