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Frank Krennrich, Physics and Astronomy, (515) 294-3736
David Carter-Lewis, Physics and Astronomy, (515) 294-8269
Skip Derra, News Service, (515) 294-4917
VIOLENT PROCESSES MARK THE EXISTENCE OF 'EXTREME' GALAXIES
AMES, Iowa -- The detection of very high-energy gamma rays from x-ray emitting galaxies may signal the existence of a new constituent in the cosmos: "extreme" galaxies.
The discovery of such galaxies, based on the observations of an international team of astrophysicists working at the Whipple Observatory, Mt. Hopkins, Ariz., confirms the hypothesis that very bright galaxies that strongly emit the highest energy x-rays also emit the most energetic gamma-rays.
"We call these galaxies 'extreme' because they emit most of their power at the highest x-ray energies and at the highest gamma-ray energies ever detected," said Frank Krennrich, an Iowa State assistant professor of physics and astronomy and a Whipple collaborator. "We believe the high energy emissions are fired out in a jet originating near a supermassive black hole. These galaxies' ability to produce such energetic gamma-rays are the result of some of the most violent physical processes in the universe."
Krennrich, Iowa State physics and astronomy professor David Carter-Lewis, and three ISU post doctoral fellows are part of the Whipple Gamma Ray Collaboration. The team presented its findings today (April 4), at the Gamma-Ray Astrophysics 2001 meeting in Baltimore.
The collaboration includes researchers from Iowa State University; Kansas State University, Manhattan; Purdue University, West Lafayette, Ind.; Smithsonian Astrophysical Observatory, Amado, Ariz.; University of California at Los Angeles; University of Chicago; University of Utah, Salt Lake City; Washington University, St. Louis; National University of Ireland, Dublin; and University of Leeds, United Kingdom.
The Whipple group has detected three extreme galaxies. The first was Markarian 501, in 1996, which was confirmed by several other observatories in 1997. The second, 1ES2344+514, gave hints of its nature during observations in 1998, but was not confirmed until recently. A third extreme galaxy, 1H1426+428, was the first selected for observation in gamma rays because of its strong x-ray signal.
With energies many billions of times greater than optical photons (which make up visible light) and wavelengths smaller than the nucleus of an atom, these gamma rays are at the far top end of the electromagnetic spectrum. Gamma rays in the tera-electron-volt, or TeV (1 x 10
eV) energies, like those being emitted from extreme galaxies, can be created on Earth only by collisions of particles in the most powerful accelerators.
"These cosmic powerhouses are more efficient in converting mass into energy than nuclear fusion, and are considered candidates for accelerating the highest-energy particles we know of, cosmic rays with energies of 10
eV," Krennrich said.
The conventional explanation for these gamma-ray galaxies, dubbed "extreme" by Italian theorists at the Osservatorio di Brera, Milan, is that the high-energy emissions are shot out in jets of particles originating in the vicinity of a supermassive black hole. The gamma-rays are emitted in narrow beams which, in the case of these three extreme galaxies, are pointed in our direction.
High-energy gamma-ray telescope observations made with NASA's orbiting Compton Gamma Ray Observatory (CGRO) produced a catalog of some 70 gamma-ray emitting galaxies. Surprisingly, none of these three "extreme" sources were in the catalog, indicating they are their brightest at very high energies, Krennrich said.
What is unusual about these galaxies is that the energies of their gamma rays is 10 to 100 times greater than the CGRO detected galaxies. They are among the highest-energy photons ever detected from cosmic sources.
"Although the galaxies are more than a billion light-years away, emission variations on time-scales of days are clearly seen," said ISU post doctoral fellow Dirk Petry.
"This is a hint to the extremely small size of the region responsible for the emission, which has to be comparable to the solar system despite the tremendous energy output we observe," added ISU postdoctoral fellow Stephan Le Bohec.
Discovery of the TeV gamma-ray emission from these galaxies was made possible by the development of an unusual detection method. The Whipple team developed a novel telescope that combines techniques borrowed from high-energy particle physics, solar energy research and optical astronomy.
"When high-energy gamma-ray photons collide with molecules of oxygen or nitrogen 20 kilometers (12 miles) up in the atmosphere, they produce a shower of electrons and positrons, which we observe as a flash of light lasting a few billionths of a second," explained Carter-Lewis.
"By focusing the light collected with a large optical reflector onto a camera made of fast, sensitive light detectors, the gamma rays are identified and their source in the sky is pinpointed," he added.
Over the past two decades, the sensitivity of these detectors has improved significantly and the technique has been copied by a number of observatories. The Whipple team is now building a new instrument, called VERITAS (Very Energetic Radiation Imaging Telescope Array System), which is an array of seven large reflectors that will provide 20 times the sensitivity of the present instrument.
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