For the first time, scientists have confirmed that the highest energy cosmic rays that bombard the Earth come from outside the Milky Way Galaxy.
In an article to be published Sept. 22 in the journal Science, a group of more than 400 scientists from 18 nations describe how they detected an anisotropy, an asymmetry in the cosmic particles' distribution of arrival directions as they approach the Earth.
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 new results are based on 12 years of data collection by the Pierre Auger Observatory, built in the plains of western Argentina in 2001 specifically to learn more about cosmic rays and where they come from. The observatory collects data from 1,600 particle detectors deployed in a hexagonal grid over 1,160 square miles - an area larger than Rhode Island. A set of telescopes also is used to observe the faint fluorescence light that the charged particles emit at night.
Snow says 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."
As the Science article explains, ultrahigh energy cosmic rays have been observed for more than 50 years, but their sources remain a mystery. The best hope of finding their origins is to study their directions of travel as they approach the Earth - but that is surprisingly difficult.
Because they are charged particles, they interact with the magnetic fields of the Milky Way and beyond. The intergalactic magnetic fields deflect the cosmic ray particles by a small amount from their directions of origin. The task is even more challenging because the highest energy particles - those with energies reaching quintillions of electron volts - reach Earth at a rate of only one particle per square kilometer each year.
"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. Some galaxies have an explosive, massive black hole in their centers and there are theories that these very violent centers accelerate particles of very high energy that eventually reach Earth."
The Auger Observatory was created under the leadership of James Cronin, a Nobel Prize-winning physicist from the University of Chicago who died in 2016, and Alan Watson from the University of Leeds in the UK.
Although Snow is the only University of Nebraska professor who is part of the Pierre Auger collaboration, he and Nebraska Physics Department Chair Dan Claes have a special interest in cosmic rays. In 2000, they started to deploy cosmic ray detectors throughout the state for the Cosmic Ray Observatory Project. The National Science Foundation-funded effort enlists high school students and their teachers to collect research quality data on cosmic rays. The project uses equipment donated by Cronin after he completed a previous cosmic-ray research project. It was that relationship that led Cronin to invite Snow to join the Auger Observatory, Snow said.