Powerful but unknown forces are at work in a small companion galaxy of the Milky Way, astronomers say in the latest issue of the journal Science.

Something is keeping the structure and magnetic field of this galaxy (the Large Magellanic Cloud) strong and ordered, even while the Milky Way's gravity works to tear them apart.

A team led by Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics used the CSIRO Australia Telescope near Narrabri in NSW to study the galaxy's magnetic field.

"This is the most detailed map ever made of another galaxy's magnetism," says Gaensler.

At just 160,000 light-years away the Large Magellanic Cloud, or LMC, is the Milky Way's closest neighbour and is being clawed apart by the Milky Way's gravity. The researchers were surprised that the LMC's magnetic field is so smooth and ordered, given the internal turmoil the galaxy must experience.

"It's like having a birthday party all afternoon for a bunch of 4-year-olds, and then finding the house still neat and tidy when they leave," Gaensler says. "Some powerful forces must be at work to keep the magnetic field from being messed up."

The Milky Way and many other large spiral galaxies have well-organised, large-scale magnetic fields. It's thought that the overall rotation of these galaxies combines and smooths out the small-scale magnetic fields created by whirls and eddies of gas. The process is called a "dynamo," and is similar to the process that produces the Earth's magnetic field.

"But if a galaxy experiences sudden bursts of star formation or supernova explosions, the energy that these processes release should completely disrupt the large-scale magnetic field,"" says Lister Staveley-Smith of the CSIRO Australia Telescope National Facility.

"And we know the LMC has had those kind of violent events over the last several thousand million years," he says.

So what keeps the LMC's magnetic field in order? There are several possibilities, the researchers say, but the process they favour is one driven by extremely energetic particles called "cosmic rays." This would take effect more quickly than the conventional dynamo mechanism. It also requires vigorous star formation to operate, so "stars bursting out at random all over would strengthen the magnetic field, not mess it up," says Gaensler. "You could say this galaxy is thriving on chaos."

Astronomers usually map the magnetic field of a galaxy by observing the polarised optical or radio emission from the galaxy itself. But in this study the researchers looked at how radio emission from background sources was altered as it passed through the Large Magellanic Cloud. More specifically, they measured how far the plane of polarised radio emission was rotated by the LMC's magnetic field ‹an effect called Faraday rotation).

Suitable background sources are few and far between, and so up until now the "rotation measure" technique had been applied only to our Milky Way galaxy and one other, M31. But because the LMC is nearby and looms large on the sky ‹seven degrees across, about 15 times the diameter of the full Moon) the researchers were able to find 291 polarised background sources they could use to probe it. About 100 of these sources lay directly behind the LMC. The others were used to correct for foreground Faraday rotation occurring in our own Galaxy.

The research used archival data from the Australia Telescope Compact Array that was originally gathered to map the location of the neutral hydrogen gas in the LMC.

Magnetic fields are found in planets, stars, galaxies, and perhaps even in the near-empty space between galaxies. They both store and release energy, create enormous electrical currents that circulate throughout space, help form stars, and convert the large-scale motions of galaxies into turbulence and heat. Understanding magnetic fields is essential for understanding how the cosmos works.

Faraday rotation is a promising technique for measuring galactic magnetism, the researchers say. "Future radio telescopes, such as the Extended Very Large Array, and the Square Kilometre Array that's now in the pipeline, will be able to pick up hundreds of times more background sources," says Gaensler.

"That'll give us a handle on the magnetism of most of the local Universe."

This work was supported by the National Science Foundation through grant AST-0307358, and by the University of Sydney through the Denison Fund. The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.