Cosmic-Ray Source Discovered

Patience is a virtue—especially when solving scientific problems. In 1912, Victor Hess confirmed that a steady stream of energetic cosmic rays bombard the top of Earth’s atmosphere. Nearly 100 years later, a team of scientists uncovered one of the prime sources of these cosmic rays. And the discovery shows one more way Earth’s location is fine-tuned to support life.

Although the last century witnessed tremendous advances in our knowledge of cosmic rays, their source(s) remained elusive. Because cosmic rays are charged, the magnetic field of the Milky Way Galaxy (MWG) bends the paths of the cosmic rays as they travel through the galaxy. Consequently the direction detected at the top of Earth’s atmosphere gives no information on the location where a cosmic ray originated. However, the processes believed to produce cosmic rays also generate gamma rays that are unaffected by the galactic magnetic field.

Recently, an international collaboration using the most powerful gamma-ray telescope in the northern hemisphere found evidence of cosmic ray production in a “starburst galaxy” known as M82. This galaxy is undergoing a large burst of star formation and, consequently, contains numerous supernovae as well as massive stars exhibiting strong winds. The explosions and winds accelerate cosmic rays that subsequently emit gamma rays.

Although the flux of gamma rays emitted in this fashion is well below background levels, observations from the VERITAS telescope array found the distinctive gamma ray signature from M82. The flux of gamma rays indicates that the cosmic ray density in M82 exceeds that of the MWG by a factor of 500.

Not only does this result go a long way toward answering the century old question of the source of cosmic rays, it also shows that not all galaxies are amenable to life. The excessive cosmic-ray density of M82 likely renders the galaxy uninhabitable. In contrast, the MWG exhibits enough star formation to permit planet formation but not so much as to generate a detrimental quantity of cosmic rays.