Artist’s impression of the bright core region of a quasar, an active galaxy. The supermassive black hole at the center is surrounded by a bright disk of gas and dust. The dust component farther away can obscure the view of the interior and shines primarily in the mid-infrared range, light that can be analyzed by the James Webb Space Telescope. A beam of high-energy particles is ejected into space from the immediate vicinity of the black hole perpendicular to the disk. Credit: © T. Müller / MPIA
Surprisingly unimpressive: the black hole already weighed over a billion solar masses in the early universe despite its average appetite.
Peering into the early stages of the universe at 13.8 billion years old, James Webb Space Telescope has spotted a galaxy as it existed only 700 million years ago Big bang. It’s puzzling how Black hole at its center it could already weigh a billion solar masses when the universe was still in its infancy. James Webb’s observations were intended to take a closer look at the feeding mechanism, but they found nothing unusual. Apparently, black holes were already growing in a manner similar to today’s. But the result is even more important: It may show that astronomers know less about how galaxies form than they thought. Still, the measurements are by no means disappointing. On the contrary.
The mystery of early black holes
The first billion years of cosmic history present a challenge: the earliest known black holes at the centers of galaxies have surprisingly large masses. How did they get so massive, so fast? The new observations described here provide compelling evidence against some proposed explanations, particularly an “ultra-efficient feeding regime” for the earliest black holes.
The growth limits of a supermassive black hole
Stars and galaxies have changed enormously over the past 13.8 billion years, the lifetime of the universe. Galaxies have grown larger and gained more mass, either by consuming the surrounding gas or (occasionally) by merging with each other. Astronomers have long assumed that supermassive black holes at the centers of galaxies would have grown gradually along with the galaxies themselves.
But the growth of a black hole cannot be arbitrarily fast. Matter falling onto a black hole forms a rotating, 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 how much matter can fall onto the black hole: Light exerts a pressure that can prevent additional matter from falling in.
How did black holes get so massive and so fast?
That’s why astronomers were surprised when, over the past twenty years, observations of distant quasars revealed many young black holes that nevertheless reached a mass 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 were in an era known as the “cosmic dawn,” less than a billion years after the Big Bang, when the first stars and galaxies formed.
Explaining these early, massive black holes is a significant challenge for current models of galaxy evolution. Could it be that early black holes were much more efficient at accreting 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 is widely accepted.
A close-up look at the early growth of black holes
Deciding which, if any, of the explanations are correct requires a more complete picture of quasars than was previously available. With the advent of the JWST space telescope, specifically the telescope’s MIRI mid-infrared instrument, astronomers’ ability to study distant quasars has taken a giant leap. For measuring spectra of distant quasars, MIRI is 4000 times more sensitive than any previous instrument.
Instruments like MIRI are created by international consortia, with scientists, engineers and technicians working closely together. Naturally, a consortium is very interested in testing whether their tool works as well as intended. In exchange for building the tool, consortia are usually given a certain amount of monitoring 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 quasar known, an object that goes by the designation J1120+0641.
Observation of one of the earliest black holes
Analyzing the observations fell to Dr Sarah Bosman, a postdoctoral fellow at the Max Planck Institute for Astronomy (MPIA) and a member of the European MIRI consortium. MPIA’s contribution to the MIRI instrument includes the construction of a number of key internal parts. Bosman was asked to join the MIRI collaboration specifically to bring expertise on how best to use the instrument to study the early universe, specifically the first supermassive black holes.
The observations were made in January 2023, during the first cycle of JWST observations, and lasted about 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 derives not from an image but from a spectrum: the arc-like decomposition of an object’s light into components of different wavelengths.
Tracking dust and fast moving gases
The overall shape of the mid-infrared spectrum (“continuum”) encodes the properties of a large dust plume that surrounds the accretion disk in typical quasars. This torus helps direct matter into the accretion disk, “feeding” the black hole. The bad news for those whose preferred solution to massive early black holes lies in alternative rapid growth modes: the fuel and by extension the feeding mechanism in this very early quasar appears to be the same as in 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 kelvins, warmer than the 1300 K found for the hottest dust in less distant quasars.
The shorter-wavelength part of the spectrum, dominated by emission from the accretion disk itself, shows that to us as distant observers, the quasar’s light is not obscured by more than the usual dust. Arguments that perhaps we are simply overestimating the masses of early black holes due to extra dust are also not a solution.
Early Quasars ‘Shockingly Normal’
The broad-line region of the quasar, where clumps of gas orbit the black hole at speeds close to the speed of light—allowing inferences about the black hole’s mass and the density and ionization of the surrounding matter—also appears normal. With almost all the properties that can be deduced from the spectrum, J1120+0641 is indistinguishable from quasars at later times.
“Overall, the new observations only add to the mystery: the early quasars were shockingly normal.” “No matter what wavelengths we observe them at, quasars are almost identical across all epochs of the universe,” Bosman says. Not only supermassive black holes themselves, but their feeding mechanisms were apparently already fully “mature” when the universe was only 5% of its current age. By ruling out a number of alternative solutions, the results strongly support the idea that supermassive black holes started out with significant masses from the start, in astronomical jargon: that they are “primordial” or “big.” Supermassive black holes did not form from the remains of early stars and then became massive very quickly. They must have formed early with initial masses of at least a hundred thousand solar masses, possibly through the collapse of massive early clouds of gas.
Reference: “Mature Cosmic Dawn Quasar Revealed by JWST Infrared Quiescent Spectroscopy” by Sarah E. I. Bosman, Javier Alvarez-Marquez, Luis Colina, Fabian Walter, Almudena Alonso-Herrero, Martin J. Ward, Goran Ostlin, Thomas R. Greve, Gillian Wright, Ariane Bick, Leindert Boogaard, Carina Caputi, Luca Costantin, Andreas Eckart, Macarena Garcia-Marin, Steven Gilman, Jens Hjort, Edoardo Yanni, Olivier Ilbert, Iris German, Alvaro Labiano , Danial Langerudy, Florian Peisker, Pierluigi Rinaldi, Martin Topinka, Paul van der Werf, Manuel Gudel, Thomas Henning, Pierre-Olivier Lagage, Tom P. Ray, Ewain F. van Dyschock, and Bart Vandenbusche, 17 Jun 2024, Natural astronomy.
DOI: 10.1038/s41550-024-02273-0
