Mysteries surrounding the JWST’s little red dots are clearing up as astronomers find evidence that some compact objects are supermassive black holes hidden inside dense cocoons of gas, with recent imaging suggesting they sit at the hearts of remarkably compact, star-forming galaxies in the early universe.
Astronomers examining images sent back by the James Webb Space Telescope have debated the identity of strange, compact celestial objects dubbed little red dots. First appearing in data from the summer of 2022, these bright points of light showed up in large numbers around 600 million years after the Big Bang before seemingly vanishing before the universe reaches 2 billion years old. Some scientists even suggested they had broken cosmology
due to their sheer abundance and unexpected characteristics.
Unlocking the Secrets of GLIMPSE-17775 Using Gravitational Lensing
A major breakthrough came when researchers studied a specific little red dot designated GLIMPSE-17775, captured as it appeared 1.8 billion years after the Big Bang. The observation relied on a massive galaxy cluster known as Abell S1063 acting as a gravitational lens, bending spacetime to magnify the distant light source behind it.
Astronomers using the James Webb Space Telescope may be close to solving the mystery of little red dots
in the early universe. The team has studied one of these strange objects, designated GLIMPSE-17775, finding evidence it is a black hole star — a ravenously feeding, growing supermassive black hole cocooned in a dense cloud of partially ionised gas. This data represents the deepest spectrum of light from a little red dot collected to date.
The Case for Black Hole Stars and Pure Light Signatures
Researchers also detected an exceptionally deep Balmer break—a spectral drop-off where light disappears below certain wavelengths—that ruled out ‘ordinary’ stars as the source. The red dot’s light contained almost no signature of metals or any elements other than hydrogen and helium, prompting researchers to run simulations of different scenarios to see what combination of astrophysical features could produce the distinctive color.
Simulations run by the research team indicated that it turns out you can make something that red using just hydrogen, without any dust, if you have an extremely dense
Connecting Little Red Dots to Compact Host Galaxies
While some objects outshine their surroundings, a broader image stacking analysis of 217 little red dots published in Nature Astronomy suggests they sit inside remarkably compact, star-forming galaxies. Little red dots (LRDs) are small, red objects seen at far distances from Earth,
Wuhan University astronomer Xuheng Ding and colleagues said in a statement. However, the nature of LRDs and the source of their light remains unclear.
Previous research found signs of material around some of these objects at ultraviolet wavelengths, possibly indicating the presence of galaxies, though there had been little evidence of this material in optical light that gives a clearer view of stellar mass.
In the study, astronomers performed an image stacking analysis of 217 little red dots and detected faint extended emission around these objects. Modelling of the data indicates that this emission likely comes from star-forming galaxies, with little red dots residing in their centers as supermassive black holes. Little red dots may reside in small galaxies with total masses equivalent to around one billion times that of the Sun. The galaxies have average radii of just 685 light-years, making them about 2.5 times more compact than other star-forming galaxies of a similar mass seen at a comparable period in the Universe’s history.
Meanwhile, separate James Webb Space Telescope observations, led by Institute of Science and Technology Austria PhD fellow Eduardo Iani, revealed unexpectedly overmassive black holes at the hearts of a pair of dwarf galaxies named Pelias and Neleus, far in excess of the expected ratio of black hole to galaxy mass. While supermassive black holes are often described as anchors to their host galaxies, data show them to account for a mere 0.1 to 0.5% of a galaxy’s mass, yet these galaxies display unusual spectral energy distributions. When images with JWST’s NIRISS and NIRSpec instruments are taken, the galaxies appear to be quite blue, indicating low amounts of dust and young, ionizing stars, while JWST’s MIRI instrument shows a large amount of mid-infrared emissions well beyond what their stellar masses should produce.
What Remains Unanswered in Early Universe Physics
Several explanations for little red dots have been proposed, but the black hole star concept has emerged as a frontrunner. If black hole stars exist, the little red dot disappearance would be the result of their intense, short-lived growth spurts that cause them to burn out, or because the growing supermassive black holes at their centers eventually clear away the dense gas and dust obscuring them, changing their appearance as they evolve into more typical active galaxies. Previous deep infrared JWST observations demonstrated that supermassive black holes could grow far beyond this size in the early universe, presenting a discovery that challenges current understanding of black hole formation with relation to galaxies.
