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Wednesday, 1 August, 2001, 20:47 GMT 21:47 UK
Astronomers see 'cosmic camp fire'
Nasa
A halo of hot gas surrounding hot stars in a nearby galaxy
By BBC News Online science editor Dr David Whitehouse

The orbiting Chandra X-ray observatory has detected the first definite evidence for a giant halo of hot gas around a spiral galaxy much like our own Milky Way.

The discovery may lead to a better understanding of our own star system, as well the structure and evolution of galaxies in general.

The astronomers observed the spiral galaxy NGC 4631 which is about 25 million light-years from Earth with both Chandra and the Hubble Space Telescope.

Previous observations of NGC 4631 have detected diffuse X-ray emission from it and other spiral galaxies. However, Chandra's detailed view enables astronomers to be able to separate the individual X-ray sources in the galaxies from the diffuse halos.

Chandra found the diffuse halo of X-ray gas in NGC 4631 to be radiating at a temperature of almost three million degrees and extending some 25,000 light-years from the galaxy.

'Gas outflows'

"Scientists have debated for over 40 years whether the Milky Way has an extended corona, or halo, of hot gas," said Daniel Wang of the University of Massachusetts, US.

"Of course since we are within the Milky Way, we can't get outside and take a picture. However, by studying similar galaxies like NGC 4631, we can get an idea of what's going on within our own galaxy."

Stsci
Hubble's image of NGC 4631
Astronomers say that one important feature of the X-ray emission from NGC 4631 is that it closely resembles the overall size and shape seen in the radio emission from the galaxy.

This indicates that there may be a close connection between the outflows of hot gas, seen in X-rays, and the galaxy's magnetic field, revealed by radio emission.

The Hubble image of NGC 4631 shows filamentary structures enclosing X-ray-emitting gas. The data shows the hot gas is heated by clusters of massive stars and is now expanding into the halo of the galaxy.

"What we see in NGC 4631 can be thought of as the bursting flames of a gigantic cosmic camp fire," said Wang.

"Using Chandra and Hubble together, we really get a complete story of what is happening in this galaxy."

See also:

15 Mar 01 | Sci/Tech
Astronomical hunt ends in success
12 Jul 00 | Sci/Tech
Dim star wakes up
13 Sep 00 | Sci/Tech
Middleweight black hole discovered
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