Giant Magnetic Bubble discovered in nearby galaxy

HILO, HAWAII -- A team of astronomers from the Joint Astronomy Centre (JAC) in Hawaii today announced the first image of a magnetic field in star-formation regions of another galaxy. M82 is one of the closest `starburst' galaxies, with dozens of very active sites around the nucleus where stars are being born. The new discovery shows a giant magnetic `bubble' 3000 light years across, apparantly blown outwards by the superwind from the galaxy's stars and supernovae.

`This is the first time we've been able to see right into the heart of the star-forming activity and image the magnetic structure', said JAC astronomer Jane Greaves, who led the research team. By observing at short radio wavelengths of about a millimetre, they can probe through obscuring interstellar dust clouds that block out the nucleus in traditional optical images.

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 centre of the galaxy.

The team was surprised to see the huge `bubble' outlined in the image. The most likely explanation 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. `One of the most exciting things', said team member Wayne Holland, `is that we see some field lines pointing right into the nucleus'. `Magnetic fields can help gas clouds fall inwards, so we may have a clue to why this galaxy has such a condensation of star-forming activity near the centre.'

The astronomers used a new technique that detects tiny differences in emission from interstellar dust, by looking at different angles on the sky. The dust grains are lined up by local magnetic fields, just like iron filings around an ordinary magnet. The differencing technique, millimetre-wavelength polarimetry, has never before been used to look at another galaxy.

M82 is one of our closer galaxy neighbours, at a distance of about 11 million light years. It is object number 82 in the famous catalogue of `fuzzy objects' compiled by Messier in 18th century. The starburst activity was most likely triggered by a close flyby of the neighbour galaxy M81.

How was the new image obtained?

The new image was obtained using the 15-metre James Clerk Maxwell Telescope at the Mauna Kea Observatory in Hawaii. The JCMT is the world's largest telescope dedicated to the study of light at `submillimetre' wavelengths. The team of astronomers used a revolutionary new camera called SCUBA (Submillimetre Common User Bolometer Array), which was built by the Royal Observatory in Edinburgh (now the UK Astronomical Technology Centre). The Polarimeter was built by Queen Mary and Westfield College in London, and funded by a joint science initiative of the UK and Japan.

SCUBA uses detectors cooled to a tenth of a degree above absolute zero (-273 degrees Celsius) to measure the tiny amounts of heat emission from small dust particles at a wavelength close to one millimetre. SCUBA by itself detects both of the two perpendicular waves (`planes of polarization') of which light of any wavelength is made up. The Polarimeter uses a very fine (6 micron spacing) grid that passes only one plane, and a bi-refringent quartz plate that rotates the source polarization. Together these produce slightly different images every 30 seconds, that are analysed to measure the magnetic field directions.

Notes

The JCMT is operated by the Joint Astronomy Centre, on behalf of the UK Particle Physics and Astronomy Research Council, the Netherlands Organisation for Scientific Research, and the National Research Council of Canada.