Galactic magnetism on a gigantic scale.

Galactic magnetism on a gigantic scale

Astronomers have long known that an entire cluster of galaxies may sometimes lie buried within a vast, dense ball of gas whose temperature exceeds 10 million kelvins. Two teams of radio astronomers have now uncovered evidence that this hot gas also appears associated with a significant magnetic field.

Although such a magnetic field appears much weaker than the Earth's, it extends over an area several million light-years across. And because cluster magnetic fields pack a lot of energy, they may play an important role in cluster evolution as galaxies merge and hot gas gradually collapses toward the clusters' center.

Richard A. Perley and Gregory B. Taylor of the National Radio Astronomy Observatory (NRAO) in Socorro, N.M., studied a bright radio source known as Hydra A. The radio waves come from an apparent massive black hole at the center of a large elliptical galaxy about 1 billion light-years from Earth. This galaxy in turn lies at the center of about 40 smaller galaxies spread across a region some 8 million light-years in diameter.

"What makes Hydra A exceptionally interesting to us is what happens to the radio emission between the time it is emitted and the time we receive it at the [Very Large Array radiotelescope]," Taylor says.

A black hole produces tightly focused beams of highly energetic electrons moving at relativistic speeds. These accelerated particles generate polarized radio waves with electromagnetic fields that tend to vibrate in a certain direction. Because an ambient magnetic field would shift the polarization direction, researchers can detect the field's presence and infer its strength by measuring how much the polarization direction changes as the radio waves pass through the hot, ionized gas.

Radio emissions from Hydra A reveal that if the magnetic field is distributed throughout the cluster, it has a strength of 10 microgauss. Earth's surface magnetic field is roughly 30,000 times stronger.

Jing-Ping Ge and Frazer N. Owen, also at NRAO, combined radio-wave, optical and X-ray observations of A1795, a radio source in a different galaxy cluster, to deduce that this particular cluster has a magnetic field of 40 microgauss. The large cluster, about 800 million light-years from Earth, contains several hundred galaxies held together by gravity.

The origin of the cluster magnetic fields, each about 100 times stronger than expected, remains unclear.

Both groups reported their findings this week at the American Astronomical Society meeting in Albuquerque, N.M.