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Seeking the source
of cosmic rays

Posted: November 03, 2009

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New results from the VERITAS telescope array and the Fermi space telescope show that cosmic rays – subatomic particles that race through space at nearly the speed of light – are likely powered by exploding stars and stellar winds.

M82, also known as the Cigar Galaxy, lies 12 million light-years away in the constellation Ursa Major. Fermi’s LAT and the ground-based VERITAS observatory have detected diffuse gamma rays from the galaxy’s core, which produces stars at a rate ten times faster than our entire galaxy. Image: NASA/ESA/Hubble Heritage Team (STScI/AURA).

Cosmic rays are made up of electrons, positrons and atomic nuclei and they constantly bombard the Earth, however, their origin has remained a mystery for nearly a century since their discovery. Instruments can infer the presence of cosmic rays by looking for the glow of gamma ray emission that results from cosmic ray particles striking interstellar gas or radiation.

Since theory holds that supernovae and stellar winds from massive stars are the dominant accelerators of cosmic-ray particles, starburst galaxies – those with excessively high rates of star formation – should be ideal candidates for generating more cosmic rays than others. Both ground-based VERITAS (Very Energetic Radiation Imaging Telescope Array System) and space-based Fermi measurements support this long-lived theory.

Fermi’s Large Area Telescope (LAT) shows that an intense star-forming region in the Large Magellanic Cloud named 30 Doradus is also a source of diffuse gamma rays. Brighter colours indicate larger numbers of detected gamma rays. Image: NASA/DOE/Fermi LAT Collaboration.

“Starburst galaxies have not been accessible in gamma rays before,” says Fermi team member Seth Digel. “Most of the galaxies Fermi sees are exotic and distant blazars, which produce jets powered by matter falling into enormous black holes. But these new galaxies are much closer to us and much more like our own.”

M82, the Cigar Galaxy, located 12 million light years away is one such galaxy studied by both Fermi and VERITAS. The ground-based array indicates that the cosmic ray density in M82 is a staggering 500 times more than the average density in our own Milky Way Galaxy. “The core of M82 forms stars at a rate ten times greater than the entire Milky Way galaxy,” says Niklas Karlsson, a VERITAS team member. “These very-high-energy gamma rays probe physical processes in other galaxies that will help us understand how and where cosmic rays become accelerated.”

This mosaic of 1,500 images highlights glowing gas clouds in the Large Magellanic Cloud, a galaxy that orbits our own. Fermi sees gamma-ray emission from the large, prolific star-forming region known as 30 Doradus (centre left). Image: C. Smith, S. Points, the MCELS Team and NOAO/AURA/NSF.

Fermi also studied starburst galaxy NGC 253 and star-forming region 30 Doradus. Located 170,000 light years away in the Large Magellanic Cloud (LMC), 30 Doradus hosts an intense source of gamma rays emanating from the heart of the star-forming factory. Moreover, Fermi showed that the LMC’s brightest diffuse emission remains close to 30 Doradus and doesn’t extend across the galaxy, implying that the stellar factory itself is the source of the cosmic rays producing the glow.

“Star-forming regions produce lots of massive, short-lived stars, which explode when they die,” says Digel. “The connection makes sense.”

Fermi collaborator Jurgen Knodlseder adds, “The tangled magnetic fields near 30 Doradus probably confine the cosmic rays to their acceleration sites.” Knodlseder presented the findings at the 2009 Fermi Symposium being held in Washington this week.

The discoveries provide fundamental insight into the origin of cosmic rays and suggests that the Universe is awash with natural particle accelerators.

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