Researchers with the IceCube Collaboration have created the first-ever image of the Milky Way Galaxy using neutrino detections. The findings confirm theoretical expectations about neutrino creation within our home galaxy. The image is a significant achievement in the blossoming field of neutrino astronomy, which enables scientists to observe the universe in a completely different physical regime than light, the mainstay of astronomy since prehistory. See also: Milky Way Galaxy; Neutrino; Neutrino astronomy

An artist's impression of a view into the center of the Milky Way Galaxy produced by IceCube researchers, with areas of neutrino emission (blue) overlaid on a black-and-white optical image capturing regions of light emission interspersed with obscuring galactic clouds of dust. (Credit: L. Le, S. Johnson, S. Brunier, IceCube Collaboration, NSF, ESO)

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As uncharged subatomic particles in the lepton family, neutrinos do not interact with matter through electromagnetism or the strong nuclear force, and instead interact only through gravity and the weak nuclear force. However, because of their extremely low masses and the weak nuclear force's influence being felt only over very small distance scales, neutrinos rarely interact with other matter, making the particles famously difficult to detect. This scarcity of interaction belies the fact that neutrinos are the most numerous particles in the universe—even more so than photons (light)—and produced naturally by stars in copious amounts. Thus, although any given neutrino has a very small chance of interacting with a target in a detector instrument, the number of incident neutrinos makes up for this low interaction probability. See also: Electric charge; Elementary particle; Fundamental interaction; Gravity; Lepton; Matter; Weak nuclear interactions

To further increase chances of detecting these ghostly neutrinos, researchers created a massive detector, known as the IceCube Neutrino Observatory, in 2010. Within a cubic-kilometer-sized gigaton of Antarctic ice near the South Pole, researchers placed thousands of photosensitive devices to serve as the detectors for the instrument. When an incident cosmic neutrino interacts with particles of ice, the interaction can produce charged particles that travel through the ice faster than the speed of light in that medium. (The speed of light varies according to material properties; for instance, light propagates more slowly through water than through air.) When this local "speed limit" is exceeded, a form of light called Cherenkov radiation is produced, analogous to the way in which a sonic boom is produced when a jet breaks the sound barrier. The pristine Antarctic ice is almost perfectly transparent, allowing for resulting Cherenkov radiation to reach the detectors embedded in the ice. By gauging the direction and brightness of this radiation, the location of the neutrino interaction and the original path of the incident neutrino can be reconstructed. See also: Cerenkov radiation; IceCube Neutrino Observatory; Light

By taking these reconstructed neutrino paths and projecting them back onto the night sky, researchers were able to determine the location within the Milky Way where the neutrinos had originated. A machine-learning algorithm assisted in selecting and determining the directions of incident neutrinos for more than 60,000 neutrino interactions collected over 10 years. The team expected the disk of the Milky Way to be a source of energetic neutrinos due to previous observations of gamma-ray emissions emanating from this region, in part due to high-energy extragalactic cosmic rays (fast-moving miniscule bits of matter) smashing into gas within the disk. The new results showing a neutrino glow in the disk confirmed this prediction. IceCube Collaboration researchers and colleagues now intend to further identify the potential sources of the Milky Way's neutrino flux. See also: Algorithm; Cosmic ray; Gamma-ray astronomy; High-energy astrophysics; Machine learning