The cosmic puzzle created by evidence of apparently enormous black holes in the very early universe continues to deepen. Observations by the JWST of one such anomaly, known as J1120+0641, indicate that the once favored explanation for how these objects could emit so much light so soon after the Big Bang is unlikely, forcing astronomers to try again.
The extraordinary power of the JWST has allowed astronomers to observe galaxies that are further away than we have ever seen before. The further we look into space, the longer we look back in time – and we see these objects as they looked not so long after the universe was formed. The fact that many of these models appear larger and more developed than existing models requires explanation.
Among these oddities at the dawn of time are quasars, enormously bright accretion disks surrounding supermassive black holes. The intense brilliance of those of these early quasars, considering the billions of light years that light had to travel, indicates very massive black holes.
The dominant model of the universe does not allow for black holes to grow so large so quickly.
One explanation is that the objects we see are particularly efficient at feeding, meaning the black holes are smaller than the quasars that produced them would suggest. This would be a very handy way out of trouble, were it not for the fact that signs of such an efficient power supply have not been spotted in J1120+0641, suggesting that the black hole at its heart contains more than a billion solar masses.
That doesn’t make J1120+0641 the most massive anomalously large black hole (some have been found to have masses as high as 10 billion solar masses), but it’s still big enough to pose a problem given its age. It’s also the first black hole that JWST has studied in a way that could test some explanations that would avoid the need to rethink our models of the universe. J1120+0641 was chosen for this task because, as of 2019, when JWST began clocking time, it was the most distant quasar known.
The repeated delays of the JWST mean that the observations did not occur until January 2023, when more distant quasars had 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-Institut für Astronomy studied the spectrum of J1120+0641, collected by the JWST, and found that it appears indistinguishable from relatively nearby quasars used as benchmarks, except that it is surrounded by somewhat hotter dust.
The dust may be hotter, but it is no different, ruling out the explanation that dust anomalies led us to overestimate the masses of old black holes.
“Overall, the new observations only add to the mystery: early quasars were shockingly normal. No matter what wavelength we observe them at, quasars are virtually identical across all epochs of the universe,” Bosman said in a statement .
We can estimate the mass of a black hole from the light emitted by nearby clumps of gas in what is known as the broad region of the spectrum. These clumps orbit the black hole at a speed close to the speed of light, and the broad beam tells us how close, which in turn allows us to calculate the black hole’s mass. Using the JWST observations, Bosman and co-authors calculate the mass of J1120+0641 to be 1.52 billion times that of the Sun.
Black holes grow as their enormous gravity consumes the surrounding matter. However, there is a limit to how quickly that can happen, known as the Eddington limit, caused by the balance between outward radiation pressure and inward gravity. There are ways in which the limit can be temporarily exceeded, but there are questions about how long this can be sustained. In recent years, many black holes have been found that appear to have reached impossible masses, and the JWST has significantly increased their number.
If these gigantic early black holes are really the size we think, they must have exceeded the Eddington limit, or started out huge. This is known as the ‘heavy seed’ scenario and requires an explanation of how black holes with masses at least a hundred thousand times that of the Sun could have appeared before there were any stars.
By definition, these cannot have been formed the way black holes now happen: through the collapse of very massive stars. Instead, the most likely explanation is that massive clouds of gas have somehow collapsed directly into black holes. However, how this could have happened remains a problem yet to be solved.
The research has been published open access in the journal Nature Astronomy.