Letter

Nature 462, 770-772 (10 December 2009) | doi:10.1038/nature08557; Received 6 July 2009; Accepted 30 September 2009; Published online 1 November 2009

A connection between star formation activity and cosmic rays in the starburst galaxy M82

The VERITAS Collaboration

  1. Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, Arizona 85645, USA.
  2. Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, Delaware 19716, USA.
  3. Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA.
  4. Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, California 95064, USA.
  5. Physics Department, McGill University, Montreal, Quebec H3A 2T8, Canada.
  6. Department of Physics, Washington University, St Louis, Missouri 63130, USA.
  7. School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK.
  8. Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA.
  9. School of Physics, University College Dublin, Belfield, Dublin 4, Ireland.
  10. School of Physics, National University of Ireland, Galway, Ireland.
  11. Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, Illinois 60605, USA.
  12. Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA.
  13. Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA.
  14. Department of Physics, Grinnell College, Grinnell, Iowa 50112-1690, USA.
  15. Department of Physics and Astronomy, Barnard College, Columbia University, New York, New York 10027, USA.
  16. Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France.
  17. Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA.
  18. Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
  19. Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, Iowa 52242, USA.
  20. Department of Physics and Astronomy, DePauw University, Greencastle, Indiana 46135-0037, USA.
  21. Department of Physics, Pittsburg State University, 1701 South Broadway, Pittsburg, Kansas 66762, USA.
  22. Department of Physics, Anderson University, 1100 East 5th Street, Anderson, Indiana 46012, USA.
  23. Department of Life and Physical Sciences, Galway-Mayo Institute of Technology, Dublin Road, Galway, Ireland.
  24. Kavli Institute for Cosmological Physics, University of Chicago, Chicago, Illinois 60637, USA.
  25. Department of Applied Physics and Instrumentation, Cork Institute of Technology, Bishopstown, Cork, Ireland.
  26. A list of participants and their affiliations appears at the end of the paper.

Correspondence to: Correspondence and requests for materials should be addressed to W.B. (Email: wbenbow@cfa.harvard.edu).

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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