neutrinos

IceCube Senses Neutrinos Streaming From an Active Galaxy 47 Million Light-Years Away

Researchers using the IceCube Neutrino Observatory have detected neutrinos emanating from the energetic core of an active galaxy millions of light-years away. Neutrinos are difficult to detect, and finding them originating from the galaxy is a significant development. What does the discovery mean?

Neutrinos are strange particles. For a long time, scientists thought these elementary particles had no mass. Now they know that neutrinos do have mass, but so little of it that the particles pass right through us and other matter. Neutrinos also have no electrical charge, a property that gives them their name and allows them to pass through electromagnetic fields.

“The unveiling of the obscured universe has just started, and neutrinos are set to lead a new era of discovery in astronomy.”

Elisa Resconi, professor of physics at Technical University of Munich.

Detecting something with almost no mass and no charge is difficult, so detectors are built in strange places, like deep in abandoned mines. Only in those isolated environments can scientists detect the rare neutrino that interacts with other matter as it passes through Earth.

The IceCube Neutrino Observatory (ICNO) is a unique facility buried deep in Antarctic ice. The ICNO is made up of strings of detectors sunk into the Antarctic ice. There are 86 strings of sensors, with each string having 60 modules. The strings are sunk between 1,450 and 2,450 meters into the ice in holes bored with hot water. The dense ice slows light below the speed of light <sort of> while the neutrino maintains its velocity. This only happens when the neutrinos are highly energetic. In those conditions, they emit Cherenkov radiation, which ICNO detects.

The IceCube Neutrino Observatory is an array of strings of detectors drilled deep into the Antarctic ice. Image Credit: University of Adelaide.

Scientists working with the ICNO have detected neutrinos emanating from the energetic core of an active galaxy millions of light-years away. Why is this important? Because almost all of the neutrinos ever detected come from the Sun.

The galaxy is M77, also known as NGC 1068. It’s a spiral galaxy about 47 million light-years away in the constellation Cetus. It’s sometimes called the Squid Galaxy.

A new paper published in the journal Science presented the findings. The paper is “Evidence for neutrino emission from the nearby active galaxy NGC 1068.” The IceCube Collaboration, an international group of more than 350 people from 14 countries, produced the paper.

Neutrinos are important for a host of reasons in particle physics. But their critical characteristic is that they rarely interact with other matter. So when we detect one on Earth, it’s largely unchanged by interactions with matter and electromagnetic fields, even when its source is hundreds of light-years away or further.

Detecting neutrinos from sources other than the Sun is challenging. But studying them can potentially answer some of our key questions about the Universe, especially if we detect multiple neutrinos from the same source.

One of the strings of detectors being lowered into its hole at the IceCube Neutrino Observatory in Antarctica. Image Credit: WISC/IceCube Collaboration.

“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,” says 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.”

Neutrino astronomy is a different way of studying astronomical objects. Typically, we observe objects with electromagnetic radiation: everything from radio waves to gamma rays. Those are all photons of different energies, but photons interact with matter and energy on their way from distant sources to our telescopes. Those interactions can be helpful because they teach us a lot about the source of the photons and whatever lies between our detectors and the source.

When a neutrino interacts with molecules in the clear Antarctic ice, it produces secondary particles that leave a trace of blue light as they travel through the IceCube detector. Image Credit: Nicolle R. Fuller, IceCube/NSF

But neutrinos rarely interact, so they allow astrophysicists to observe things electromagnetic telescopes can’t observe, like the interior of the Sun. Or, in this case, an active galaxy.

NGC 1068 is similar to the Milky Way. It’s a barred spiral galaxy, and also like the Milky Way, it has a supermassive black hole (SMBH) at the center. When SMBHs actively take in gas and dust, they emit energy jets and are called Active Galactic Nuclei (AGN.) From our viewpoint, NGC 1068’s central region is obscured by a torus of dust.

The environment around an AGN is complex. Neutrino astronomy is one way to study this complex object.

“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.”

“As we observe neutrinos emitted by <NGC 1968>, we will be able to learn more about the extreme particle acceleration and production processes occurring inside the galaxy, which hasn’t been possible up to now as other high energy emissions can’t escape from it,” said Associate Professor Gary Hill, from the University of Adelaide’s Department of Physics, School of Physical Sciences and member of the international IceCube Collaboration.

Astrophysicists and astronomers are very familiar with NGC 1068. It’s one of our most well-studied galaxies and can be seen with backyard telescopes. (A search in Google Scholar produces over 14,000 results for NGC 1068.) This familiarity helps scientists understand new developments like this neutrino detection.

“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,” said Theo Glauch, a postdoctoral associate at the Technical University of Munich and another of the paper’s main analyzers. According to Glauch, NGC 1068 could become a “standard candle” in neutrino astronomy. In astronomical terms, a standard candle is an object with a known luminosity, meaning its distance can be accurately determined.

Neutrino astronomy is poised to take a step forward in the near future. There are plans for an expanded IceCube Neutrino Observatory, dubbed IceCube-Gen2. It’s a next-generation extension of the original IceCube Observatory. The design will add optical and radio instruments to the original system, making it a multi-messenger observation facility. The new instrument will increase the neutrino detection rate by an order of magnitude while also being five times more sensitive to neutrino point sources.

“IceCube-Gen2 will build upon two discoveries by IceCube,” says Albrecht Karle, an IceCube-Gen2 coordinator based at the University of Wisconsin–Madison. “One is the presence of a large cosmic neutrino flux at high energies; the other is the exceptional clarity of the ice. By optimizing the design, we can scale the detector up by one order of magnitude with very similar instrumentation.”

“The unveiling of the obscured universe has just started, and neutrinos are set to lead a new era of discovery in astronomy,” says Elisa Resconi, a professor of physics at TUM and another of the paper’s main analyzers.

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Evan Gough

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