Scientists on Thursday confirmed that cosmic rays with the highest energies that bombard the Earth originated from outside our Milky Way galaxy.
In a paper published in the journal Science, a group of more than 400 scientists from 18 nations, reported that these cosmic rays do not come uniformly from all directions.
Actually, the prominent arrival direction is from a broad area of the sky about 120 degrees away from the direction that points to the center of our Milky Way galaxy, where some scientists have hypothesized the rays may originate.
"There have been other pieces of evidence, but I would say this paper really confirms that most of the highest energy cosmic ray particles are not coming from the Milky Way galaxy," said Gregory Snow, a University of Nebraska-Lincoln physics professor who serves as education and outreach coordinator for the Pierre Auger Observatory project.
"The Sun emits low-energy cosmic ray particles that are detected here on Earth, but they are nowhere near as high energy as the particles detected at the Auger Observatory," Snow said. "The particles we detect are so energetic they have to come from astrophysical phenomena that are extremely violent."
The new results are based on 12 years of data collection by the largest cosmic-ray observatory ever built, the Pierre Auger Observatory in Argentina.
The observatory collects data from 1,600 particle detectors deployed in a hexagonal grid over 3,000 square kilometers near the town of Malargue in western Argentina.
Although this discovery clearly indicated an extragalactic origin for the particles, the specific sources of the cosmic rays are still unknown.
The new study pointed to a broad area of sky rather than to specific sources because even such energetic particles are deflected by a few tens of degrees in the magnetic field of our galaxy.
The task is even more challenging because the highest energy particles -- those with energies reaching quintillions of electron volts -- reach the Earth at a rate of only one particle per square kilometer each year.
Snow said cosmic rays are clues to the very structure of the universe.
"By understanding the origins of these particles, we hope to understand more about the origin of the Universe, the Big Bang, how galaxies and black holes formed and things like that," he said. "These are some of the most important questions in astrophysics."