Supermassive black holes 'too big' for the early universe may not be so massive after all

In the four years that the James Webb Space Telescope (JWST) has been making observations of the early universe, it has been delivering one scientific milestone after another. However, the $10 billion telescope has also left astronomers with a massive problem  — literally.

The JWST has been routinely spotting supermassive black holes in the early universe that, at hundreds of millions of times the mass of the sun, are too big to have grown to such monstrous sizes before the universe was even 1 billion years old. These cosmic titans are also too massive in comparison to the masses of their small host galaxies, challenging what astronomers know about the relationships between

and their galactic homes that are usually observed in the modern universe.

"We estimate masses of roughly one to ten million

, compared with the tens or hundreds of millions previously inferred," team leader Alessandro Trinca of the Italian National Institute for Astrophysics (INAF) Astronomical Observatory of Rome told Space.com. "These are still enormous objects, comparable to the four-million-solar-mass black hole at the center of the

or a few times larger, but they lie at the lower end of the supermassive black hole population rather than among the most extreme examples.

"Importantly, these masses are much more consistent with the small galaxies that host them."

While considering the mystery of

, Trinca and colleagues picked up on another puzzle associated with these objects.

To reach such sizes, these black holes must be voraciously feeding on surrounding gas and dust. Such feeding black holes and the cosmic larders of gas and dust that surround them are usually extremely bright emitters of X-rays. But this doesn't seem to be the case with the supermassive black holes in the early universe seen by the JWST. The team looked at 14 X-ray-silent supermassive black holes, finding that there is another way of considering this lack of emission.

Trinca explains that this apparent contradiction is related to what happens when a black hole accretes matter at extremely high rates. The disk of gas around the black hole becomes geometrically thick, forming a donut-like structure with a narrow funnel along the rotation axis. The hot inner region that would normally produce strong X-rays becomes surrounded by this thick disk.

"As the X-rays escape, they are repeatedly scattered by the surrounding gas, losing energy before they can emerge. As a result, the black hole appears much fainter in X-rays than we would normally expect," Trinca continued. "Therefore, the lack of X-rays is not unexpected in this scenario; it is actually a prediction of the model.

"In addition, this very high feeding might help explain how these black holes could already have grown to millions of times the sun's mass when the universe was only a few hundred million years old."

One of the reasons that supermassive black holes are so troubling in the early universe is the fact that they grow by consuming matter and through merging with other black holes. However, there exists a limit to how rapidly a black hole, or any astronomical body, can gather or "accrete" matter.

Called the "

" this exists because the more matter a body accretes, the more radiation it blasts out. So as the disk around a feeding black hole gathers enough mass to feed a rapidly growing black hole, it should emit so much radiation that it pushes away more matter, preventing it from being replenished.

Eventually this cuts off the food supply to the black hole and curtails its growth. However, this research suggests that supermassive black holes can enter brief phases of "super-Eddington" feeding.

"If these

are up to thirty times less massive than previously estimated, there is much less mass to build up within the available time, and their growth history becomes much less extreme," Trinca said. "Instead of requiring continuous, uninterrupted growth over hundreds of millions of years, the picture becomes one of shorter episodes of very rapid accretion, which is exactly the kind of behavior we might expect in the gas-rich and dynamically active galaxies of the early universe."

Confirming the team's theory about bouts of extreme feeding for supermassive black holes will require astronomical evidence, which may be tough to obtain at least for now.

"The most direct test would be to study similar objects that are accreting very rapidly but are located at lower redshifts, where we can observe and characterize their environments in much greater detail. However, such objects are quite rare," Trinca said. "For the more distant population observed with JWST, future missions providing deeper X-ray observations and improved spectra will be of crucial importance and might potentially detect the weak X-ray emission predicted by this scenario."

"What was interesting was seeing how much information could be extracted from something we did not observe," Trinca concluded. "A non-detection may appear to be the least informative type of observation, but in this case the absence of X-rays provided a strong constraint and helped define the physical picture more clearly than the detections alone."

The team's research was published in the June edition of the journal

.