Iowa State researchers contribute to discovery of gamma rays from starburst galaxy

November 02, 2009

AMES, Iowa - Iowa State University astrophysicists contributed to the recent discovery that a galaxy quickly creating new stars is also a source of high energy gamma rays.

The discovery has just been published by the journal Nature. The study reports that researchers using the VERITAS array of four telescopes at the Fred Lawrence Whipple Observatory in Arizona have detected gamma rays of a trillion electron volts coming from the M 82 galaxy. The corresponding author of the article is Wystan Benbow of the Harvard-Smithsonian Center for Astrophysics and the Whipple Observatory.

Researchers discovered cosmic rays (mostly hydrogen nuclei) from space nearly a century ago and have developed theories about their origins in supernova remnants and star-forming galaxies, but hadn't found evidence to support those theories.

"This is a step toward solving a 100-year-old puzzle in cosmic ray physics," said Frank Krennrich, an Iowa State professor of physics and astronomy and a collaborator on the VERITAS project.

Gamma rays are high energy electromagnetic radiation. The rays discovered by the VERITAS researchers have a trillion times the energy of visible light. M 82 is a galaxy in the direction of the Ursa Major constellation that's 12 million light years from Earth. It is classified as a starburst galaxy. Such galaxies are colliding with other galaxies, causing shockwaves that compress gases and create stars at very high rates.

"What this shows is that there is a strong connection between a galaxy with high star formation, high gas density and the production of cosmic rays," Krennrich said.

But Krennrich said there's more work to be done to definitively trace gamma rays to cosmic rays in starburst galaxies.

Researchers believe more knowledge of gamma rays could help them explore distant regions of space, help them look for evidence of dark matter, determine how much electromagnetic radiation the universe has produced and answer questions about the formation of stars and galaxies.

Krennrich said one key to current gamma ray research is the VERITAS telescope system (that's the Very Energetic Radiation Imaging Telescope Array System). The $20 million instrument started operating in 2007 and is the world's most sensitive instrument for detecting gamma rays.

It's not easy to detect those rays. Even with their high energies, gamma rays can't penetrate the earth's atmosphere. When they hit the atmosphere, they create showers of electrons and positrons that create a blue light known as Cerenkov radiation. Those showers move very fast. And they're not very bright.

VERITAS looks for the rays with four reflector disks 12 meters across that look like satellite dishes. The reflectors are covered with mirrors that direct light into cameras attached to the front of each disk. Each camera is about 7 feet across and contains about 500 tube-shaped photon detectors or pixels.

All those detectors were built in a laboratory on the fourth floor of Iowa State's Zaffarano Physics Addition. The assembly took about $1 million and a lot of work by Iowa State post-doctoral researchers Martin Schroedter and Tomoyuki Nagai.

The telescope system is based on techniques Iowa State researchers Richard Lamb and David Carter-Lewis helped develop in the 1980s.

And now Krennrich says researchers are contemplating the next generation of gamma ray detection systems.

Krennrich said researchers are assembling a worldwide collaboration to plan and build a $300 million, 36-telescope array. The new instrument would be known as AGIS (the Advanced Gamma-ray Imaging System) and would be 10 times more sensitive than VERITAS.

Krennrich said Iowa State researchers are working on image-recognizing technology for the AGIS system that would help researchers by automatically separating gamma ray events from background events.

The new instrument, Krennrich said, might finally produce the data that establishes the origins of gamma rays and cosmic rays.
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Iowa State University

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