The cosmic puzzle posed by evidence of apparently giant black holes in the early universe is growing. Observations of one such anomaly, known as J1120+0641, by the JWST suggest that the once-favored explanation of how these objects could emit so much light so soon after the Big Bang is unlikely, so astronomers must try again.
The extraordinary achievement of the JWST has allowed astronomers to observe galaxies that are farther away than anything we have ever seen before. The further we look into space, the further back in time we see – and we see these objects as they were not long after the universe was formed. The fact that many of them appear to be larger and more evolved than existing models seem to allow for requires an explanation.
Among these curiosities from the early days of human history are quasars, enormously bright accretion disks that surround supermassive black holes. The intense brightness of these early quasars, which takes into account the billions of light years that the light had to travel, indicates the presence of very massive black holes.
The prevailing model of the universe does not provide a way for black holes to reach such a size so quickly.
One possible explanation is that the objects we observe are particularly efficient at feeding, meaning that the black holes are smaller than the quasars they produce would suggest. This would be a very convenient way out of the mess if some signs of such efficient feeding had not been discovered in J1120+0641, suggesting that the black hole at its center contains more than a billion solar masses.
While that doesn’t make J1120+0641 the heaviest anomalous large black hole—some have as much as 10 billion solar masses—it’s still big enough to pose a problem given its age. It’s also the first black hole that the JWST has studied in a way that can test some explanations that would avoid rethinking our models of the universe. J1120+0641 was chosen for this task because it was the most distant quasar known in 2019, when time was booked at the JWST.
Due to repeated delays to the JWST, observations did not take place until January 2023. By that time, more distant quasars had already been observed, but J1120+0641 was still a suitable choice. We see it as it was 770 million years after the Big Bang.
Dr. Sarah Bosman of the Max Planck Institute for Astronomy examined the spectrum of J1120+0641 collected by JWST and found that it is optically no different from the relatively nearby quasars that serve as comparison objects, apart from being surrounded by slightly hotter dust.
The dust may be hotter, but otherwise shows no differences. This rules out the explanation that dust anomalies caused us to overestimate the masses of ancient black holes.
“Overall, the new observations only add to the mystery: early quasars were frighteningly normal. No matter what wavelength ranges we observe them at, quasars are almost identical in all eras of the universe,” Bosman said in a statement.
We can estimate the mass of a black hole by looking at the light emitted by nearby gas clumps in the so-called broad-line region of the spectrum. These clumps orbit the black hole at nearly the speed of light, and the broad-line radiation tells us how close they are, which in turn allows us to calculate the mass of the black hole. Using the JWST observations, Bosman and his co-authors calculate the mass of J1120+0641 to be 1.52 billion times the mass of the Sun.
Black holes grow as their enormous gravity captures the surrounding matter. However, there is a limit to how fast this can happen, called the Eddington limit, caused by the balance of outward radiation pressure and inward gravity. There are ways to temporarily exceed the limit, but there are doubts about how long this can be maintained. In recent years, many black holes have been found that have reached seemingly impossible masses, and the JWST has significantly increased their number.
If these giant early black holes are really the size we suspect, they must have exceeded the Eddington limit or been huge to begin with. This is known as the “heavy seed” scenario and requires an explanation of how black holes with masses of at least a hundred thousand solar masses could have formed before stars even existed.
By definition, they could not have been formed in the same way that black holes are formed today – through the collapse of very massive stars. The most likely explanation is that huge gas clouds somehow collapsed directly into black holes. How this happened, however, is still an unsolved problem.
The study is published in open access in the journal Nature Astronomy.