JWST spots a strange red dot so extreme scientists can’t explain it

Follow-up data made it clear that these objects were extraordinarily distant. Even the ones closest to us had taken 12 billion years for their light to arrive. Since looking across space is also looking back in time, we see those objects as they appeared 12 billion years ago, roughly 1.8 billion years after the Big Bang.

Early Theories Point to Massive, Young Galaxies

This discovery raised difficult questions. To interpret any astronomical observation, researchers rely on models that describe what different types of objects should look like. Astronomers can confidently identify a star only because they understand stars as giant plasma spheres held together by gravity, generating energy through nuclear fusion. They also know how stars should appear in images and in detailed measurements of their light known as spectra. When an object matches both appearance and spectrum, it can be classified reliably.

The little red dots did not align with any familiar category, so astronomers began considering more extreme explanations. One early proposal suggested that these objects were unusually dense galaxies filled with enormous numbers of stars, with their reddish color caused by thick layers of dust. To visualize this density, imagine placing the solar system inside a cube one light-year on each side. In our region of space, that cube would contain only the Sun. In the proposed galaxies, the same cube would hold several hundred thousand stars.

In the Milky Way, only the central nucleus has star densities remotely comparable, and that region still contains only about one thousandth of the stars needed for the little-red-dot models. If these galaxies truly packed hundreds of billions of solar masses worth of stars less than a billion years after the Big Bang, it would challenge basic theories of how galaxies form. As co-author Bingjie Wang (Penn State University) notes, “The night sky of such a galaxy would be dazzlingly bright. If this interpretation holds, it implies that stars formed through extraordinary processes which have never been observed before.”

Galaxies or Active Galactic Nuclei? A Scientific Divide

Debate quickly emerged. Some researchers favored the star-rich, dust-heavy galaxy idea, while others argued that the little red dots were actually active galactic nuclei obscured by large amounts of dust. Active galactic nuclei occur when material spirals into a galaxy’s central black hole, forming an extremely hot accretion disk. However, this interpretation also ran into problems. The spectra of the little red dots differed significantly from known dust-reddened active galactic nuclei. The scenario also required these objects to host supermassive black holes with extremely large masses, and far more of them than expected, considering how many little red dots JWST detected.

Despite their disagreements, astronomers agreed on one point. To solve the mystery, they needed more data. The initial JWST findings offered images, but understanding the physics required spectra, which reveal how much light the objects emit at different wavelengths. Securing such observations is challenging because time on major telescopes is highly competitive. Once the significance of the little red dots became clear, many groups began requesting observing time. One of those successful proposals was the RUBIES program, led by Anna de Graaff of the Max Planck Institute for Astronomy, short for “Red Unknowns: Bright Infrared Extragalactic Survey.”

The RUBIES Survey Uncovers an Extreme Example

Between January and December 2024, the RUBIES team used nearly 60 hours of JWST time to collect spectra for 4500 distant galaxies, producing one of the largest JWST spectroscopic data sets so far. According to Raphael Hviding (MPIA), “In that data set, we found 35 little red dots. Most of them had already been found using publicly available JWST images. But the ones that were new turned out to be the most extreme and fascinating object.” The most striking discovery came in July 2024: an extraordinarily distant example they named “The Cliff,” whose light traveled 11.9 billion years to reach us (redshift z=3.55). Its properties suggested it was an especially intense representative of the little-red-dot population and therefore a crucial object for testing any theories about them.

The Cliff earned its name because of a dramatic feature in its spectrum. In what would normally be the ultraviolet region, the spectrum showed a very steep rise. Due to the expansion of the universe, that wavelength was stretched to nearly five times its original value, placing it in the near-infrared, a process called cosmological redshift. This sudden rise is known as a “Balmer break.” Balmer breaks appear in ordinary galaxies, especially in those forming few or no new stars, but they are much weaker than what was seen in The Cliff.

Testing Every Known Explanation

The unusually sharp Balmer break put The Cliff at odds with both of the leading interpretations for the little red dots. De Graaff and her colleagues tested a wide range of galaxy and active galactic nucleus models against the object’s spectrum, attempting to reproduce its features. Every model failed.

Anna de Graaff says, “The extreme properties of The Cliff forced us to go back to the drawing board, and come up with entirely new models.” Around this time, a September 2024 study from researchers in China and the UK suggested that some Balmer-break features might come from sources other than stars. De Graaff’s team had begun considering a related idea themselves. Balmer breaks can appear in the spectra of single, very hot, young stars, as well as in galaxies containing many such stars. Strangely, The Cliff resembled the spectrum of one very hot star more than that of an entire galaxy.

A New Model Emerges: The Black Hole Star (BH)*

Building on that idea, de Graaff and her collaborators introduced a new concept they refer to as a “black hole star,” written as BH*. In this model, the central engine is an active galactic nucleus containing a supermassive black hole with an accretion disk, but instead of dust, the entire system is enshrouded in a thick layer of hydrogen gas that reddens the emitted light. BH* objects are not true stars because they lack nuclear fusion in their centers. The gas around them is also far more turbulent than anything found in a normal star’s atmosphere. Still, the basic physical situation is comparable. The active galactic nucleus heats the surrounding gas envelope in a way that resembles how fusion heats the outer layers of a star, producing a similar outward appearance.

The models presented by the team serve as early proofs of concept. They are not yet perfect matches to the data, but they reproduce the observed features more successfully than any previous model. The steep rise in the spectrum that inspired the name The Cliff can be explained by a dense, spherical, turbulent gas envelope around an active galactic nucleus. If this interpretation is correct, The Cliff would represent an extreme case dominated by the central black hole star, while the other little red dots would contain varying mixes of BH* light and light from surrounding stars and gas.

Implications for Fast Early Galaxy Growth

If BH* objects are real, they could help clarify another long-standing puzzle. Earlier theoretical work on somewhat smaller intermediate-mass black holes had suggested that a gas-enshrouded configuration like this could enable very rapid black hole growth in the early universe. JWST has already revealed evidence for unusually massive black holes at early times. If supermassive black hole stars grow in a similar way, they could provide a new mechanism for explaining that rapid growth. It remains uncertain whether BH* objects can achieve this, but if they can, it would significantly influence models of early galaxy evolution.

Even with these promising insights, caution is needed. The results are brand-new and follow the standard practice of reporting scientific work only after acceptance by peer-reviewed journals. Whether these ideas will become widely accepted depends on further evidence gathered in the years ahead.

Remaining Mysteries and Future Observations

The new findings mark a major step, offering the first model capable of explaining The Cliff’s extreme Balmer break. However, they also raise new questions. How could such a black hole star form in the first place? What allows its unusual gas envelope to persist over long periods (especially since the black hole consumes the gas and must somehow be replenished)? How do the other spectral characteristics of The Cliff arise?

Addressing these issues will require both theoretical modeling and more observations. De Graaff’s team already has JWST follow-up observations scheduled for next year, targeting The Cliff and other especially interesting little red dots.

These future studies will help determine whether black hole stars truly played a role in shaping the earliest galaxies. The possibility is intriguing, but far from settled.

Background and Research Team

The work described here has been accepted for publication as A. de Graaff et al., “A remarkable Ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a Little Red Dot at z = 3.5” in Astronomy & Astrophysics. A companion paper led by Raphael Hviding, presenting the broader sample of Little Red Dots from the RUBIES survey, has also been published in the same journal under the title “RUBIES: A spectroscopic census of little red dots — All point sources with v-shaped continua have broad lines.”

Researchers involved include Anna de Graaff, Hans-Walter Rix and Raphael E. Hviding from the Max Planck Institute for Astronomy, along with Gabe Brammer (Cosmic Dawn Center), Jenny Greene (Princeton University), Ivo Labbe (Swinburne University), Rohan Naidu (MIT), Bingjie Wang (Penn State University and Princeton University), and other collaborators.