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

The James Webb Space Telescope observed a galaxy located at a particularly young stage in the universe. Looking back, it became clear that light from the galaxy called J1120+0641 took almost as long to reach Earth as it took the universe to expand to the present day. It is inexplicable that the black hole at its center could have weighed more than a billion solar masses at the time, as independent measurements have shown. The results are published in the journal Natural astronomy.

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

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

Limits to the growth of supermassive black holes

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

But the growth of black holes cannot be arbitrarily rapid. Matter falling on a black hole forms a hot, shiny, swirling “accretion disk.” When this happens around a supermassive black hole, the result is an active galactic core. The brightest objects, called quasars, are among the brightest astronomical objects in the entire cosmos. But this brightness limits the amount of matter that can fall on the black hole: the light exerts pressure that can prevent any additional matter from falling.

How did black holes become so massive and so fast?

This is why astronomers were surprised when, over the last twenty years, observations of distant quasars revealed very young black holes which had nevertheless reached masses of up to 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 known quasars as they looked during the era known as the “cosmic dawn,” less than a billion years after the Big Bang, when the first stars and galaxies formed. trained.

Explaining these first massive black holes poses a considerable challenge for current models of galaxy evolution. Could it be that early black holes were much more efficient at accumulating gas than their modern counterparts? Or could the presence of dust affect quasar mass estimates in a way that would cause researchers to overestimate the masses of early black holes? There are currently many proposed explanations, but none are widely accepted.

A Closer Look at the Early Growth of Black Holes

Deciding which, if any, of the explanations is correct requires a more complete picture of quasars than previously available. With the advent of the JWST space telescope, particularly its mid-infrared MIRI instrument, astronomers’ ability to study distant quasars took a giant step forward. For measuring the spectra of distant quasars, MIRI is 4,000 times more sensitive than any previous instrument.

Instruments like MIRI are built by international consortia, with scientists, engineers and technicians working closely together. Naturally, one consortium is very interested in testing whether their instrument works as well as expected.

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

Observe one of the first black holes

The analysis of the observations was entrusted to Dr Sarah Bosman, 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 the construction of a number of key internal components. Bosman was invited to join the MIRI collaboration specifically to provide expertise on how best to use the instrument to study the early universe, particularly early supermassive black holes.

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

Tracking fast-moving dust and gases

The overall shape of the mid-infrared spectrum (“continuum”) encodes the properties of a large dust torus that surrounds the accretion disk in typical quasars. This torus helps guide matter onto the accretion disk, “feeding” the black hole.

The bad news for those whose preferred solution to the first massive black holes lies in alternative, rapid growth modes: the torus, and by extension the power mechanism of this very first quasar, appears to be the same as that of its counterparts more modern. The only difference is one that no model of the rapid growth of early quasars had predicted: a slightly higher dust temperature, about a hundred Kelvin, hotter than the 1,300 K found for the hottest dust. hot from less distant quasars.

The part of the shorter wavelength spectrum, dominated by emissions from the accretion disk itself, shows that for us, as distant observers, the quasar light is not attenuated by a quantity more dust than usual. Arguments that we may simply be overestimating the first masses of black holes due to the presence of extra dust are also not the solution.

The first quasars are ‘incredibly normal’

The large quasar region, where clumps of gas orbit the black hole at speeds close to the speed of light, allowing us to infer the mass of the black hole, as well as the density and ionization of surrounding matter , also seems normal. In almost all properties that can be inferred from the spectrum, J1120+0641 is no different from quasars of later epochs.

“Overall, the new observations only add to the mystery: the first quasars were incredibly normal. Regardless of the wavelengths in which we observe them, quasars are almost identical in all epochs of the universe,” Bosman explains. Not only the supermassive black holes themselves, but also their feeding mechanisms, were apparently already completely “mature” when the Universe was only 5% of its current age.

Ruling out a number of alternative solutions, the results strongly support the idea that supermassive black holes started with considerable masses from the start, in astronomy jargon: that they are “primordial” or “seeded large size “. Supermassive black holes did not form from the remains of early stars and then became massive very quickly. They must have formed very early with initial masses of at least one hundred thousand solar masses, probably via the collapse of early massive gas clouds.