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See Inside April 2008

Catching No Rays

Missing in action: ultraenergetic cosmic rays from the Virgo cluster

If you’re being shot at, you could do worse than diving into a swimming pool. As the Discovery Channel show MythBusters once demonstrated, even a few feet of water can slow a bullet to nonlethal speeds. A similar drama plays itself out in the cosmos. To particles moving faster than a certain velocity, just shy of the speed of light, the tenuous haze of microwave radiation that fills space might as well be a dense sea. Passing through a few hundred million light-years of it should slow a particle to more moderate speeds.

So astronomers have long puzzled over why such particles, known as ultrahigh-energy cosmic rays (UHECRs), are hitting Earth. If they can punch their way through space without losing speed, something must be wrong with scientists’ understanding of them, perhaps a failure of Einstein’s special theory of relativity [see “Cosmic Power,” by George Musser; Scientific American, January 1999]. Last fall, though, the world’s largest array of cosmic-ray detectors, the Pierre Auger Observatory in Argentina, matched the directions of incoming UHECRs with fairly nearby galaxies. Such an alignment would be unlikely if the particles came from more distant sources and barreled through the universe unimpeded. It looks like Einstein has dodged a bullet.

But what is it about these galaxies that makes them fire off ultrafast particles? On that front a new mystery has arisen. At the American Astronomical Society conference in January, Auger team member Vasiliki Pavlidou of the University of Chicago described how the observatory has seen not one single UHECR coming from the direction of the Virgo cluster of galaxies, a clump of at least 1,000 large galaxies about 60 million light-years away. You name it, Virgo has it: black holes, collapsing stars, dark matter. “Whatever your personal preference might be for the possible sources of ultrahigh-energy cosmic rays, Virgo was guaranteed to have plenty of those,” Pavlidou says.

The lack of UHECRs from Virgo might just be a statistical fluke; another year’s data might bring some to light. “We must be patient,” says another Auger team member, Paul Sommers of Pennsylvania State University. “Virgo may appear.” Cosmic rays are so rare, and the number of possible alignments with galaxies so large, that the team is wary of pinning a number to the statistical significance of their absence. Still, if you live near a Gatling gun, you are bound to hear at least a couple of bullets whiz by, no? “The Virgo deficit is already becoming uncomfortable,” Pavlidou says.

Many researchers are taking the deficit as a much needed clue to what the ultrapowerful sources might be. Supermassive black holes have long been a prime suspect. These monsters gather swirling disks of matter around them, and as they munch away, they shoot out fast-moving jets of gas that flick particles to epic speeds. Most of the galaxies from which Auger detects UHECRs are thought to contain actively feeding holes. A pioneer of this hypothesis, Peter Biermann of the Max Planck Institute for Radio Astronomy in Bonn, Germany, attributes the Virgo deficit to strong magnetic fields in the outskirts of our own galaxy. These fields deflect incoming charged particles and could have caused particles from Virgo to be misattributed to some other source. The amount of deflection may indicate that UHECRs consist not of protons but of highly charged atomic nuclei, argues Susumu Inoue of the National Astronomical Observatory of Japan.

Glennys Farrar of New York University and her colleagues note that supermassive holes in Virgo tend to have undersize disks. Few generate the power required to sling UHECRs. Virgo may be an impressive clump of galaxies, but it is nothing special when it comes to supermassive black holes.

In other ways, too, Virgo could be an unexpectedly wimpy generator of ultra rays. Pavlidou says that the dense packing of galaxies could suppress various conceivable sources. For instance, interactions between galaxies could strip away gas, choke off star formation and reduce the number of exploding stars. Magnetic fields in Virgo itself may trap particles attempting to exit the cluster.

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