New scenario for cosmic neutrino events

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA), with help from a ‘natural telescope’ formed by gravity, have identified the source of a neutrino burst. The team expected a supermassive black hole to be the engine driving the remarkably bright galaxy. Instead, the team found that the galaxy was driven by vigorous star formation. This result provides important observational evidence to help explain the mysterious origin of cosmic neutrinos.

Of all the signals reaching us from the Universe, neutrinos are some of the most enigmatic. The few galaxies which have been identified as sources of neutrinos cannot explain the relatively large number of high-energy neutrinos that have been observed.

An international team including researchers at MITOS Science Co., LTD., National Central University, Chung Yuan Christian University, Tohoku University, Fukui University of Technology, and the National Astronomical Observatory of Japan, carried out follow-up observations using ALMA and other telescopes to search for the source of the high-energy neutrino event IC 210922A detected by the IceCube Neutrino Observatory at the South Pole. They found an extremely bright galaxy, JCMT0402−0424, located about 11 billion light-years away.

The few neutrino sources previously identified are powered by supermassive black holes. However, this time the team found no energetic emission indicating such a black hole. The galaxy is also heavily obscured by dust and faint in visible light, while shining brightly at submillimeter wavelengths, so the team nicknamed it ‘Shadow Blaster.’

The team was able to study the internal structure of Shadow Blaster thanks to the serendipitous alignment of a foreground galaxy. The gravity of the foreground galaxy bent and concentrated the radio waves from Shadow Blaster, acting as a natural telescope to create brighter, enlarged images of Shadow Blaster that ALMA could study in detail. ALMA’s radio observations of the internal environment also showed no sign of a powerful black hole, but suggested an alternative explanation. The observations indicate that the gas and dust in the galaxy are most likely heated by intense star formation. Analysis revealed that the central region of Shadow Blaster contains a “compact core,” where a large amount of gas and dust is packed into a region only about 1,500 light-years across. Such an extremely high-density environment can produce neutrinos.

These results point to an alternative pathway for high-energy neutrino production. A population of compact dusty starburst galaxies undergoing intense star formation may contribute a significant fraction, possibly up to around 20%, of the high-energy neutrino background.