Image of the emission from M82 at a wavelength of 0.45 mm, and polarization results at 0.85 mm wavelength. The red and blue arrows show the observed magnetic field directions and the white dashed curves outline the magnetic bubble structure. The long white arrows depict the direction of the wind from the center of the galaxy. Photo: JAC

A giant magnetic 'bubble' measuring 3000 light years (18 trillion miles) across has been discovered in a nearby galaxy by a team of astronomers in Hawaii. Nothing similar has ever been seen before and it sheds new light on our understanding of how "starburst" galaxies evolve.

The astronomers from the Joint Astronomy Centre were mapping the magnetic structure of galaxy M82 in order to see stars being born in the smouldering gas clouds at the very heart of this active starburst region when they detected the magnetic bubble.

"We were really surprised to see the huge bubble," said British astronomer Jane Greaves," this is a new feature of galaxies that we didn't know about before and could show how magnetic fields help shape the evolution of starburst regions.

"The most likely explanation for the bubble is that enormously energetic winds -- outflows of interstellar gas powered by stars and supernovae -- are forcing the magnetic field out into the halo of the galaxy.

Galaxy M82 makes up to fifty times more stars than other galaxies, but the reason for this remains unexplained. "One of the most exciting things," said team member Wayne Holland, "is that we see some magnetic field lines pointing right into the nucleus of the galaxy, and since the (ionised) particles in gas clouds tend to flow along the lines of magnetic force, then we may have a clue as to why this galaxy has such a predominance of star-forming activity at its centre."

Optical image of M82.

The team used the 15 metre James Clerk Maxwell Telescope equipped with a revolutionary new camera SCUBA, which was built by the Royal Observatory Edinburgh, now the UK Astronomy Technology Centre. The polarimeter used was built by Queen Mary and Westfield College, London. The scientists used a new technique that detects tiny differences in emission from interstellar dust. They discovered that the dust grains are lined up around local magnetic fields, just like iron filings around an ordinary magnet.

The next step will be to use this 'sub-millimetre' technique to look at other galaxies, to see whether M82 is a freak or if other nearby starburst galaxies show the same thing.

Professor Ian Halliday, Chief executive of the Particle Physics and Astronomy Research Council, PPARC, said "British scientists are leading the field of sub-millimetre astronomy. The JCMT/SCUBA combination represents the largest telescope in the world dedicated to this type of observation. The UK is continuing its commitment to this field by investing in the new ALMA submillimetre array in the Atacama Desert. ALMA, which begins construction in 2002, will enable scientists to see 10 times more clearly at this wavelength and will take our understanding of galaxy formation to a new level."