The complex puzzle known as little red dots has become more complete since their initial discovery by NASA\u2019s James Webb Space Telescope in 2022. Now a particular little red dot\u2019s spectrum is helping connect many of the pieces.<\/p>\n
A team of astronomers led by Vasily Kokorev at the University of Texas at Austin identified the lucky dot in question: GLIMPSE-17775. By carefully analyzing the dot\u2019s spectrum captured by Webb \u2014 the deepest spectrum<\/a> to date of a little red dot \u2014 the research team has identified multiple lines of evidence, all of which support the interpretation that GLIMPSE-17775 is a supermassive black hole enveloped in a dense cocoon of partially ionized<\/a> gas, a model referred to as the BH* (black hole star) scenario. A paper describing the results was published today in The Astrophysical Journal<\/a>.<\/p>\n \u201cI think part of the scientific community is converging on a singular picture \u2014 that little red dots can be explained by black hole star models. But none of the previous little red dots have all of the pieces of evidence in the same place,\u201d said Kokorev, lead author of the study. \u201cWith GLIMPSE-17775 we can test these models because of how deep and amazing this source\u2019s spectrum is.\u201d<\/p>\n While the primary purpose of NASA\u2019s James Webb Space Telescope\u2019s observations of galaxy cluster Abell S1063 was to look for a certain population of stars, scientists obtained a detailed spectrum of GLIMPSE-17775 from the dataset. This little red dot is located behind Abell S1063.<\/p>\n Image: NASA, ESA, CSA, Vasily Kokorev (UT Austin); Image Processing: Alyssa Pagan (STScI)<\/p>\n<\/figcaption> Soon after Webb first began science operations, it discovered a new, mysterious type of object in the very early universe \u2013 abundant red objects that emerged about 600 million years after the big bang. Scientists have explored multiple explanations<\/a> for these little red dots, including the black hole star scenario<\/a>.<\/p>\n A set of fortunate circumstances brought about this new, elaborate spectrum of a little red dot. The little red dot that would come to be known as GLIMPSE-17775 was fortunately included in Webb\u2019s imaging and spectroscopy efforts for a project that sought to look for Population III stars<\/a> and faint galaxies in galaxy cluster Abell S1063. This little red dot is more distant than the galaxy cluster and magnified by gravitational lensing<\/a>. (GLIMPSE-17775 has a cosmological redshift<\/a> of 3.5, meaning it existed about 1.8 billion years after the big bang.)<\/p>\n While Webb provided a 30-hour spectrum of the little red dot, the effect of gravitational lensing made it equivalent to 80 hours of telescope time. This combination of Webb\u2019s infrared sensitivity and nature\u2019s own \u201cmagnifying glass\u201d amplified the amount of detail that could be gleaned from GLIMPSE-17775. The result was more than 40 spectral lines<\/a> from this small, red source, which is the most detailed little red dot spectrum to date.<\/p>\n \u201cWhen we saw the spectrum for the first time, it was like having all the pieces of a puzzle scattered on the floor,\u201d said Kokorev. \u201cWe 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.\u201d<\/p>\n The spectroscopic data collected by Webb contains multiple lines of evidence that support the interpretation that little red dot GLIMPSE-17775 is a black hole star: a rapidly accreting, or growing, black hole enveloped in a dense gas cocoon, which is reprocessing the light emitted from near the black hole and producing the features seen in the spectrum.<\/p>\n Illustration: NASA, ESA, CSA, Vasily Kokorev (UT Austin); Designer: Leah Hustak (STScI)<\/p>\n<\/figcaption> Among the 40-plus lines that the team detected in GLIMPSE-17775\u2019s spectrum were various independent indicators that all align with the BH* scenario. For example, the team found that many of the spectral lines, such as hydrogen, oxygen, and helium, do not fit a simple model of a rotating gas cloud. Instead, the best fit model includes a broadening effect known as electron scattering, a telltale sign that a dense, layered gas cocoon is enshrouding this source.\u00a0<\/p>\n The strength and ratios of certain lines to each other, most notably the 16 iron lines that compose what the team has dubbed an \u201ciron forest\u201d and certain oxygen lines, require a high-energy source to produce them, like a rapidly accreting black hole. Additionally, astronomers noted the fluorescence and absorption of helium in the spectrum, both of which individually suggest that there is a dense medium enveloping a powerful source.<\/p>\n The BH* scenario not only fits GLIMPSE-17775; it also accounts for why most little red dots are faint in X-rays, since any such emission is likely absorbed by the dense gas cocoon.<\/p>\n One missing element of the GLIMPSE-17775 puzzle piece is the part of the spectrum that would reveal what\u2019s known as a Balmer break, or a strong dip in the emitted light that\u2019s a signature characteristic of little red dots. To build a more comprehensive understanding of this little red dot, the team incorporated ancillary data from two observing programs that used NASA\u2019s Hubble Space Telescope: the Frontier Fields and BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy Observations) programs.<\/p>\n The Webb and Hubble data together help explain why the Balmer break is weaker than what typically is found in other little red dots: A giant host galaxy is surrounding GLIMPSE-17775. Although a little red dot\u2019s host galaxy is not something that has been usually seen at such scale before, it isn\u2019t inconsistent with the dense gas cocoon model. The black hole star model of little red dots attributes excess blue light to stars in the host galaxy.<\/p>\n When Webb first discovered little red dots, some researchers thought these objects had \u201cbroken cosmology,\u201d unsure how galaxies could have grown so big so quickly in the early universe to account for all this light coming from their stars. However, the team believes the GLIMPSE-17775 puzzle piece fits nicely in the existing framework of the universe\u2019s evolutionary history, because black hole masses don\u2019t need to be as high in order to explain the broad emission lines.<\/p>\n \u201cEverything fits, nothing is broken, and I think that makes the puzzle that is our universe even better,\u201d said Kokorev. \u201cLooking ahead, I\u2019m eager to dive deeper and learn about what is powering the central engines of little red dots. While we think it\u2019s a black hole, there are some other interesting theories being proposed, which is exciting. Maybe in a year or two, we\u2019ll have the final answer to what powers these sources.\u201d\u00a0<\/p>\n The James Webb Space Telescope is the world\u2019s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).<\/p>\n To learn more about Webb, visit:<\/p>\n![\"A](\"https:\/\/assets.science.nasa.gov\/dynamicimage\/assets\/science\/missions\/webb\/science\/2026\/06\/STScI-01KTM4PHQTQEBK0TWEFSNGHHMC.png?w=4199&h=4199&fit=clip&crop=faces%2Cfocalpoint\")
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