Webb finds strongest evidence yet for "black hole stars”

 The complex puzzle of the objects known as little red dots (LRDs) has gradually become more complete since their initial discovery by the NASA/ESA/CSA 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 analysing 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 ionised gas. 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.”

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 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 [1] 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).

"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 in France, who is a co-author of the study 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.”

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 [2] from this small, red source, which is the most detailed LRD 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.

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 black hole star 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 black hole star 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 programmes that used the NASA/ESA Hubble Space Telescope: Frontier Fields and BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy Observations) programmes.

The Webb and Hubble data together help explain why the Balmer break is weaker than typically found in other little red dots: a giant host galaxy is surrounding GLIMPSE-17775. Although an LRD’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.”

Notes

[1] Astronomers know that the first stars, officially known as Population III stars, must have been made almost solely of hydrogen and helium — the elements that formed as a direct result of the Big Bang. They would have contained none of the heavier elements like carbon, nitrogen, oxygen, and iron that are found in stars shining today. In other words, Population III stars were metal-free (astronomers refer to any element heavier than helium as a metal).

[2] In a spectrum, light emitted or absorbed at a specific frequency by an atom or molecule. Each ion, atom, and molecule emits and absorbs light at specific wavelengths, making it possible to identify the makeup of a star or other celestial body. Emission lines produce bright features, absorption lines dark features, and each line represents light given off or absorbed by one or more substances.

GLIMPSE-17775 in Abell S1063 (NIRCam image annotated)

The little red dot that would come to be known as GLIMPSE-17775 was fortunately included in the NASA/ESA/CSA James Webb Space Telescope’s field of view as it was observing galaxy cluster Abell S1063 for a different scientific purpose. GLIMPSE-17775 is located behind the galaxy cluster and has a cosmological redshift of 3.5, meaning it existed about 1.8 billion years after the Big Bang.

Since galaxy clusters like Abell S1063 are some of the most massive objects in the Universe, light emitted by objects farther away can become distorted as it reaches the telescope. This effect is known as gravitational lensing. The combination of Webb’s 30 hours of observing time and gravitational lensing enabled scientists to obtain the deepest spectrum to date of a little red dot. The result: the strongest evidence to date of a hot, dense gas cocoon known as a “black hole star.”

[Image description: A field of galaxies against the black background of space. In the centre 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.]

Credit:

NASA, ESA, CSA, V. Kokorev (University of Texas at Austin), A. Pagan (STScI)

GLIMPSE-17775 spectrum

The NASA/ESA/CSA James Webb Space Telescope’s spectroscopic data on little red dot GLIMPSE-17775 contains more than 40 spectral lines. The spectrum contains multiple independent indicators that support the theory that this little red dot is a black hole star: a rapidly accreting, or growing, black hole enveloped in a hot, dense gas cocoon. This layered, shell-like environment is reprocessing the light emitted from near the black hole and producing the features seen in the spectrum.

For example, scientists found that many of the spectral lines, such as hydrogen, oxygen, and helium, do not match a simple, rotating gas cloud model. The best fit model includes a broadening effect known as electron scattering, a telltale sign that a dense, layered gas cocoon is enshrouding the source.

[Image description: A spectrum graphic showing the amount of light blocked on the y-axis versus wavelength of light, in 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.]

Credit:

NASA, ESA, CSA, V. Kokorev (University of Texas at Austin), A. Pagan (STScI)

Abell S1063 galaxy cluster

The little red dot that would come to be known as GLIMPSE-17775 was fortunately included in the NASA/ESA/CSA James Webb Space Telescope’s field of view as it was observing this galaxy cluster Abell S1063 for a different scientific purpose.

Since galaxy clusters like Abell S1063 are some of the most massive objects in the Universe, light emitted by objects farther away can become distorted as it reaches the telescope. This effect is known as gravitational lensing. The combination of Webb’s 30 hours of observing time and gravitational lensing enabled scientists to obtain the deepest spectrum to date of a little red dot. The result: the strongest evidence to date of a hot, dense gas cocoon known as a “black hole star.”

[Image description: A field of galaxies against the black background of space. In the centre 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.]

Credit:

NASA, ESA, CSA, V. Kokorev (University of Texas at Austin), A. Pagan (STScI)

Fuente: ESA/Hubble/Webb Information Centre