M82 - TeV gamma-ray emission from a new class of source: starburst galaxies
Reference: V. A. Acciari et al. (The VERITAS Collaboration), Nature, volume 472, 770-772, 2009.
ArXiv version: ArXiV:0911.0873
Contact person: Wystan Benbow
M 82 is the prototype small starburst galaxy and is widely predicted to be the brightest starburst galaxy in gamma rays. This article reports the discovery of hard spectrum (photon index of 2.5), >700 GeV gamma-ray emission from M 82 during an intense (~140 h) observation campaign carried out over two years. M 82 is among the weakest VHE sources ever detected (~0.9% Crab) and is only one of two starburst galaxies known to emit gamma-rays. It is also one of the few extragalactic gamma-ray sources not associated with an active galactic nucleus.
The active regions of starburst galaxies have exceptionally high rates of star formation. The cosmic rays produced in the formation, life, and death of their massive stars are expected to eventually produce diffuse gamma-ray emission via their interactions with interstellar gas and radiation. From the VERITAS data we determine a cosmic-ray density of 250 eV cm-3 in the starburst core of M 82, or about 500 times the average Galactic density. This result strongly supports that cosmic-ray acceleration is tied to star formation activity, and that supernovae and massive-star winds are the dominant accelerators.
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