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Tuesday, April 13, 1999 Published at 15:27 GMT 16:27 UK


Missing mass hiding in galaxy clusters

The galaxy cluster seen in optical light

By BBC News Online Science Editor Dr David Whitehouse

Some of the Universe's missing mass has been revealed hiding in clusters of galaxies.

Astronomers have discovered previously-unseen clouds of hot gas being pulled into the clusters.

The gas has far greater mass than the observable stars in the galaxies and so may make up an appreciable fraction of the mass of the Universe.

[ image: The galaxy cluster seen in ultraviolet light]
The galaxy cluster seen in ultraviolet light
Astronomers have puzzled over the 'missing mass' of the Universe for decades. The problem is that what we can see in the Universe only accounts for about 10% of its the total mass.

Ninety per cent of the Universe is invisible and can only be detected by its gravitational effects.

A team of astronomers used the highly sensitive Nasa Extreme Ultraviolet Explorer satellite to detect radiation coming from the vast spaces between galaxies.

They believe the UV radiation is coming from hot gas that is being sucked into the core of the galaxy cluster from outside.

It is something not seen before. According to Dr Richard Lieu, of University of Alabama: "The UV radiation represents a genuine component of intracluster space."

One object, the Abel 1795 cluster, seems to have enough hot gas to cause the entire cluster to collapse in on itself, pulled by gravity.

Dr Lieu speculates that as the cluster of galaxies is intact, then the gas cannot have been in the cluster for very long. It must therefore have been sucked in from the space in-between the clusters of galaxies.

The new gas clouds are not massive enough to be all of the much sought after missing mass. But it does provide a few clues.

If a new component of galaxy clusters has been identified, and that component is just warm gas, then what else might be lurking out there between the clusters?

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