The mystery of the universe's 'little red dots' is becoming clearer thanks to the James Webb Space Telescope. Soon after Webb began scientific operations in 2021, it discovered a previously unknown type of object in the early universe: abundant red sources appearing around 600 million years after the Big Bang.
New Findings on GLIMPSE-17775
A team led by Vasily Kokorev at the University of Texas at Austin identified GLIMPSE-17775 as a key example. By studying its spectrum in detail, the team found multiple lines of evidence suggesting the object is a supermassive black hole surrounded by a dense cocoon of partially ionized gas.
'I think part of the scientific community is converging on a singular picture — that little red dots can be explained by black hole star models,' said Kokorev, lead author of the study published in The Astrophysical Journal. 'But none of the previous little red dots have all the pieces of evidence in the same place. With GLIMPSE-17775 we can test these models because of how deep and amazing this source's spectrum is.'
Gravitational Lensing Reveals Details
GLIMPSE-17775 was found under particularly favorable conditions. It was included in Webb observations designed to search for Population III stars and extremely faint galaxies in the galaxy cluster Abell S1063. Although it appears near the cluster, the object is much farther away, and its light has been magnified by gravitational lensing — the effect of gravity acting as a natural telescope. The object has a cosmological redshift of 3.5, meaning it existed around 1.8 billion years after the Big Bang.
'The source was discovered from the GLIMPSE programme, designed to reveal the faintest sources in the early Universe,' said Hakim Atek of the Institut d'Astrophysique de Paris, co-author and Principal Investigator of the GLIMPSE programme. 'In addition, the magnification by gravitational lensing enables a more detailed characterization of brighter objects, including LRDs such as GLIMPSE-17775.'
Detailed Spectrum Analysis
Webb captured a 30-hour spectrum of the object, but gravitational lensing made the observations equivalent to around 80 hours of telescope time. The combination of Webb's infrared capabilities and natural magnification allowed astronomers to detect more than 40 spectral lines — the most detailed spectrum yet obtained for a little red dot.
'When we saw the spectrum for the first time, it was like having all the pieces of a puzzle scattered on the floor,' said Kokorev. 'We picked up each piece, measured the lines, and started combining them into a mosaic. Maybe a few pieces looked like nothing at first, but then a couple came together, and we realized that there was something there.'
Evidence for Black Hole Star Model
The observations provide several independent clues supporting the idea that GLIMPSE-17775 is a 'black hole star' — a rapidly accreting black hole surrounded by a dense gas cocoon that absorbs and reshapes light. Among the spectral features detected were signatures from hydrogen, oxygen, and helium that did not match a simple rotating gas cloud model. Instead, the best explanation involved electron scattering, suggesting layers of dense gas surround the source. The strength and combination of certain spectral lines, including 16 iron lines forming an 'iron forest,' as well as oxygen features, indicate a powerful high-energy source such as a growing black hole.
Astronomers also detected signs of helium fluorescence and absorption, both suggesting a dense environment surrounding an energetic central object. The black hole star model may also explain why many little red dots appear faint in X-rays — any X-ray emission from the black hole would likely be absorbed by the surrounding cocoon of gas.
Missing Balmer Break Explained
One remaining puzzle was the missing spectral feature known as the Balmer break — a strong dip in emitted light commonly associated with little red dots. The team combined Webb observations with data from the Hubble Space Telescope's Frontier Fields and BUFFALO programmes. Together, the observations revealed that GLIMPSE-17775 is surrounded by a large host galaxy. While such a galaxy has not commonly been seen around a little red dot at this scale, researchers say it is still consistent with the dense gas cocoon model. The black hole star explanation suggests that excess blue light from little red dots may come from stars in their surrounding host galaxies.
Implications for Cosmology
When Webb first revealed little red dots, some researchers suggested they might challenge existing ideas about cosmology, questioning how galaxies could have become so large so quickly in the early universe. However, the team behind GLIMPSE-17775 believes the object fits within current models of cosmic evolution because the black holes involved do not need to be as massive to explain the observed features.
'Everything fits, nothing is broken, and I think that makes the puzzle that our Universe is even better,' said Kokorev. 'Looking ahead, I'm eager to dive deeper and learn about what is powering the central engines of little red dots. While we think it's a black hole, there are some other interesting theories being proposed, which is exciting. Maybe in a year or two, we'll have the final answer to what powers these sources.'
