Kyoto, Japan -- Peering far into the distant, high-redshift universe, the James Webb telescope has discovered an abundance of small red galaxies known as the Little Red Dots. From their observations, astronomers believe that at least some of these galaxies may be home to growing supermassive black holes at their centers, objects which they believe are embedded in dense gaseous envelopes, an environment that is suitable for producing high-energy neutrinos.
Neutrinos are electrically neutral elementary particles with masses near zero. High-energy neutrinos from across the universe have been detected on Earth, but the origin of all-sky high-energy neutrino background radiation has remained a mystery.
Neutrino production involves the collision of high-energy particles such as protons with surrounding photons or matter, and the resulting neutrinos can escape even if they are produced inside thick gas. Sources that produce high-energy neutrinos generally also produce gamma rays, yet if all sources that produced neutrinos also produced gamma rays, the result would exceed observed gamma ray background levels.
Promising source candidates for these neutrinos must therefore be hidden objects from which gamma rays cannot easily escape. Certain features of the Little Red Dots prompted a team of researchers at Kyoto University to suspect they may be hidden neutrino sources. Since most Little Red Dots show little emission associated with jets or outflows, such as radio or X-ray emission, the researchers hypothesized a scenario in which the jets are buried within dense gas envelopes.
"In the scenario we considered, abundant photons and dense gas are expected to exist around the central black hole in a Little Red Dot, which may allow such collisions to occur efficiently," says first author Riku Kuze.
With this in mind, the researchers used typical luminosity and number density to analytically estimate the extent to which the Little Red Dots could contribute to the all-sky high-energy neutrino background. They also performed complex numerical calculations that estimated particle acceleration, the secondary particles produced from them, and their cooling processes to evaluate the neutrino spectrum expected from the Little Red Dots.
The team's results revealed that, if particle acceleration occurs in the buried black-hole environments of the Little Red Dots, they could produce high-energy neutrinos while suppressing gamma rays. In that case, they could be contributing to a fraction of the high-energy neutrino background observed on Earth.
"Although it is difficult to observe the individual objects directly, we believe this study is significant because it is the first to demonstrate that, given their abundance, these little red galaxies could account for a part of the observed high-energy neutrinos," says Kuze.
Moving forward, the challenge is to estimate the ratio of different types of neutrinos -- also called neutrino "flavors" -- and to investigate the conditions under which the jets became buried within the envelopes.
