They are known as Little Red Dots, or LRDs. We find them in deep field images of the James Webb Space Telescope (JWST), and they remain a bit of a mystery. But a new study finds that they are not super-Eddington objects, so while they are unusual, they don’t break the known rules of astrophysics.
We’re pretty sure that LRDs are young supermassive black holes at the hearts of early galaxies. Previous observations strongly suggest that they have many of the properties of Active Galactic Nuclei (AGNs) such as quasars, blazars, and radio galaxies. For example, the emission lines of their spectra are very broad, which means they come from material that is rapidly moving around a dense mass. They also don’t emit much x-ray or radio light, suggesting the source is surrounded by a dense cloud of ionized gas. This is exactly what you’d expect to see from a rapidly growing black hole in a primordial galaxy.
There is, however, a possible snag in this model. Since the light from AGNs comes from the super-hot accretion disk surrounding the black hole, the amount of x-rays they emit is tremendous. In order to block the x-rays, the ionized cloud surrounding the AGN would need to be very thick, almost galactic in size. But this cloud should also block much of the visible and infrared light we see from Little Red Dots. In order to be as bright as we observe, these things would need to emit an incredible amount of energy. Perhaps too much energy according to what’s known as the [Eddington Limit].(https://briankoberlein.com/post/take-it-to-the-limit/)
To consume matter, black holes have to pull gas and dust into the small region surrounding the black hole. All of the material trying to enter the black hole is squeezed tremendously, which is why it superheats and emits light. Of course, the heat and pressure from the matter push other stuff away from the black hole. As a result, there is a kind of self-regulating maximum growth rate for black holes, known as the Eddington limit. If a black hole captures too much matter too quickly, the resulting heat and pressure would push away incoming material, thus damping down the growth rate.
It is possible that a black hole could break this limit for a short time, becoming a super-Eddington black hole. Eat really fast, then clear the room with a hot burp. This could also explain the lack of intense x-rays, since the black hole is eating material too quickly for x-rays to build in intensity. But a new study suggests this isn’t the case.
X-rays can be difficult to detect, and we only see a handful of x-ray photons from any given LRD. So the team compiled observations from the Chandra Deep Field South, combining observations from 55 different LRDs into a simulated single one. This gave the team the ability to look at the optical depth of the surrounding material statistically, as well as the overall intensity of a typical LRD. What they found was that the amount of ionized gas surrounding an LRD is significant, but the overall intensity doesn’t break the Eddington limit.
One way to explain these results is to assume the supermassive black holes at the center of Little Red Dots aren’t as massive as they initially appear. Thus, they would emit plenty of light at wavelengths that aren’t heavily obscured without emitting the kind of high-energy x-rays we observe in more modern AGNs. There is still a great deal to learn about LRDs, but we now know they aren’t as mysterious as we once thought.
Reference: Sacchi, Andrea, and Akos Bogdan. “Chandra Rules Out Super-Eddington Accretion For Little Red Dots.” arXiv preprint arXiv:2505.09669 (2025).