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Cosmic mystery “solved” after decades

Nov. 8, 2007
Courtesy University of Utah
and World Science staff

The most en­er­get­ic par­t­i­cles known, called ul­tra­high-en­er­gy cos­mic rays, pro­b­ab­ly come from su­per­mas­sive black holes in the hearts of bright near­by ga­lax­ies, a new study con­cludes. If cor­rect, that solves a dec­ades-long mys­tery: the source of these sub­a­tom­ic par­t­i­cles, which can slam in­to our at­mos­phere with the en­er­gy of a speed­ing base­ball.

Centaurus A, an active galaxy. (Courtesy European Southern Observatory)
“Ga­lax­ies which host vi­o­lent black holes,” also called act­ive ga­la­xies, now seem to be the cul­prit, said Mi­guel Mos­ta­fa, a Un­ivers­ity of Utah phys­i­cist col­la­bo­rat­ing in the work. This puts sci­en­tists “one step clos­er to know­ing what phys­i­cal pro­cess can ac­cel­er­ate par­t­i­cles” so pow­er­ful­ly, he added. “Right now, we don’t know.”

Mostafa is part of a 17-na­t­ion col­labora­t­ion that oper­ates the $54 mil­lion Pierre Au­ger Ob­serv­a­to­ry in Ar­gen­ti­na, which was used for the stu­dy. The find­ings are to ap­pear in the Nov. 9 is­sue of the re­search jour­nal Sci­ence. 

Black holes are ex­tremely com­pact ob­jects with gra­vity so strong that noth­ing—not even light—can es­cape them. Sci­en­tists be­lieve the cores of most ga­lax­ies, in­clud­ing ours, con­tain su­per­mas­sive black holes, which can con­tain the weight equiv­a­lent of bil­lions of our suns crammed in­to a ti­ny space. They gobble near­by ma­te­ri­al, which in the pro­cess heats up and spews out par­t­i­cles and light be­fore va­nish­ing. Par­tic­u­larly bright ga­la­xy cores are known as ac­tive ga­lac­tic nu­clei.

Cos­mic rays, dis­cov­ered in 1912 by the Aus­tri­an Vic­tor Hess, aren’t really rays at all. They are sub­a­tom­ic par­t­i­cles, in­clud­ing nu­clei of cer­tain atoms, that en­ter the at­mos­phere from space at nearly light speed. Low­er-en­er­gy cos­mic rays come from or­d­in­ary stars. Medium-en­er­gy rays are thought to come from ex­plod­ing stars. But the source of the most pow­er­ful ones—around 100 mil­lion times more en­er­get­ic than any that lab­o­r­a­to­ries on Earth can pro­duce—has been un­ex­plained. 

The highest-en­er­gy cos­mic ray de­tected was meas­ured in 1991 by the Un­ivers­ity of Utah’s Fly’s Eye ob­serv­a­to­ry, ac­cord­ing to sci­en­tists in­volved in the new stu­dy. Its en­er­gy was logged at 300 bil­lion bil­lion elec­tron volts. It thus would have felt like a fast-pitched base­ball had it hit some­one on the head, though it would­n’t have, as the at­mos­phere ab­sorbs most cos­mic rays. They are de­tect­ed by ground in­stru­ments based on show­ers of sec­ond­ary part­i­cles that they give off on im­pact.

In the new stu­dy, sci­en­tists at the Au­ger Ob­serv­a­to­ry—the world’s larg­est for cos­mic rays—found that of the 27 most en­er­get­ic ones de­tected, 20 came from the di­rec­tion of ac­tive ga­lac­tic nu­clei. There’s only a one per­cent chance that such a cor­rela­t­ion could have hap­pened ran­dom­ly, Mos­ta­fa said. Most like­ly, he added, if the rays had been com­ing ran­domly from all di­rec­tions, only five or six would have seemed to come from such ob­jects.

Super-en­er­get­ic cos­mic rays al­so have to come from ga­lax­ies rel­a­tively close by, with­in 326 mil­lion light-years, Mos­ta­fa said. A light-year is the dis­tance light trav­els in a year; but even that many light-years “is our lo­cal neigh­bor­hood in cos­mic terms,” he added.

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The most energetic particles in the universe—ultrahigh-energy cosmic rays—likely come from supermassive black holes in the hearts of bright nearby galaxies, a new study concludes. If correct, the finding solves a decades-long mystery. Researchers have been trying to determine the source of these subatomic particles, which can plunge into our atmosphere with the energy of a speeding baseball. The study found that these come from the hearts of “galaxies which host violent black holes,“ said Miguel Mostafa, a University of Utah physicist collaborating in the new work. The finding, he added, puts scientists “one step closer to knowing what physical process can accelerate particles to these ultrahigh energies. Right now, we don't know.“ Mostafa is one of 370 scientists and engineers belonging to a 17-nation collaboration that operates the $54 million Pierre Auger Observatory in Argentina, which was used for the study. The findings are to appear in the Nov. 9 issue of the research journal Science. Black holes are extremely compact objects with gravity so strong that nothing—not even light—can escape them. Scientists believe most galaxies, including ours, host supermassive black holes, which can contain the weight equivalent of billions of our Suns packed into a tiny space. When they suck in material, the infalling matter heats up and spews out particles and light before disappearing. Particularly bright ones are known as active galactic nuclei. Cosmic rays, discovered in 1912 by the Austrian Victor Hess, are subatomic particles that enter the atmosphere from space, including nuclei of certain atoms. Low-energy cosmic rays come from the sun and other stars. Medium-energy rays are thought to come from exploding stars. But the source of ultra-energetic rays—which are around 100 million times more energetic than any particles that laboratories on Earth can produce—has been unexplained. The highest-energy cosmic ray ever detected was measured in 1991 by the University of Utah's Fly's Eye observatory, according to scientists involved in the new study. Its energy was logged at 300 billion billion electron volts. It would have felt like a fast-pitched baseball had it hit someone on the head, though it wouldn't, as the atmosphere absorbs most cosmic rays. In the new study, scientists at the Auger Observatory—the world's largest for cosmic rays—found that of the 27 most energetic rays detected, 20 came from the direction of active galactic nuclei. There's only a one percent chance that such a correlation could have happened randomly, Mostafa said. Most likely, he added, if the rays had been coming randomly from all directions, only five or six would have seemed to come from these galactic nuclei. Super-energetic cosmic rays also have to come from galaxies relatively close to our own, within 326 million light-years, Mostafa said. A light-year is the distance light travels in a year. “This is our local neighborhood in cosmic terms,“ he added.