IceCube neutrinos provide new view of nearby galaxy

IceCube neutrino detector at the South Pole.

Photograph: Martin Wolf, IceCube/NSF

For the first time, an international team of scientists has found evidence of high-energy neutrino emission from a nearby galaxy. The galaxy, NGC 1068 (also known as Messier 77) is an active galaxy in the constellation Cetus and one of the most familiar and well-studied active galaxies. The results are published in Science.

The neutrinos were detected using the IceCube Neutrino Observatory, a massive neutrino telescope encompassing 1 billion tons of instrumented ice at depths of 1.5 to 2.5 kilometers below Antarctica's surface at the South Pole. Members of the IceCube Collaboration at Stockholm University and Uppsala University have been involved in the analysis of the data acquired during ten years of observations and in the interpretation of the results.

"This represents the slow and steady accumulation of a neutrino signal, until it has become the single brightest spot in the neutrino sky," says Chad Finley, associate professor at Stockholm University and previously the coordinator for neutrino point-source searches in IceCube.

In 2018, IceCube together with gamma-ray observatories identified the distant blazar galaxy TXS 0506+056 as a neutrino source after the detection of a very high-energy neutrino in coincidence with gamma-rays during the same period of time. In contrast, the emission attributed to NGC 1068 consists of approximately 80 lower-energy neutrino events recorded by IceCube since it became fully operational in 2011.

First spotted in 1780, NGC 1068 is located 47 million light-years away from us, and can be seen with large binoculars. Like 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 also an "active galaxy" where most radiation is not emitted by stars but by material falling into the black hole at its center. This black hole is millions of times more massive than our Sun and even more massive than the inactive black hole at the center of our galaxy.

"What is surprising about seeing NGC 1068 with neutrinos is that it has not been detectable with gamma rays in the same energy range, 1 to 10 tera-electron volts," says Chad Finley. Both neutrinos and gamma-rays are generally expected to be produced in the same processes, in cosmic ray interactions. "The lack of gamma rays suggests that the neutrinos are emerging from a dense region where the gamma rays get absorbed."

The black hole is located is hidden

Unlike photons, neutrinos interact extremely weakly with matter, and can thus escape from extremely dense environments in the universe. NGC 1068 is not only an active galaxy, but is classified as a Seyfert II-type active galaxy, meaning that it is seen from Earth at an angle such that the central region where the black hole is located is hidden within a torus of dust that encircles it.

"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," says Hans Niederhausen, a postdoctoral associate at Michigan State University and one of the main analyzers of the paper. "This neutrino detection from the core of NGC 1068 will improve our understanding of the environments around supermassive black holes."

The most extreme particle accelerators in nature

The IceCube Collaboration presently consists of about 350 scientists at 58 institutions around the world. IceCube counts Stockholm University and Uppsala University among its founding member institutions.

"IceCube has again pinpointed an active galaxy as a source of highly energetic neutrinos." Says Olga Botner, Professor at Department of Physics and Astronomy at Uppsala University. "NGC 1068 is well known to astronomers but for the first time we've succeeded to observe it in a new "light". This discovery will lead to an increased understanding of processes close to the most extreme particle accelerators in nature and bodes well for our plans to build a 10 times larger and more sensitive IceCube detector. We have now taken another step towards neutrino astronomy! It is exciting to realize that there is much more in the high energy universe just waiting to be discovered!"

The IceCube Neutrino Observatory is supported by the US National Science Foundation, as well as the Swedish Research Council and national funding agencies of other member countries.

Åsa Malmberg