At the center of the Little Red Dot, there may be a black hole surrounded by a thick outer gaseous envelope. In this environment, photons produced near the center are absorbed and scattered by the gas, so neutrinos can escape the envelope without interacting with the surrounding gases. If there are many Little Red Dots, they may account for a part of the high-energy neutrinos arriving from the universe.
Image Credit: KyotoU / Riku Kuze
Scientific Frontline: Extended "At a Glance" Summary: Little Red Dots as Hidden Neutrino Sources
The Core Concept: "Little Red Dots" are abundant, high-redshift, small red galaxies recently observed by the James Webb Space Telescope. Researchers hypothesize that these galaxies harbor growing supermassive black holes enveloped in dense gas, making them a primary candidate for the universe's mysterious all-sky high-energy neutrino background.
Key Distinction/Mechanism: High-energy neutrinos are produced when accelerated particles collide with surrounding matter or photons. Unlike typical high-energy neutrino sources, which also emit detectable gamma rays, the dense gaseous envelopes surrounding the black holes in Little Red Dots suppress gamma-ray emissions while allowing neutrinos to escape, thereby matching observed cosmic background levels.
Major Frameworks/Components:
- Supermassive Black Holes: Central celestial objects generating the extreme energetic forces required for particle collisions.
- Particle Acceleration: The mechanism by which protons and other particles achieve high velocities within buried jets, leading to the production of secondary particles.
- Gaseous Envelopes: Thick, dense layers of gas surrounding the central black hole that absorb scattered photons (gamma rays) while permitting electrically neutral neutrinos to escape.
- Neutrino Spectrum Analysis: Complex numerical modeling utilized to evaluate cooling processes, particle collisions, and the expected neutrino output from these distant galaxies.
Branch of Science: Astrophysics, Particle Physics, and Cosmology.
Future Application: Further refinement of this model will enable scientists to estimate the ratios of different neutrino "flavors," as well as deduce the specific astrophysical conditions under which galactic jets become buried within dense gaseous envelopes.
Why It Matters: This research provides the first analytically supported demonstration that the sheer abundance of these newly discovered galaxies could resolve a long-standing cosmological mystery regarding the origin of the all-sky high-energy neutrino radiation observed on Earth.
Peering far into the distant, high-redshift universe, the James Webb Space Telescope has discovered an abundance of small red galaxies known as Little Red Dots. From their observations, astronomers believe that at least some of these galaxies may host growing supermassive black holes at their centers—objects that they believe are embedded in dense gaseous envelopes, an environment 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 the 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; however, if all sources that produced neutrinos also produced gamma rays, the result would exceed observed gamma-ray background levels.
Promising candidate sources for these neutrinos must therefore be hidden objects from which gamma rays cannot easily escape. Certain features of 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 emissions, 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," said first author Riku Kuze.
With this in mind, the researchers used typical luminosity and number density to estimate analytically the extent to which 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, and their cooling processes to evaluate the neutrino spectrum expected from Little Red Dots.
The team's results revealed that if particle acceleration occurs in the buried black-hole environments of Little Red Dots, they could produce high-energy neutrinos while suppressing gamma rays. In that case, they could contribute 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," said 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.
Published in journal: Physical Review D
Title: Little red dots as hidden neutrino sources
Authors: Riku Kuze, Kunihito Ioka, Kohta Murase, Shigeo S. Kimura, and Kohei Inayoshi
Source/Credit: Kyoto University
Edited by: Scientific Frontline
Reference Number: asph063026_01