Intergalactic magnetism runs deep and wide

Hunting for magnetic energy in intergalactic space, researchers have found an unexpected motherlode of it. Both in the gaps between galaxies that are clustered and in the lonelier neighborhoods outside those clusters, magnetic fields are remarkably strong, a scientific team reports.

Computer-enhanced radio sky near the Coma cluster (arrow) shows intergalactic magnetic fields (red, weakest; blue, strongest) and more distant radio sources (compact blue dots). Kronberg et al.

As their hunt widens, the scientists find that they are detecting more extensive fields, says Philipp P. Kronberg of the University of Toronto, leader of the decades-long search. Kronberg reported the results April 29 at an American Physical Society meeting in Long Beach, Calif.

This is evidence of a tremendous energy source that astronomers have overlooked, comments theorist Stirling A. Colgate of Los Alamos (N.M.) National Laboratory. At the meeting, he argued that enormous dynamos powered by black holes have cranked up these intergalactic magnetic fields. Other researchers suspect instead that the fields arose during the early history of the universe.

The magnetic field observations might help researchers understand the origins of the highest-energy cosmic rays and, ultimately, sort out whether magnetic fields have helped shape the universe.

One way that astronomers measure cosmic magnetic field strengths is by detecting how light from more distant quasars rotates its angle of polarization as it travels through a region of interest, say a galaxy or galaxy cluster. The stronger the magnetism there or the denser the gas that the field pervades, the more the field rotates the light’s polarization. To estimate the magnetic field strength, researchers compare the polarizations of quasar light that does and doesn’t pass through the region, and they use data from an X-ray telescope to determine the density of the region’s gas.

A decade ago, Kronberg and other coworkers made such a calculation for Earth’s largest nearby galaxy cluster—the Coma cluster in the constellation Coma Berenices. This cluster lies about 300 million light-years away. The scientists found to their surprise that the cluster’s dilute intergalactic gas had magnetic fields of 2 to 3 microgauss (µG), similar in strength to those in the Milky Way.

Because the Coma cluster has some unusual traits, the investigators remained uncertain about whether most clusters have potent magnetic fields. Other research groups have since measured other cluster fields. Some have reported still higher magnetic fields in regions where gravity strongly compresses a cluster’s gas.

At this week’s meeting, Kronberg described extending the Coma cluster experiment to 24 clusters near Earth, purposely avoiding compressed regions. He finds on the average even higher field strengths, about 5 ?G, than he did a decade ago. “That tells us there’s significant energy in space contained in the [intergalactic] magnetic fields,” he says.

“I’m surprised, very surprised,” says Russell M. Kulsrud of Princeton University, adding that he harbors some doubts that the strengths “are quite as high as [Kronberg] said.” But even if the field strengths are a bit smaller, he adds, “they are still . . . very difficult to explain.”

To investigate the spatial extent of intergalactic fields, the researchers took a different tack. In the presence of a magnetic field, charged particles moving at velocities near the speed of light give off so-called synchrotron radiation. Mapping synchrotron radiation from a patch of sky with a radio telescope indicates both where and how strong magnetic fields are.

About a decade ago, Kronberg and his coworkers had picked up synchrotron-radiation signals indicating field strengths outside the Coma cluster of a hundredth to a few tenths of the cluster’s field strengths. Those first fields to be detected outside a cluster extended millions of light-years beyond the cluster’s bounds.

Radio-telescope improvements since then have made it possible for researchers to search for magnetic fields across a much larger patch of sky and to do so at lower frequencies, which are sensitive to weaker radiation, Kronberg says. In a test of that capability, he and his colleagues used the Very Large Array of radio dishes at Socorro, N.M., to look again at the environs of the Coma cluster.

The new radio image shows essentially the same pattern of extended fields. However, it also contains patches of fields having roughly equivalent strength much further from Coma and extending into the surrounding population of galaxies. The image “confirms that these magnetic fields really exist in intergalactic space,” Kronberg claims.

Calling both sets of findings “very intriguing,” Eugene N. Parker of the University of Chicago insists they offer no easy answers about the origins and influences of cosmic magnetic fields. Rather, he says, they are “a warning flag” indicating that scientists don’t really understand how magnetic fields work.

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