A black hole of unexplained mass: JWST observations reveal a mature quasar at cosmic dawn

The James Webb Space Telescope observed a galaxy at a particularly young stage of the universe. Looking back into the past, it became clear that the light from the galaxy J1120+0641 took almost as long to reach Earth as the universe has taken to develop to this day. It is inexplicable how the black hole at its center could have weighed more than a billion solar masses at the time, as independent measurements have shown. The findings are published in the journal Nature Astronomy.

Recent observations of the material in the immediate vicinity of the black hole should reveal a particularly efficient feeding mechanism, but they have found nothing special. This result is all the more remarkable: it could mean that astrophysicists understand less about the development of galaxies than they thought. And yet they are by no means disappointing.

The first billion years of cosmic history pose a challenge: the earliest known black holes at the centers of galaxies have surprisingly large masses. How did they grow so big so quickly? The new observations described here provide strong evidence against some proposed explanations, especially against an “ultra-effective feeding mode” for the earliest black holes.

The limits of supermassive black hole growth

Stars and galaxies have changed enormously over the past 13.8 billion years, the lifespan of the universe. Galaxies have grown larger and gained more mass, either by consuming surrounding gas or (sometimes) by merging with each other. For a long time, astronomers assumed that the supermassive black holes at the centers of galaxies would have gradually grown along with the galaxies themselves.

But black hole growth cannot be arbitrarily fast. Matter falling onto a black hole forms a swirling, hot, bright ‘accretion disk’. When this happens around a supermassive black hole, the result is an active galactic nucleus. The brightest such objects, known as quasars, are among the brightest astronomical objects in the entire cosmos. But that brightness limits the amount of matter that can fall on the black hole: light exerts a pressure that can prevent additional matter from falling in.

How did black holes get so big and so fast?

That’s why astronomers were surprised when observations of distant quasars over the past two decades revealed very young black holes that had nevertheless reached masses as large as 10 billion solar masses. Light takes time to travel from a distant object to us, so looking at distant objects means looking into the distant past. We see the most distant quasars as they were in an era known as “cosmic dawn,” less than a billion years after the Big Bang, when the first stars and galaxies formed.

Explaining these early, massive black holes poses a significant challenge to current models of galaxy evolution. Could it be that early black holes were much more efficient at sucking in gas than their modern counterparts? Or could the presence of dust affect estimates of quasar masses in such a way that researchers overestimate the masses of early black holes? There are numerous proposed explanations at this point, but none are widely accepted.

A closer look at the early growth of black holes

Deciding which explanations – if any – are correct will require a more complete picture of quasars than previously available. With the advent of the JWST space telescope, in particular the telescope’s mid-infrared instrument MIRI, astronomers’ ability to study distant quasars took a quantum leap forward. For measuring quasar spectra at a distance, MIRI is 4,000 times more sensitive than any other instrument.

Instruments such as MIRI are built by international consortia, with scientists, engineers and technicians working closely together. Naturally, a consortium is very interested in testing whether their instrument performs as well as planned.

In exchange for building the instrument, consortia are usually given a certain amount of observation time. In 2019, years before JWST was launched, the MIRI European Consortium decided to use some of this time to observe what was then the farthest known quasar, an object designated J1120+0641.

Observation of one of the earliest black holes

Analyzing the observations was the job of Dr. Sarah Bosman, a postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) and member of the European MIRI consortium. MPIA’s contributions to the MIRI instrument include building a number of key internal components. Bosman was specifically asked to join the MIRI collaboration to bring expertise on how best to use the instrument to study the early universe, especially the first supermassive black holes.

The observations were conducted in January 2023, during JWST’s first observing cycle, and lasted approximately two and a half hours. They represent the first mid-infrared study of a quasar in the cosmic dawn period, just 770 million years after the Big Bang (redshift z=7). The information comes not from an image, but from a spectrum: the rainbow-like decomposition of the light from the object into components of different wavelengths.

Detecting dust and fast-moving gas

The general shape of the mid-infrared spectrum (“continuum”) encodes the properties of a large torus of dust surrounding the accretion disk in typical quasars. This torus helps guide matter into the accretion disk and “feed” the black hole.

The bad news for those whose preferred solution to the massive early black holes lies in alternative rapid growth modes: the torus, and by extension the feeding mechanism in this very early quasar, appears to be the same as that of its more modern counterparts. The only difference is one that no model of rapid early quasar growth predicted: a slightly higher dust temperature of about a hundred Kelvin warmer than the 1300 K found for the hottest dust in less distant quasars.

The shorter wavelength part of the spectrum, dominated by the emissions from the accretion disk itself, shows that for us as distant observers the light from the quasar is not dimmed by more than normal dust. Arguments that we might just be overestimating the masses of early black holes because of extra dust aren’t the answer either.

Early quasars ‘shockingly normal’

The broad region of the quasar, where clumps of gas orbit the black hole at speeds close to the speed of light – allowing statements about the mass of the black hole and the density and ionization of the surrounding matter – also looks normally off. Based on almost all properties that can be inferred from the spectrum, J1120+0641 is no different from quasars at later times.

‘Overall, the new observations only add to the mystery: early quasars were shockingly normal. Regardless of what wavelength we observe them at, quasars are virtually identical in all epochs of the universe,” says Bosman. Not only the supermassive black holes themselves, but also their feeding mechanisms were apparently already fully ‘mature’ when the universe was only 5% of its current age.

Ruling out some alternative solutions, the results strongly support the idea that supermassive black holes started out with significant masses, in astronomy jargon: that they are “primordial” or “large seeded.” Supermassive black holes did not form from the remains of early stars, but subsequently grew to enormous sizes very quickly. They must have formed early with an initial mass of at least a hundred thousand solar masses, probably from the collapse of huge early gas clouds.