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NASA Webb Finds Strongest Evidence Yet for ‘Black Hole Stars’

A field of galaxies against the black background of space. In the center is a bright-white elliptical galaxy that is the core of the Abell S1063 galaxy cluster. Around the core are short, curved red lines, which are distant background galaxies magnified and warped by gravitational lensing. A couple of foreground stars appear large and bright with Webb’s signature eight-point diffraction spike pattern. Toward the very bottom, slightly off center toward the right, is a small red dot that is highlighted by an orange square outline. A larger orange square in the top right corner shows the object in more detail. The object, labeled “GLIMPSE-17775” looks like a fuzzy red dot with a yellow core.

While the primary purpose of NASA’s James Webb Space Telescope’s 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.

Credits:
Image: NASA, ESA, CSA, Vasily Kokorev (UT Austin); Image Processing: Alyssa Pagan (STScI)

The complex puzzle known as little red dots has become more complete since their initial discovery by NASA’s James Webb Space Telescope in 2022. Now a particular little red dot’s spectrum is helping connect many of the pieces.

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’s spectrum captured by Webb — the deepest spectrum to date of a little red dot — 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 gas, a model referred to as the BH* (black hole star) scenario. A paper describing the results was published today in The Astrophysical Journal.

“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. But none of the previous little red dots have all of the pieces of evidence in the same place,” said Kokorev, lead author of the study. “With GLIMPSE-17775 we can test these models because of how deep and amazing this source’s spectrum is.”

Image: Abell S1063 with Pullout of GLIMPSE-17775 (NIRCam Image)

Image: NASA, ESA, CSA, Vasily Kokorev (UT Austin); Image Processing: Alyssa Pagan (STScI)

Connecting puzzle pieces

Soon after Webb first began science operations, it discovered a new, mysterious type of object in the very early universe – abundant red objects that emerged about 600 million years after the big bang. Scientists have explored multiple explanations for these little red dots, including the black hole star scenario.

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’s imaging and spectroscopy efforts for a project that sought to look for Population III stars and faint galaxies in galaxy cluster Abell S1063. This little red dot is more distant than the galaxy cluster and magnified by gravitational lensing. (GLIMPSE-17775 has a cosmological redshift of 3.5, meaning it existed about 1.8 billion years after the big bang.)

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’s infrared sensitivity and nature’s own “magnifying glass” amplified the amount of detail that could be gleaned from GLIMPSE-17775. The result was more than 40 spectral lines from this small, red source, which is the most detailed little red dot spectrum to date.

“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 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.

Image: Evidence of a ‘Black Hole Star’

Graphic titled “GLIMPSE-17775, Evidence of a ‘Black Hole Star, NIRSpec, Multi-Object Spectroscopy’” shows a graph of amount of light blocked on the y-axis versus wavelength of light, microns. The bottom of the y-axis is labeled “fainter,” and the top is labeled “brighter.” The x-axis starts with 2.80 microns at left and continues in increments of five, ending with 3.05 microns at right. A key at top left has a white line labeled “Data” and a small blue square labeled “Model of light scattered through hot dense gas.” The white data line is stepped with a large bell-like curve that peaks at 2.95 microns. It is labeled “hydrogen” and highlighted by a semi-transparent purple. The data also forms small peaks highlighted with different colors: around 2.84 microns, oxygen, green; 3.0 microns, helium, red; and 3.02 microns, sulfur, orange. The blue filling, representing the model, approximately fills the bell-like curve that marks hydrogen. A smaller peak of blue also approximately fills the data’s peak of helium.

NASA’s James Webb Space Telescope captured the deepest spectrum to date of a little red dot. More than 40 spectral lines have been discerned from the data, many of which independently support the theory that GLIMPSE-17775 is a black hole enshrouded by a hot, dense gas cocoon.

Illustration: NASA, ESA, CSA, Vasily Kokorev (UT Austin); Designer: Leah Hustak (STScI)

Lines of evidence

Among the 40-plus lines that the team detected in GLIMPSE-17775’s 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.

The strength and ratios of certain lines to each other, most notably the 16 iron lines that compose what the team has dubbed an “iron forest” 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.

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.

One missing element of the GLIMPSE-17775 puzzle piece is the part of the spectrum that would reveal what’s known as a Balmer break, or a strong dip in the emitted light that’s 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’s Hubble Space Telescope: the Frontier Fields and BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy Observations) programs.

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’s host galaxy is not something that has been usually seen at such scale before, it isn’t 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.

When Webb first discovered little red dots, some researchers thought these objects had “broken cosmology,” 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’s evolutionary history, because black hole masses don’t need to be as high in order to explain the broad emission lines.

“Everything fits, nothing is broken, and I think that makes the puzzle that is our universe 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.”

The James Webb Space Telescope is the world’s 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).

To learn more about Webb, visit:

https://science.nasa.gov/webb

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