At the heart of nearby galaxy NGC 1068, a supermassive black hole is feasting on gas and dust. In the process, it gives off energy and neutrinos — tiny, lightweight subatomic particles with no electric charge, which pass through most matter without leaving a trace.
Recently, a few dozen of those neutrinos found their way across the vastness of space to set off detectors buried in the ice of Antarctica, astronomers from the IceCube observatory reported in the journal Science.
What’s New — It’s the first time astronomers have ever detected something they consider a steady source of the elusive particles — as opposed to the unidentified sources that create a general background of neutrinos zipping around the cosmos.
Detecting neutrinos requires a new — and very strange — kind of particle detector. IceCube is a giant trap for neutrinos in the Antarctic ice, embedding instruments in an area that spans a square kilometer on the surface and extends 1,500 meters deep.
When a team of astronomers mapped the origins of all the neutrinos they’d detected with IceCube, the resulting map looked like a TV static pattern spread across the sky. In general, neutrinos seem to be coming and going from all directions, all the time; astronomers call that “flux.”
But a closer look at the neutrino sky map could reveal occasional hotspots, like the one that turned out to be NGC 1068, a barred spiral galaxy about 47 million light years away.
“This is a little bit like a topographic map in which you are looking for elevations,” said Technical University of Munich astronomer Elisa Resconi, a member of the IceCube team and a co-author on the recent paper, during a press conference. And just north of the cosmic equator, the spiral galaxy NGC 1068 looks like Mount Everest.
Here’s the background — Astronomers almost always use light to see the universe — it’s not always light we can see, but it’s always energy traveling in waves, from the lowest-frequency radio waves, through the spectrum of what we think of as light, to the highest-energy gamma rays. Neutrino astronomy is different because neutrinos are matter, not energy. Using them to “see” distant objects, like a ravenous supermassive black hole, is a new kind of astronomy.
Neutrinos are so small that they tend to slip unnoticed through the spaces between bigger subatomic particles like protons, neutrons, and electrons — the more familiar pieces that make up atoms.
Once in a while, a neutrino happens to bump into a proton; when that happens, the neutrino is moving so fast that the collision triggers a mini-explosion, giving off a bright blue flash called Cherenkov radiation. IceCube’s instruments are designed to detect that radiation. That’s why the observatory had to be in Antarctica — because the instruments needed extremely clear ice in order to detect the Cherenkov flashes.
By measuring those flashes, IceCube can calculate where a passing neutrino came from and how much energy it’s carrying. That’s how the IceCube astronomers produced their sky map. And in 2018, it’s how IceCube traced several neutrinos back to another active galaxy, TXS-0506+056. And now they’ve pinned roughly 80 more on NGC 1068.
Why It Matters — Neutrinos offer the first real glimpse into the core of NGC 1068. It’s an active galaxy, meaning the supermassive black hole at its center is still actively drawing in new material, and belching out high-energy radiation and accelerated particles. A donut-shaped ring of dust around the galaxy’s core hides it from our view by blocking most types of radiation, including the high-energy gamma rays that models predict should be blasted into space alongside neutrinos.
And now that the IceCube team has found two galaxies pumping out neutrinos, they can start to compare them.
“Even though they're both active galaxies, their properties appear to be quite different,” said Georgia Tech astronomer Ignacio Taboada, a member of the IceCube team and a co-author on the recent paper, during the conference.
For instance, one galaxy is what astronomers call a time-variable source, blasting neutrinos out into space in periodic bursts, while the other’s neutrino output is steadier over time. “This could be telling us that there are least two different types of populations that are driving the extragalactic neutrino flux,” Taboada says.
But to understand more about how that works, IceCube will need to detect many more neutrinos and trace them to more galaxies. Most of the background flux of neutrinos probably comes from active galaxies like NGC 1068, says Taboada.
“There must be many other sources that are far dimmer than NGC 1068 that are hiding somewhere to be found,” he says.
What’s Next — A few dozen fast-moving particles from NGC 1068 won’t answer all our questions about neutrinos, or about what’s going on in the dust-shrouded heart of the distant galaxy. But it’s a start, and it’s a key step for astronomers in learning how to get good data from neutrino astronomy.
That’s important because in the long run, neutrinos could help us understand another type of weird subatomic particle that flies through space: cosmic rays.
Cosmic rays are electrically-charged bits of atoms (a positively-charged nucleus or a negatively-charged electron) that get blasted into space at high speeds by violent astronomical events like solar flares, supernovae, or the ongoing cataclysm of an active galactic core – the same kinds of cosmic-scale particle accelerators that fire neutrinos out into the universe.
But cosmic rays are hard to trace back to their sources, because they tend to bounce all over the place; they’re easily deflected by magnetic fields — and there are a lot of magnetic fields in the universe. Neutrinos just keep going in the direction they started on, however, and that makes them good proxies for studying the kinds of events that also produce cosmic rays.