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

Unlike light, neutrinos can escape in large numbers from extremely turbulent environments in the universe and reach Earth undisturbed by matter and the electromagnetic fields that permeate extragalactic space. And although scientists envisioned neutrino astronomy more than 60 years ago, the weak interaction of neutrinos with matter and radiation makes their detection extremely difficult. It took nine years, from IceCube’s completion in 2011 to May 2020, for the world’s largest neutrino detector to record the nearly 80 events that provide evidence for neutrino emission from this nearby active galaxy. Each neutrino was registered after its interaction with an atomic nucleus in the ice near or inside IceCube.   

As is the case with our home galaxy, NGC 1068 is a barred spiral galaxy, with loosely wound arms and a relatively small central bulge. However, unlike our own Milky Way galaxy, NGC 1068 is an active galaxy where most of its radiation is not produced by stars. Instead, the core of the galaxy shines across the entire electromagnetic spectrum due to material falling into a black hole about 15 million times more massive than our sun and four times more massive than the inactive black hole in the center of our galaxy.

This is not exactly the case for NGC 1068, which is what scientists call a Seyfert II galaxy. The name signifies that the view from Earth is not head-on but at an angle that obscures the central region of the galaxy where the black hole is located. 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. This inflow, known as the accretion disk, produces a continuous growth in the mass of the central black hole.

Unlike with TXS 0506+056, multimessenger observations are expected to be more of a challenge in Seyfert II galaxies. The latest modeling of the black hole environments in these objects suggests that gas and dust should block the gamma rays and X-rays that would otherwise accompany the neutrinos. Thus, the astronomical community is excited about the low level of gamma radiation from NGC 1068, as it confirms the strong absorption of electromagnetic radiation in the region around the central black hole. With this neutrino detection, the core of NGC 1068, obscured for centuries, may provide a comprehensive understanding of the environments around supermassive black holes in the not too distant future.

“We are finally going beyond all the clouds,” said Elisa Resconi, a professor of physics at the Technical University of Munich, in Germany, who led the analysis team. “I think NGC 1068 could become a standard candle for future neutrino telescopes. It already is 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.”

Now efforts focus in two directions: Achieving a joint observation of the highest energy sources in our cosmos in the new field of multimessenger astronomy and gaining a better understanding of what neutrinos tell us about the environments where they are produced.

“The observation of NGC 1068 is particularly fascinating, as it points to other potential neutrino sources being opaque to high-energy gamma rays,” explains Ignacio Taboada, a physics professor at the Georgia Institute of Technology and the spokesperson of the IceCube Collaboration.

IceCube continues to accumulate more neutrinos every year, providing a unique contribution to multimessenger studies of the universe. Future improvements can be applied to both new and old data to better understand emission from NGC 1068. IceCube collaborators are also developing an enlarged and improved neutrino observatory, hoping to speed the collection of high energy neutrino events for further study.