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 discovery. The active regions of starburst galaxies have exceptionally high rates of star formation, and their large sizeĝ€"more than 50 times the diameter of similar Galactic regionsĝ€"uniquely enables reliable calorimetric measurements of their potentially high cosmic-ray density. 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 predicted 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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Acknowledgements 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.