A high-speed jet of material shoots out of Centaurus A, a nearby galaxy whose central black hole is devouring its surroundings. At least four of the 27 ultra-high-energy cosmic rays detected by Auger appear to originate in Centaurus A.The most energetic particles in the universe have regained some of their former mystery. Last year, it seemed that the origin of these particles had finally been tracked down to a set of giant black holes in nearby galaxies, but a new study casts doubt on that conclusion.
Ultra-high-energy cosmic rays, or UHECRs, are individual sub-atomic particles with energies up to about 1020 electron volts, far beyond anything achieved in particle accelerators.
When they hit the Earth's atmosphere, they produce a shower of other particles, and the Pierre Auger Observatory in Argentina has spotted more of these events than any other detector.
In November 2007, an Auger team looked at the arrival directions of the 27 highest-energy cosmic rays, and found that they fit a suggestive pattern. Most came from within 3° of the directions of nearby active galaxies, which hold supermassive black holes at their cores and emit many kinds of radiation. So it seemed that the galaxies were emitting UHECRs too.
Now researchers led by Igor Moskalenko of Stanford University in California, US, have looked more closely at these particular active galaxies, as well as others along the same line of sight. They find that they are an unremarkable bunch. "The sample consists mainly of low-power active galaxies," says Moskalenko.
Such weak active galaxies are common – so common in fact that astronomers expect to find several within 3° of any random direction on the sky. "The correlation found by the Auger group is likely to be a chance coincidence," Moskalenko told New Scientist.
Too weak
Furthermore, Moskalenko believes that the small, weak active galaxies simply lack the firepower to generate the highest energy cosmic rays. For one thing, they show no signs of high-energy gamma-ray emission, which he believes should go along with cosmic-ray acceleration.
Instead, he suggests that UHECRs are more likely to come from the more energetic breeds of active galaxy, such as quasars and radio galaxies, especially those that squirt out high-speed jets of material, which are already known to emit gamma rays.
Some of the cosmic rays seen by Auger do coincide with examples of this tooled-up type of active galaxy. At least four of them could have been fired at us by Centaurus A, a radio galaxy only 12 million light years away.
Moskalenko and colleagues say the other UHECRs could also be from nearby active galaxies with jets, even if their arrival directions don't coincide with such sources.
Bent paths
That's because these cosmic rays are probably protons or heavier charged particles, whose paths are bent by magnetic fields. Moskalenko points out that the strength and alignment of intergalactic magnetic fields is not well known, so they might be able to bend the paths of cosmic rays beyond 3°. Then, radio galaxies such as Centaurus A could in effect be shooting around the corner at us.
But Auger team member Jim Hinton of Leeds University, UK, disagrees with Moskalenko's interpretation of the data. He says the fact that the galaxies are weak suggests that most should not be able to produce even a single ultra-high energy cosmic ray that could make its way to a detector on Earth.
That explains why Auger has detected only 27 UHECRs, when there are far more active galaxies that theoretically produce them. "Which ones are seen is largely a matter of chance," Hinton told New Scientist.
"This also implies that UHECR-accelerating [active galaxies] are common in nature – ie, they are not just the powerful radio galaxies," Hinton continued. "This implication is something that has surprised (and led to criticism by) many authors – but nature may just be like that!"
Even if the UHECR puzzle is not yet solved, it might be soon – the Auger Observatory, which released its first results in 2005, is almost completed and is amassing cosmic ray data faster than before.
Stephen Battersby
