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 that appeared around 600 million years after the Big Bang.
Scientists have explored several explanations for these little red dots, including the idea that they are powered by rapidly growing black holes hidden inside dense clouds of gas. 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 ionised gas.
Converging on a Singular Picture
'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. 'But none of the previous little red dots have all of 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.'
The findings, published in The Astrophysical Journal, explain that GLIMPSE-17775 was found under particularly favourable conditions. The object 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, GLIMPSE-17775 is actually 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, that was 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 also enables a more detailed characterization of brighter objects, including LRDs such as GLIMPSE-17775.'
Detailed Spectrum Reveals Clues
Webb captured a 30-hour spectrum of the object, but the gravitational lensing effect made the observations equivalent to around 80 hours of telescope time. The combination of Webb's infrared capabilities and the natural magnification provided by the lensing effect 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 of the puzzle, measured the lines, and started combining the different pieces into a mosaic. Maybe a few pieces looked like nothing at first, but then a couple of them came together, and we realized that there was something there.'
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 the light produced near the black hole. Among the more than 40 spectral features detected were signatures from hydrogen, oxygen and helium that did not match a simple model of a rotating gas cloud. Instead, the researchers found that the best explanation involved a process known as electron scattering, suggesting that layers of dense gas surround the source.
The strength and combination of certain spectral lines, including 16 iron lines forming what the team calls an 'iron forest', as well as oxygen features, indicate the presence of a powerful high-energy source such as a growing black hole. Astronomers also detected signs of helium fluorescence and absorption, both of which suggest a dense environment surrounding an energetic central object.
Explaining Missing X-ray Emissions
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. One remaining part of the GLIMPSE-17775 puzzle was the missing spectral feature known as the Balmer break – a strong dip in emitted light commonly associated with little red dots.
To investigate further, the team combined Webb observations with additional data from the NASA/ESA 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.
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.'
