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Young Galaxy's Magnetism Surprises Astronomers

October 02, 2008

Astronomers have made the first direct measurement of the magnetic field in a young, distant galaxy, and the result is a big surprise.

Looking at a faraway protogalaxy seen as it was 6.5 billion years ago, the scientists measured a magnetic field at least 10 times stronger than that of our own Milky Way. They had expected just the opposite.

The scientists made the discovery using the National Science Foundation's ultra-sensitive Robert C. Byrd Green Bank Telescope (GBT) in West Virginia.

"This new measurement indicates that magnetic fields may play a more important role in the formation and evolution of galaxies than we have realized," said Arthur Wolfe, of the University of California-San Diego (UCSD).

At its great distance, the protogalaxy is seen as it was when the Universe was about half its current age. According to the leading theory, cosmic magnetic fields are generated by the dynamos of rotating galaxies -- a process that would produce stronger fields with the passage of time. In this scenario, the magnetic fields should be weaker in the earlier Universe, not stronger.

The new, direct magnetic-field measurement comes on the heels of a July report by Swiss and American astronomers who made indirect measurements that also implied strong magnetic fields in the early Universe.

"Our results present a challenge to the dynamo model, but they do not rule it out," Wolfe said.

There are other possible explanations for the strong magnetic field seen in the one protogalaxy Wolfe's team studied. "We may be seeing the field close to the central region of a massive galaxy, and we know such fields are stronger toward the centers of nearby galaxies. Also, the field we see may have been amplified by a shock wave caused by the collision of two galaxies," he said.

The protogalaxy studied with the GBT, called DLA-3C286, consists of gas with little or no star formation occurring in it. The astronomers suspect that the strong magnetic field may prevent the gravitational collapse that is needed for stars to form.

To make their measurement, the scientists studied radio waves emitted by an even more-distant object, the quasar 3C 286, behind the protogalaxy. As these electromagnetic waves passed through the protogalaxy, some were absorbed by Hydrogen atoms in the protogalaxy. Normally, the atoms would absorb only a single, specific frequency. However, because the atoms were affected by the protogalaxy's magnetic field, they absorbed at two closely-spaced frequencies. This phenomenon, called the Zeeman Effect, allows scientists to measure the strength of the magnetic field affecting the Hydrogen gas through which the waves passed.

The GBT observations of the protogalaxy were the first measurements using the Zeeman Effect made on a celestial object at such a great distance.

Wolfe worked with Regina Jorgenson, a UCSD graduate student; Carl Heiles of the University of California-Berkeley; Timothy Robishaw, a graduate student at UC-Berkeley; and Jason X. Prochaska of the University of California-Santa Cruz. The scientists reported their findings in the October 2 issue of the journal Nature. Their research was supported by grants from the National Science Foundation.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

The National Radio Astronomy Observatory