Galaxy's Dying Stars Confirm Cosmic Ray Source
An image of the Cigar Galaxy, or M82, composed from data from the Hubble Space Telescope, Chandra X-Ray Observatory, and Spitzer Space Telescope.
Credit: NASA, ESA, CXC, and JPL-Caltech

Recent photographs of an active galaxy offer new confirmation for the idea that supernova explosions are the creators of surprisingly energetic particles called cosmic rays.

Cosmic rays are protons that whiz through space at almost the speed of light. They originate far beyond the solar system, but when some make it past Earth's atmosphere they can carry such an energetic punch that they knock out electronics systems. Scientists have long been unsure of the process that ramps the particles up to such great speeds, though gathering evidence suggests that these super-fast protons originate during the death of a star

When a very large star dies, it implodes into a black hole while expelling its outer layers in a powerful explosion called a supernova. Scientists think that some of the energy released during these blasts acts as a sort of supercharger, accelerating particles to extremely fast speeds and creating cosmic rays.

New proof of this idea has come from recent observations of the Cigar Galaxy (M82) by the VERITAS gamma ray telescope array in Amado, Ariz. The galaxy sits about 12 million light-years from Earth in the direction of the constellation Ursa Major (also known as the Big Dipper). It is a starburst galaxy, meaning it has exceptionally high levels of star formation.

With so many stars forming in M82, there are also high numbers of massive stars and supernovas, making the galaxy a perfect testing ground for the cosmic ray explanation. If supernovas are responsible for cosmic rays, scientists figured, then a place with more of the stellar explosions will also have higher concentrations of the energetic particles.

Indeed, researchers found that the density of cosmic rays in M82 was 500 times that of the Milky Way, confirming the hypothesis.

"This discovery provides fundamental insight into the origin of cosmic rays," said Rene Ong, a professor of physics at the University of California, Los Angeles, and the spokesperson for the VERITAS collaboration.

Though the telescope couldn't image cosmic rays directly in the distant galaxy, the researchers looked for gamma rays, a very energetic form of light produced when cosmic rays interact with gas and radiation within the galaxy. They used this measurement to extrapolate how many cosmic rays lie within M82.

The findings are detailed in the Nov. 1 online issue of the journal Nature.