Although Galactic cosmic rays (protons and nuclei) are widely believed to be mainly accelerated by the winds and supernovae of massive stars, definitive evidence of this origin remains elusive nearly a century after their discovery1. The active regions of starburst galaxies have exceptionally high rates of star formation, and their large size—more than 50 times the diameter of similar Galactic regions—uniquely enables reliable calorimetric measurements of their potentially high cosmic-ray density2. The cosmic rays produced in the formation, life and death of massive stars in these regions are expected to produce diffuse γ-ray emission through interactions with interstellar gas and radiation. M82, the prototype small starburst galaxy, is predicted3,4 to be the brightest starburst galaxy in terms of γ-ray emission. Here we report the detection of >700-GeV γ-rays from M82. From these data we determine a cosmic-ray density of 250 eV cm-3 in the starburst core, which is about 500 times the average Galactic density. This links cosmic-ray acceleration to star formation activity, and suggests that supernovae and massive-star winds are the dominant accelerators.
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This research is supported by grants from the US Department of Energy, the US National Science Foundation and the Smithsonian Institution, and by the National Science and Engineering Research Council of Canada, Science Foundation Ireland and the UK Science and Technology Facilities Council. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and the institutions that collaborated in the construction and operation of the VERITAS array.
Author Contributions VERITAS is a collaboration of scientists who jointly participate in all aspects of the scientific effort: the collecting of data, the development of software for analysis and simulations, the analysis of data and the interpretation of the results. Every author has read the paper and agrees with the results.
A list of participants and their affiliations appears at the end of the paper.
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The VERITAS Collaboration. A connection between star formation activity and cosmic rays in the starburst galaxy M82 . Nature 462, 770–772 (2009). https://doi.org/10.1038/nature08557
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