A radio telescope that stands taller than the Statue of
Liberty has detected an unexpectedly strong magnetic field as it appeared 6.5
billion years ago in a young galaxy.
Astronomers have long thought that magnetic fields grow
very gradually from slowly rotating galaxies over 5 billion to 10 billion years,
but the strong field finding may force some rethinking. Detected in a distant
protogalaxy, it measures at least 10 times greater than the average
magnetic field in the Milky Way.
"This was a complete surprise," said Arthur Wolfe, a
physicist at the University of California, San Diego, who headed the team that
hailed from several UC campuses. "The magnetic field we measured is at least an
order of magnitude larger than the average value of the magnetic field detected
in our own galaxy."
Magnetic fields help control the rate of star formation
and the dynamics of interstellar gas within galaxies. But until recently,
astronomers knew very little about magnetic fields outside our own galaxy. They
had previously measured a weaker magnetic field in only one nearby galaxy.
However, a different team of Swiss and American
astronomers previously made indirect measurements suggesting that the magnetic
fields of young galaxies were as strong back
in the early universe as they are in mature galaxies today.
The team behind the new study, detailed in the Oct. 2
issue of the journal Nature, used the world's largest fully steerable
radio telescope for their measurements — the Robert C. Byrd Green Bank
Telescope in Green Bank, W. Va., operated by the National Science Foundation's
National Radio Astronomy Observatory. The finding represents the first direct
measurement of an early galaxy's magnetic field.
The young protogalaxy they probed, DLA-3C286, is located
in a region of the northern sky that is directly overhead during the spring in
the northern hemisphere.
Wolfe said the earlier indirect measurements and his team's
latest direct measurement of a distant galaxy's magnetic field do not
necessarily cast doubt on the current leading theory of magnetic field
creation. That mean-field-dynamo model predicts that magnetic field strengths
started out much weaker than they are today.
"Our results present a challenge to the dynamo
model, but they do not rule it out," he added. "Rather the strong
field that we detect is in gas with little, if no, star formation, and an
interesting implication is that the presence of the magnetic fields is an
important reason why star
formation is very weak in these types of protogalaxies."
Wolfe said his team has two other plausible explanations
for what they observed.
"We speculate that either we are seeing a field
toward the central regions of a massive galaxy, since magnetic fields are known
to be larger towards the centers of nearby galaxies," Wolfe said. "It
is also possible that the field we detect has been amplified by a shock wave
generated by the collision between two galaxies."
Astronomers hope to extend their magnetic field
observations further to better understand their influence on the evolution
of galaxies.
"The challenge now is to perform observations like
these on galaxies throughout the universe," said J. Xavier Prochaska,
another team member and a UC Santa Cruz astronomer.
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