Key takeaways
- In space, energetic neutrinos are usually paired with energetic gamma rays. Galaxy NGC 1068, however, emits strong neutrinos and weak gamma rays, which presents a puzzle for scientists to solve.
- A new paper posits that helium nuclei collide with ultraviolet photons emitted by the galaxy's central region and fragment, releasing neutrons that subsequently decay into neutrinos without producing gamma rays.
- The finding offers insight into the extreme environment around the supermassive black holes at the center of galaxies like NGC 1068 and our own and enhances our understanding of the relationships between radiation and elementary particles that could lead to technological advances we haven't yet imagined.
Buried deep in the ice in the Antarctic are "eyes" that can see elementary particles called neutrinos, and what they've observed is puzzling scientists: a remarkably strong neutrino signal accompanied by a surprisingly weak gamma-ray emission in the galaxy NGC 1068, also known as the Squid Galaxy.
The "eyes" are a collection of detectors buried in a cubic kilometer of ice called the IceCube Neutrino Observatory. Theoretical physicists from UCLA, the University of Osaka, and the University of Tokyo Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) are using their observations of NGC 1068 to propose a completely new route by which neutrinos can be produced.
Neutrinos are subatomic particles that interact only very weakly with gravity and can pass through matter. This makes them even harder to detect than other particles, such as electrons. The IceCube Neutrino Observatory consists of 5,160 sensors buried in clear, compressed Antarctic ice that look for events that could be produced by neutrinos when they pass through the ice, interact with it and create charged particles.
"We have telescopes that use light to look at stars, but many of these astrophysical systems also emit neutrinos," said Alexander Kusenko, professor of physics and astronomy at UCLA and a senior fellow at Kavli IPMU. "To see neutrinos, we need a different type of telescope, and that's the telescope we have at the South Pole."
The IceCube neutrino telescope detected very energetic neutrinos coming from NGC 1068 accompanied by a weak gamma-ray flux, hinting that these neutrinos may have been produced in a different way than previously thought. The NGC 1068 data is perplexing because, typically, energetic neutrinos from active galactic centers are thought to originate from interactions between protons and photons, producing gamma rays of comparable intensity. Thus, energetic neutrinos are usually paired with energetic gamma rays.
NGC 1068's gamma-ray emission is significantly lower than expected and shows a distinctly different spectral shape. Traditional models, including those based on proton-photon collisions and emission from the galaxy's hot plasma region known as the "corona," have been widely used to explain such neutrino signals, but they have faced theoretical limitations, prompting the search for a new explanation.
In a new paper published in Physical Review Letters, Kusenko and colleagues suggest that the high-energy neutrinos from NGC 1068 primarily result from the decay of neutrons when helium nuclei in the galaxy's jet break apart under intense ultraviolet radiation. When these helium nuclei collide with ultraviolet photons emitted by the galaxy's central region, they fragment, releasing neutrons that subsequently decay into neutrinos. The energies of the resulting neutrinos match the observations.
Additionally, electrons generated by these nuclear decays interact with surrounding radiation fields, creating gamma rays consistent with the observed lower intensity. This scenario elegantly explains why the neutrino signal dramatically outshines the gamma-ray emission and accounts for the distinct energy spectrum observed in both neutrinos and gamma rays.
The breakthrough helps scientists understand how cosmic jets in active galaxies can emit powerful neutrinos without a corresponding gamma-ray glow, shedding new light on the extreme, complex conditions surrounding supermassive black holes, including the one at the center of our own galaxy.
"We don't know very much about the central, extreme region near the galactic center of NGC1068," said Kusenko. "If our scenario is confirmed, it tells us something about the environment near the supermassive black hole at the center of that galaxy."
The new paper proposes that if a helium nucleus accelerates in the jet of a supermassive black hole, it crashes into photons and breaks apart, spilling its two protons and two neutrons into space. The protons can fly away, but the neutrons are unstable and fall apart, or decay, into neutrinos without producing gamma rays.
"Hydrogen and helium are the two most common elements in space," said first author and UCLA doctoral student Koichiro Yasuda. "But hydrogen only has a proton, and if that proton runs into photons, it will produce both neutrinos and strong gamma rays. But neutrons have an additional way of forming neutrinos that don't produce gamma rays. So helium is the most likely origin of the neutrinos we observe from NGC 1068."
The work reveals the existence of hidden astrophysical neutrino sources, whose signals may previously have gone unnoticed due to their faint gamma-ray signatures.
"This idea offers a new perspective beyond traditional corona models. NGC 1068 is just one of many similar galaxies in the universe, and future neutrino detections from them will help test our theory and uncover the origin of these mysterious particles," said co-author and the University of Osaka professor of astrophysics Yoshiyuki Inoue.
Like NGC 1068, our galaxy also has a supermassive black hole at its center, where unfathomably immense gravity and energy literally tear atoms apart, and the neutrino discovery holds for our galaxy, too. Although there's not necessarily a straight line from understanding the galactic center to improvements in human welfare, knowledge gained through the study of particles like neutrinos and radiation like gamma rays has a tendency to lead technology down surprising and transformative paths.
"When J.J. Thompson received the 1906 Nobel Prize in physics for the discovery of electrons, he famously gave a toast at a dinner after the ceremony, saying that this was probably the most useless discovery in history," said Kusenko. "And, of course, every smartphone, every electronic device today, uses the discovery that Thompson made nearly 125 years ago."
Kusenko also said that particle physics gave birth to the World Wide Web, which originated as a network developed by physicists who needed to move large amounts of data between labs. He pointed out that the discovery of nuclear magnetic resonance seemed obscure at the time but led to the development of magnetic resonance imaging technology, which is now used routinely in medicine.
"We stand at the very beginning of the new field of neutrino astronomy, and the mysterious neutrinos from NGC 1068 are one of the puzzles we have to solve along the way," said Kusenko. "Investment in science is going to produce something that you may not be able to appreciate now but could produce something big decades later. It's a long-term investment, and private companies are reluctant to invest in the kind of research we're doing. That's why government funding for science is so important, and that's why universities are so important."
The research was funded by the Department of Energy, the World Premier International Research Center Initiative (WPI), and the Japan Society for the Promotion of Science.
