Seeking Galaxies by Gaslight: Hydrogen Gas Is Telling Tales on Hundreds of Galaxies Once Shielded by Interstellar Dust. Wendy Pyper Enters the Realm of Dark Matter, Dark Clouds, and the Great Attractor

By Pyper, Wendy | Ecos, October-December 2000 | Go to article overview

Seeking Galaxies by Gaslight: Hydrogen Gas Is Telling Tales on Hundreds of Galaxies Once Shielded by Interstellar Dust. Wendy Pyper Enters the Realm of Dark Matter, Dark Clouds, and the Great Attractor


Pyper, Wendy, Ecos


Galaxies, stars and mysterious perturbations of space have long tantalised our imaginations and inspired Hollywood directors. But thanks to the stars and dust in our own galaxy -- the Milky Way -- our view of the Universe and the secrets within, have Been partly obscured. Now astronomers at the CSIRO Australia Telescope National Facility (ATNF) at Parkes in New South Wales have found a way to look through the Earth's galactic curtain.

`For the first time we've searched behind the Milky Way and found hundreds of new galaxies no-one has seen before,' ATNF astronomer, Dr Raymond Haynes, says.

The discovery came after ATNF astronomers designed a special radio wave `receiver', for the Parkes telescope. Like a transistor radio, only more sensitive, the receiver picks up radio waves emitted by hydrogen gas, a basic building block of galaxies and stars. These radio waves can penetrate the dust and starlight of the Milky Way, enabling astronomers to detect faint, small or distant galaxies, which would otherwise remain hidden.

`In the past, astronomers have looked for galaxies by the starlight they give out,' Haynes says. `That's fine for nearby galaxies, but as you go deeper into the Universe, the light from faint, small or distant galaxies is usually absorbed by interstellar dust. By looking at the hydrogen gas, rather than stars, we've been able to detect those galaxies.'

So far, the `HIPASS' -- HI Parkes All Sky Survey (HI for neutral hydrogen) -- has unveiled more than 600 new galaxies behind the Milky Way, which obscures about 15% of the southern sky, and 900 new galaxies in the remainder of the southern sky and part of the northern sky.

HIPASS astronomers, including scientists from Australia, the UK, US, Mexico and the Netherlands, have also calculated the distance to these galaxies.

Because our Universe is expanding, (like the cloud from an explosion), the further a galaxy is away from us, the faster it recedes. As a result, radio waves emitted by distant galaxies are more stretched out than those emitted by galaxies closer to us.

`As with optical astronomy, we've been able to observe galaxies on the two-dimensional plane of the sky,' Haynes says. `But we've also been able to determine the distance to each galaxy using the velocity information we gather from their, radio emissions. So now we can effectively see out through space.'

This advance has enabled astronomers to draw the first three-dimensional picture of the distribution of galaxies in the `southern sky. The survey has also been conducted 13 times faster than normal,' thanks to the `multibeam' design of the receiver.

`We've put 13 receivers on the telescope at once, so we cart cover more of the sky in one bite,' Haynes says. `So if we look at a given area in the sky, instead of seeing something that's 14 minutes of are across (half the size of the moon) as we would observe with a single receiver system, we see 13 times that area simultaneously.'

In just over three years the astronomers have mapped the entire southern sky and part of the northern sky. This map will help them understand the structure, distribution and formation of stars, galaxies and extra-galactic matter and answer questions that have puzzled astronomers for decades.

One such question concerns the amount of matter in the Universe. Dr Lister Staveley-Smith, the project scientist in charge of the multibeam instrument, says one of the main aims of the survey was to put limits on the amount of `dark matter' in the Universe.

Dark matter, as the name suggests, can't be seen, as it gives off no light, radio waves or any other kind of radiation. However, astronomers know it exists because it influences the rotation of galaxies. …

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Seeking Galaxies by Gaslight: Hydrogen Gas Is Telling Tales on Hundreds of Galaxies Once Shielded by Interstellar Dust. Wendy Pyper Enters the Realm of Dark Matter, Dark Clouds, and the Great Attractor
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