sealPurdue News

April 18, 1997

New gamma-ray count challenges astronomical theories

WEST LAFAYETTE, Ind. -- Scientists have discovered recently that there are fewer low-energy photons in the universe than previously thought, an observation that may alter the way astronomers think about how galaxies were formed.

An international collaboration of astrophysicists has found that more high-energy gamma rays are reaching Earth than expected from a distant galaxy, Markarian 421, indicating that the gamma rays are not interacting with and being absorbed by quite as many low-energy photons as they travel through space.

"Using a new observational technique, we saw many more very-high-energy gamma-ray photons from this source than we thought we would," says James Gaidos, professor of astrophysics at Purdue University and a member of the research team.

"We had believed there were more low-energy photons out there to absorb the gamma rays, but so many are getting through to us from such a large distance that it appears there's much less interaction taking place," he says.

John Finley, a member of the research team and assistant professor of physics at Purdue, says, "Low-energy photons were created in the universe during the time of galaxy formation, and the number of such photons we observe, directly or indirectly, tells us how the galaxies formed.

"A reduced number of such photons has direct implications for current theories of the history of the universe, and in particular for galaxy formation," he says.

Gamma rays are highly energetic photons, and very-high-energy gamma rays carry with them energies of trillions of electron volts, a level that is hundreds of billions of times greater than visible light.

Gaidos says the latest observations present a mystery -- explaining how the gamma rays, once produced, can escape from the environs of the galaxy without interacting with lower-energy photons and degrading in energy. Once clear of the galaxy, the gamma rays must traverse vast regions of intergalactic space where they could be absorbed by interacting with infrared radiation from galaxies formed in the early universe.

The results were presented today (Friday, 4/18) at the meeting of the American Physical Society in Washington, D.C., by Jeff Zweerink of Iowa State University for the Whipple Collaboration. The collaboration is a team of scientists from the Smithsonian Astrophysical Observatory, Iowa State and Purdue Universities, University College in Dublin, Ireland, and the University of Leeds in the United Kingdom.

Markarian 421 is a giant elliptical galaxy 400 million light-years from Earth. Scientists presume that the energy engine powering Markarian 421 is a super-massive rotating black hole at the core of the galaxy, and they theorize that the gamma rays may originate in very fast-moving jets of matter that erupt from the region surrounding the black hole.

"Fortunately for us, the jet from this galaxy just happens to be pointed right at Earth, which gives us a much better picture of the true numbers of gamma rays associated with the jet," says Finley.

In the past four years, the team of astronomers working at the Smithsonian Institution's Fred Lawrence Whipple Observatory in Arizona has observed high-energy gamma-ray bursts coming from Markarian 421. The results presented today are based on data from an unusually powerful flare of very-high-energy gamma rays from Markarian 421 that was observed last May.

The Whipple research team pioneered a technique that differentiates these gamma rays from background radiation originating from various sources in space.

"Instead of using ground-based instruments to look at Markarian 421 when it's directly overhead, we looked at the galaxy while it was on the horizon," Gaidos explains. "From this position, the atmosphere is thicker and absorbs more of the background radiation, which has much less energy than these gamma rays.

"This technique allowed us to get a much cleaner signal and a more accurate count of the number of very-high-energy gamma rays coming from this source. It turned out to be much more than we expected," he says.

The gamma rays are detected as they strike the upper atmosphere. Their collision with an air molecule generates a cascade of light-emitting particles that can then be detected by large optical detectors, such as the Whipple Observatory's 10-meter optical reflector.

Sources: James Gaidos, (765) 494-5171; e-mail,
John Finley, (765) 494-5048; e-mail,
Writers: Amanda Siegfried, (765) 494-4709; e-mail,
Dan Brocious, Smithsonian Astrophysical Observatory, (520) 670-5706
Purdue News Service: (765) 494-2096; e-mail,

NOTE TO JOURNALISTS: News releases announcing the Whipple Observatory Gamma Ray Collaboration's discovery are being issued simultaneously by the Harvard-Smithsonian Center for Astrophysics, Iowa State University and Purdue University.

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