Astronomers studying a population of unusually X-ray-silent and overmassive black holes discovered by the James Webb Space Telescope have found that they may not be as massive as they appear. The new paper, outlining a plausible scenario that would produce such black holes, was published in Astronomy & Astrophysics on June 19.
X-ray mute giants
Ever since the advent of JWST, astronomers have been able to detect massive black holes at galaxy centers within the first billion years of the universe. They are detected by the light they emit as they actively feed on infalling material. To calculate the mass of these black holes, astronomers look at the gas swirling around them at extreme speeds, which is detected as spectral lines. Faster swirling indicates stronger gravity, which in turn suggests a heavier black hole. This method has worked well for decades in the nearby universe.
However, when applied to the earliest black holes found by JWST, this method produces black holes that look far too massive compared with their host galaxies. The inferred mass relations are much different from what we see in the local universe. Because it’s difficult to explain how a black hole could grow that large so quickly, astronomers have been finding ways to explain the emergence of these monstrous objects.
Additionally, many of these black holes are not detected in X-rays. This is a puzzle because typically a separate region of extremely hot plasma sits above the swirling disk of the black hole, called the corona, which is responsible for the X-ray emission.
Two problems, one solution
Astronomers have a solution for both puzzles: super-Eddington accretion. When a black hole feeds faster than the theoretical limit at which its own radiation should push infalling gas away, it is called super-Eddington accretion. Feeding this aggressively could suppress X-ray output, and it might also distort the very emission lines used to estimate mass in the first place.
The team led by Alessandro Trinca of the INAF Astronomical Observatory of Rome built a model combining the physics of super-Eddington feeding with a detailed accretion disk spectrum to test this idea. They used it to reanalyze 14 X-ray-silent black holes previously studied using standard methods.
They found that every single black hole in their sample could be explained in two very different ways. In one case, the black holes are enormous but nearly dormant. That means they are feeding on so little gas that they simply don’t emit much at any wavelength. In the other case, the black holes are relatively small but feeding at extreme, super-Eddington rates. This scenario naturally produces weak X-rays as a side effect.
Lightweight black holes
When comparing the statistical likelihood of each solution, almost all 14 objects strongly preferred the small, fast-feeding solution. The “giant but dormant” solution was disfavored for physical reasons.
“The second scenario, involving super-Eddington accreting systems, would instead produce an intrinsically red spectrum, which may more naturally account for other observed properties of these sources,” the team writes in the paper.
The researchers note that their model does not account for thick clouds of gas that could also block X-rays. “It should also be emphasized that our results assume the absence of extremely high gas column densities capable of absorbing the X-ray emission from the AGN,” they note.
They say that future observations, particularly across different wavelengths and detailed spectral data, will reveal more reliable estimates of these black holes’ masses and accretion rates. If confirmed, the findings of this study would ease one of the biggest tensions in early black hole evolution by showing that rapidly growing black holes can mimic much larger ones.
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Publication details
Alessandro Trinca et al, You can’t see me: Super-Eddington growth hindering X-ray detection in high-z broad-line active galactic nuclei, Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202659544
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JWST’s ‘overmassive’ early black holes may not be so massive after all (2026, July 8)
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