Young galaxy’s magnetism surprises astronomers
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 02, 2008

Astronomers have made the first direct measurement of the magnetic field in a young, distant galaxy, and found it to be a surprising 10 times stronger than that of our own Milky Way. The results are reported in the 2 October issue of the journal Nature.

The young ‘protogalaxy’ was seen as it was 6.5 billion years ago, about half the Universe’s current age, by scientists using the National Science Foundation's ultra-sensitive Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. In order to study the galaxy, called DLA-3C286, the astronomers focussed on the radio waves emitted by quasar 3C 286, located behind the galaxy. As the electromagnetic waves passed through the protogalaxy, some were absorbed by hydrogen atoms in DLA-3C286. Normally, the atoms would absorb only a single, specific frequency, but 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 100m Green Bank Telescope is located at the National Radio Astronomy Observatory's site in Green Bank, Pocahontas County, West Virginia. Image: NRAO.

The GBT observations of the protogalaxy were the first measurements using the Zeeman Effect made on a celestial object at such a great distance. "This new measurement indicates that magnetic fields may play a more important role in the formation and evolution of galaxies than we have realised," says Arthur Wolfe, of the University of California-San Diego.

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, and therefore weaker magnetic fields in the earlier Universe. The new results present a challenge to this dynamo model, but there could be a reasonable explanation for the protogalaxy’s out of character behaviour. "We may be seeing the field close to the central region of a massive galaxy, and we know such fields are stronger toward the centres of nearby galaxies,” says Wolfe. “Also, the field we see may have been amplified by a shock wave caused by the collision of two galaxies.”

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.

"In either case," says Wolfe, "our detection indicates that magnetic fields may be important factors in the evolution of galaxies, and in particular may be responsible for the low star formation rates detected throughout the gaseous progenitors of young galaxies in the early Universe."

Another team of astronomers reported similar results in the 17 July issue of Nature, detailing indirect measurements of the magnetic fields of 20 distant galaxies using the bright light from quasars, suggesting that the magnetic fields of young galaxies were as strong when the Universe was only a third of its current age as they are in mature galaxies today.

The challenge now will be to perform observations like these on galaxies throughout the Universe.