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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.
B.M. Gaensler, M. Haverkorn, L.Staveley-Smith, J.M. Dickey, N.M. McClure-Griffiths, J.R. Dickel and M. Wolleben. “The magnetic field of the Large Magellanic Cloud revealed through Faraday rotation.” Science, vol. 307, no. 5715 (11 March 2005).
Bryan Gaensler, Harvard-Smithsonian Center for Astrophysics
+1-617-869-7153 (mob)
+1-617-496-7854 (office)
bgaensler@cfa.harvard.edu
Lister Staveley-Smith, CSIRO Australia Telescope National Facility
+61-2-9372-4271 (office)
+61-425-212-592 (mob.)
Lister.Staveley-Smith@csiro.au
Helen Sim, CSIRO Australia Telescope National Facility
+61-419-635-905 (mob)
Helen.Sim@csiro.au