Tiny dwarfs aside, the nearest galaxies to our own are the Large and Small Magellanic Clouds. Between them is a stream of hydrogen known as the Magellanic Bridge. A new study has found that the bridge has a magnetic field. Both the discovery itself, and the way in which it was made, could help us understand why the universe is magnetized on a vast scale.
In the 1950s it was discovered that the Milky Way galaxy has a magnetic field, affecting the stars and dust within it. Gradually it has become clear that this is no anomaly. Not only are other galaxies also magnetized, but vast clusters of galaxies share a common magnetism for reasons we do not understand. Explaining this may provide insight into the formation and evolution of the universe.
This magnetization has been known to extend to the nearby Magellanic Clouds, which orbit our Milky Way, and provide us with the best view of other galaxies. Jane Kaczmarek, a PhD student at the University of Sydney, measured the hydrogen bridge between the two galaxies and found a magnetic field strength of 0.3 x10-6 Gauss. At a little more than a millionth of Earth's average field strength, this isn't the sort of thing you could use to make a dynamo, but its existence is a puzzle worth explaining.
Although the bridge has a few stars within it, the neutral hydrogen that makes up most of its matter doesn't emit light. The field was detected from its effect on light passing through it, in the form of radio waves from 124 enormously distant galaxies that pass through the bridge to get to our telescopes. Comparing these with 45 other galaxies whose radiation didn't have to pass through the bridge indicated a consistent difference in polarization that only makes sense if the bridge carries a magnetic field.
“It's only recently that we have become able to detect a field this weak," Kaczmarek told IFLScience. "This is the first time [an astronomical] field has been detected not through magnetization's effect on its own light.” The discovery has been published in Monthly Notices of Royal Astronomical Society.
Kaczmarek explained to IFLScience that the only explanation for the magnetization she and her co-authors think makes sense is for it to have originated in the clouds themselves. The bridge is thought to have formed from hydrogen pulled out of each cloud by the gravity of the other, possibly with tidal interactions with the Milky Way playing a part. “We think the magnetic fields get pulled out of the clouds as well, this changes the alignment of the field, but the field itself stays,” she said. Kaczmarek hopes the technique will shed light on the magnetization of superclusters.