Scientists in China have revealed a new map of the Milky Way, painted in gamma glow, which challenges conventional theories about the universe while also adding a piece to the century-old puzzle of the origins of cosmic radiation.
Using data collected by the Large High Altitude Air Shower Observatory (LHAASO) on Haizi Mountain, 4,400m (14,435 feet) above sea level in western China’s Sichuan province, a team led by researchers from the Purple Mountain Observatory in Nanjing found that our galaxy’s disc was shining with gamma rays at ultra-high energies of up to one quadrillion electron volts (PeV).
The radiation is mainly produced by cosmic rays as they zip through space at nearly the speed of light and collide with interstellar gas. But the scientists wrote in the journal Physics Review Letters on October 9 that instead of being cut off beyond certain points as theoretical models have predicted, the radiation spans a continuous energy spectrum.
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“The discrepancy means existing models cannot fully account for LHAASO’s observation results and may need major modifications,” the researchers wrote in a Chinese press release.
Kazumasa Kawata, of the University of Tokyo’s Institute for Cosmic Ray Research in Kashiwa, Japan, said the findings would “give new insight into the propagation, interaction processes and origin of the highest energy cosmic rays in our galaxy”.
Ever since cosmic rays were discovered by Austrian physicist Victor Hess in 1912, scientists have built detectors in space and on the ground to hunt for these mysterious, powerful particles from outer space, Kawata wrote in Physics magazine.
The highest energy cosmic rays ever detected carried over a quintillion electron volts – which is equivalent to the kinetic energy of a baseball travelling at 100 miles per hour (160km/h), or millions of times more energetic than the particles created in the largest particle collider on Earth.
Today, scientists still do not have a clear answer as to where cosmic rays actually come from. As they mainly consist of protons, their electrically charged nature means they do not travel in a straight line, instead being deflected by the universe’s magnetic fields. This means key information about their birthplace is lost by the time they arrive at Earth, Kawata wrote.
However, if cosmic rays collide with gas clouds on their way out, they generate electrically neutral gamma ray light particles that are not affected by magnetic fields. These light particles are about one-tenth as energetic as their parent cosmic rays.
So by studying the number, distribution and energy spectrum of these gamma rays, scientists can peek into the origins of cosmic radiation.
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Due to the limited sensitivity of detectors, past observation of the Milky Way’s gamma-ray haze was mostly done by space telescopes at energies lower than one trillion electron volts.
When LHAASO was completed in 2021, it became the world’s largest and most sensitive observatory to detect gamma rays and cosmic rays at ultra-high energies. A major component of LHAASO is called the Square Kilometre Array, or KM2A, comprising about 5,200 surface electromagnetic detectors and nearly 1,200 underground muon detectors.
The researchers first removed dozens of point-like – instead of diffuse – gamma ray sources in the galaxy. Then they made one of the most comprehensive and accurate measurements of the gamma glow’s distribution over a wide energy range of 0.1-1 PeV and across a large swathe of the galaxy, including both the inner and outer galactic planes.
To their surprise, the number of diffuse gamma rays measured by LHAASO is two to three times higher than the predicted cosmic ray collisions with the interstellar gas.
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Also different from expectations, the energy spectrum of the gamma emissions could be expressed by a single power law without any cut-off, contradicting a popular theory that says ultra-high energy cosmic rays would be trapped by the galaxy’s magnetic fields for a long time, anywhere between 10,000 to 10 million years, before they finally escape the pool.
“The discrepancy suggests that either additional gamma-ray sources are hidden in our galaxy, or the cosmic ray densities change depending on the location in our galaxy,” said Kawata, who worked on a China-Japan collaboration project called the Tibet AS-gamma experiment for many years.
Kawata pointed out that it would be important for the world’s major cosmic ray detectors to confirm each other’s observation results in the future.
For instance, a recent Milky Way galaxy map released by the US-led IceCube collaboration, which studies cosmic neutrinos from thousands of metres below the South Pole, gave strong evidence for the interactions between cosmic rays and interstellar gas.
Putting these pieces together, scientists should be able to gain much insight into the mysterious origins of cosmic radiation, he said.
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