IceCube neutrinos give us first glimpse into the inner depths of an active galaxy

Mainz-based scientists have been members of the IceCube consortium since 1999

4 November 2022


For the first time, an international team of scientists found evidence of high-energy neutrino emission from NGC 1068, also known as Messier 77, an active galaxy in the constellation of Cetus and one of the most familiar and well-studied galaxies to date. First spotted in 1780, this galaxy, located 47 million light-years away from us, can be observed with large binoculars. The results have now been published in Science.

The detection was made at the National Science Foundation-supported IceCube Neutrino Observatory, a massive neutrino telescope encompassing one billion tons of instrumented ice at depths from 1.5 to 2.5 kilometers below Antarctica's surface near the South Pole. This unique telescope, which explores the farthest reaches of our universe using neutrinos, reported the first observation of a high-energy astrophysical neutrino source in 2018. The source, TXS 0506+056, is a known blazar located off the left shoulder of the Orion constellation and four billion light-years away. In 2018, confirmation by optical telescopes was still necessary to identify the source with certainty. This time, however, enough neutrinos were detected by IceCube alone during a measuring period of ten years.

"One neutrino can single out a source. But only an observation with multiple neutrinos will reveal the obscured core of the most energetic cosmic objects," said Francis Halzen, a professor of physics at the University of Wisconsin-Madison and principal investigator of IceCube. "IceCube has accumulated some 80 neutrinos of teraelectronvolt energy from NGC 1068, which are not yet enough to answer all our questions, but they definitely are the next big step towards the realization of neutrino astronomy," he said.

Scientists from the research groups of Professor Sebastian Böser and formerly Professor Lutz Köpke from the Institute of Physics and the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) have been members of the IceCube consortium, which is also funded by the German Ministry of Education and Research, since 1999. "Tracing back neutrinos to distant sources in the universe is exactly what IceCube was originally planned for. After this long time, it is great to see our goals realized. We are very proud that after a whole series of remarkable results, we have now also achieved this outstanding result as a collaboration," said Professor Sebastian Böser.

Neutrinos move unhindered in space

Unlike light, neutrinos can escape in large numbers from extremely dense environments in the universe and reach Earth largely undisturbed by matter and the electromagnetic fields that permeate extragalactic space. Although scientists envisioned neutrino astronomy more than 60 years ago, the weak interaction of neutrinos with matter and radiation makes their detection extremely difficult. Neutrinos could be the key to our queries about the workings of the most extreme objects in the cosmos.

As is the case with our home galaxy, the Milky Way, NGC 1068 is a barred spiral galaxy, with loosely wound arms and a relatively small central bulge. However, unlike the Milky Way, NGC 1068 is an active galaxy where most radiation is not produced by stars but is due to material falling into a black hole millions of times more massive than our Sun and even more massive than the inactive black hole in the center of our galaxy. NGC 1068 is an active galaxy, a Seyfert II type in particular, seen from an angle such that the central region, where the black hole is located, is obscured. In a Seyfert II galaxy, a torus of nuclear dust obscures most of the high-energy radiation produced by the dense mass of gas and particles that slowly spiral inward towards the center of the galaxy.

In many models, one would expect the neutrinos emitted to be accompanied by high-energy gamma rays. But this – in accordance with new models – has not been observed in the case of NGC 1068. "Recent models of the black hole environments in these objects suggest that gas, dust, and radiation should block the gamma rays that would otherwise accompany the neutrinos," said Hans Niederhausen, a postdoctoral associate at Michigan State University and a member of IceCube. "This neutrino detection from the core of NGC 1068 will improve our understanding of the environments around supermassive black holes."

Unlike the source TXS 0506+056 which was active only for a short time, NGC 1068 emits a continuous flow of neutrinos. "I think NGC 1068 could become a standard candle for future neutrino telescopes," said Dr. Theo Glauch, a postdoctoral associate at the Technical University of Munich (TUM) and a member of IceCube. "It is already a very well-studied object for astronomers, and neutrinos will allow us to see this galaxy in a totally different way. A new view will certainly bring new insights."

"This result is a significant improvement on a prior study on NGC 1068 published in 2020," emphasized Ignacio Taboada, a physics professor at the Georgia Institute of Technology and the spokesperson of the IceCube Collaboration. "Part of this improvement came from enhanced techniques and part from a careful update of the detector calibration. Work by the detector operations and calibrations teams enabled better neutrino directional reconstructions to precisely pinpoint NGC 1068 and enable this observation. Resolving this source was made possible through enhanced techniques and refined calibrations, an outcome of the IceCube Collaboration's hard work."

"It is great news for the future of our field. It means that with a new generation of more sensitive detectors there will be much to discover. The future IceCube-Gen2 observatory could not only detect many more of these extreme particle accelerators but would also allow their study at even higher energies. It is as if IceCube handed us a map to a treasure trove," emphasized Professor Marek Kowalski, a senior scientist at Deutsches Elektronen-Synchrotron in Germany.

With the neutrino measurements of TXS 0506+056 and NGC 1068, IceCube is one step closer to answering the century-old question of the origin of cosmic rays. Even more interesting is that these results also imply that there may be many more similar objects yet to be identified. "The unveiling of the obscure universe has just started, and neutrinos are set to lead a new era of discovery in astronomy," emphasized Elisa Resconi, a professor of physics at TUM.