The "Shadow Blaster" Galaxy's Role in High-energy Cosmic Neutrinos
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On September 22, 2021, the IceCube Neutrino Detector in Antarctica caught a blast of neutrinos as it passed through the solar system.
These neutrinos were remarkably high-energy and came from a galaxy 11 billion light-years away. That's a period of the Universe's history known as "Cosmic Noon". It's when star formation in galaxies was at its most active and that provided an interesting clue to their origin.
The source of the neutrinos was nicknamed "Shadow Blaster" because the event that created the neutrinos was hidden by a dense cloud of dust, which made it invisible to optical observations. Scientists suspect that this outburst from stellar activity could be a major contributor to the cosmic neutrino background, since the stars produced large numbers of cosmic rays. However, that epoch of time is extremely distant and many targets are hidden behind dust clouds, which challenges any observations of these distant events.
Also, how could star formation emit such energetic neutrinos? To answer that, scientists turned to multi-messenger observations, using the Atacama Large Millimeter/Submillimeter Array (ALMA) to look for radio emissions from the region and the Neil Gehrels Swift space-based observatory to see if there were any x-rays or gamma-rays emitted at the same time as the neutrinos. *The IceCube Neutrino Observatory in Antarctica.
Its detectors are buried deep in the ice to catch evidence of neutrinos as they pass through. Credit: Ilya Bodo, IceCube/NSF* Neutrinos are fundamental, nearly massless natural particles that speed across space and pass through normal matter quite easily. They are emitted from sources such as the Sun, supernova explosions, gamma-ray bursts, extragalactic sources such as supermassive black holes and active galaxies.
These extragalactic sources are often referred to as cosmic accelerators because they accelerate the particles to incredibly high speeds. Neutrinos hard to study, which is why scientists have built special detectors such as IceCube, Super-Kamiokande (in Japan), and others. Their contributions to neutrino science include IceCube's neutrino map of the Milky Way (which traces these particles back to their energetic origins in our galaxy and in active galaxies with supermassive black holes.
The KM3NET Neutrino Telescope deep under the Mediterranean Sea has found extremely high-energy neutrinos whose sources are still not well-defined. Some of the most energetic neutrinos seen by IceCube have measured as high as a thousand trillion electrovolts. That's an incredible amount of energy for such tiny particles.
Just to give you an idea of how strong that is, on Earth we routinely measure atmospheric neutrinos formed by cosmic ray interactions with other atmospheric particles at between 1 and ten trillion electrovolts. So, this means that something very energetic and hitherto unknown released swarms of extremely energetic neutrinos at cosmic distances. And, those neutrinos are still energetic even after travelling across billions of light-years of space.
What could be powering the large numbers of strong ones found at IceCube? It turns out that the neutrino release (an event called IC 210922A) from the distant early galaxy is being gravitationally lensed by a foreground elliptical galaxy called JCMT0402-0424. ALMA observations (in radio emissions) showed a fairly typical-looking set of arc-like bands.
The Gehrels observations didn't show any evidence of x-ray or gamma-ray emissions, which would indicate the presence of a supermassive black hole activated in the galaxy. The team then used the lensing to study the internal characteristics of the dusty galaxy emitting the neutrinos. It appears that the gas in the central "compact core" of the galaxy is being heated by repeated rounds of very intense starbirth activity taking place in a region only about 1,500 light-years across.
Some of that gas is emitting signals detectable by ALMA, in addition to the neutrinos that IceCube saw. A multi-wavelength view of the bright, gravitationally-lensed, star-forming galaxy "Shadow Blaster". It's the likely source of the high-energy neutrino event IC 210922A, detected by the IceCube Neutrino Observatory in 2021.
Credit: International Gemini Observatory/NOIRLab/NSF/AURA/
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