ASTRONOMERS in the US have discovered magnetic fields which extend over
a greater volume of space than any known before. The field lines thread
through a cluster of 40 galaxies, a billion light years from Earth. Such
fields may influence the evolution of the cluster.
Richard Perley and Gregory Taylor of the National Radio Astronomy Observatory
in Soccoro, New Mexico, learnt of the existence of the extensive magnetic
fields indirectly, by observing the effect they have on radio waves coming
from a radio source called Hydra A, which is situated in part of the same
Hydra A is associated with a large elliptical galaxy which dominates
the centre of the cluster. From Hydra A, twin jets of plasma – ionised gas
– emerge, forming a two-lobe radio source two million light years across,
spanning a third of the cluster.
Astronomers can use radio emission from Hydra A to detect magnetic fields
because it is polarised; that is, the electric field in the radio waves
oscillates predominantly in a particular plane. When radio waves pass through
a plasma, the plasma’s magnetic field causes the plane of polarisation of
the radio waves to rotate, a phenomenon known as Faraday rotation. The amount
of the rotation is proportional to the strength of the magnetic field, the
distance the radio waves have travelled through the field, and wavelength
of the radio waves.
Taylor and Perley observed polarisation angles at four or five radio
frequencies, and were able to calculate the total rotation as the radio
waves from Hydra A passed through the cluster on their way to Earth.
Taylor says that if magnetic fields are uniform throughout the cluster,
then the average strength of the fields is about 10 microgauss, which is
1/20 000th that of the Earth’s field, but about three times the field strength
within our own Milky Way.
Two other astronomers at NRAO, Jing-Ping Ge and Frazer Owen, have measured
similar magnetic fields in other galaxy clusters. Unlike Hydra A, however,
the fields are not on a scale comparable to that of the clusters.
‘The origin of these cluster magnetic fields is very speculative at
this point,’ says Perley. They could have been stripped from galaxies, or
have been created by dynamo processes within the intergalactic medium in