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Thursday, 8 March, 2001, 15:42 GMT
Galaxy survey solves cosmic riddle
Plate AAO
Each prism is placed on the plate by the robot
By BBC News Online science editor Dr David Whitehouse

Astronomers have obtained their most comprehensive map yet of our local part of the cosmos.

With this survey, cosmology is entering an era of large-scale studies and precision measurements

Professor Marc Davis, University of California
The so-called 2dF survey covers 141,000 galaxies within three billion light-years of Earth.

The map has allowed researchers from the UK and Australia to determine the origin of the vast filaments and sheets of galaxies seen in space.

The study, published in the journal Nature, confirms these structures were formed by the action of gravity when the Universe was young.

Special equipment

Just obtaining the distances to all the galaxies was a technological triumph. The key was to get their redshifts. Because the Universe is expanding, the Doppler effect causes the light from distant objects to be "stretched" and shifted into the red end of the spectrum. The greater the redshift, the greater the distance.

Movie AAO
The astronomers have already used their data to make a fly-through movie of the local cosmos
But getting the redshifts for so many galaxies required the use of a specially designed instrument at the Anglo-Australian telescope in Australia.

The light hitting the telescope's 3.9m (12.8ft) primary mirror was focussed on to a detector plate. This plate contained more than 400 tiny, movable prisms hooked to optical fibres that were positioned by a robot. Each prism was zeroed on a particular galaxy in the frame of the image.

From there, the light from each galaxy passed through a pair of spectrographs that revealed the object's redshift. The distance was then calculated, which when combined with the galaxy's position on the sky gave its three-dimensional location in space.

Robot control

The 2dF's robot picks up each prism, which is attached to a small magnet, and places it on the steel detection plate with astonishing precision - to within five millionths of a metre - to collect the light from a particular galaxy.

The robot can position 400 prisms in under 30 minutes, and an ingenious tumbler system enables astronomers to reset the instrument for the next shot while observations are continuing on the previous one.

The survey still has another two years to go. Astronomers hope to have analysed the light from 250,000 galaxies by the end of this year.

New era

Even now, the survey has produced a "comprehensive" map of our cosmic locale, John Peacock of the University of Edinburgh, UK, and his co-workers say in the journal Nature.

From the map, "we can measure some of the fundamental parameters that define the structure of the entire cosmos," they write.

The data indicates that the 100-milion-light-year-long filaments of galaxies seen in the survey were indeed formed by gravity moulding clouds of gas.

Commenting on the work, Professor Marc Davis, of the University of California, said: "With this survey, cosmology is entering an era of large-scale studies and precision measurements."

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See also:

19 May 00 | Sci/Tech
Voyage through the Universe
29 Jan 99 | Sci/Tech
Universe put on the map
09 Mar 00 | Sci/Tech
New light on dark matter
28 Apr 00 | Sci/Tech
Pictures of the early Universe
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