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 further away can obscure the view of the interior and shines mainly in the mid-infrared range, light that can be analyzed by the James Webb Space Telescope. A focused, high-energy particle beam protrudes into space from the immediate vicinity of the black hole perpendicular to the disk. Credit: © T. Müller / MPIA
Surprisingly unspectacular: Black hole weighed more than a billion solar masses in the early universe, despite average appetite.
If we look at the early stages of the 13.8 billion year old universe, James Webb Space Telescope has discovered a galaxy as it existed just 700 million years after the Big BangIt is puzzling how the black hole at its center could already weigh a billion solar masses when the universe was still in its infancy. The James Webb observations were intended to investigate the feeding mechanism in more detail, but they found nothing unusual. Apparently, black holes were already growing in a similar way as they do today. But the result is all the more significant: it could show that astronomers know less about how galaxies form than they thought. And yet, the measurements are by no means disappointing. On the contrary.
The mystery of early black holes
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 get so huge so quickly? The new observations described here provide strong evidence against some proposed explanations, particularly an “ultra-effective feeding mode” for the earliest black holes.
The limits to the growth of supermassive black holes
Stars and galaxies have changed dramatically over the past 13.8 billion years, the lifetime of the universe. Galaxies have grown larger and more massive, either by consuming surrounding gas or (sometimes) by merging with each other. Astronomers have long assumed that the supermassive black holes at the centers of galaxies grew gradually, along with the galaxies themselves.
But black hole growth can’t 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 how much matter can fall onto the black hole: light exerts a pressure that can prevent more matter from falling in.
How did black holes get so big so quickly?
That’s why astronomers were surprised when observations of distant quasars over the past two decades revealed very young black holes that nevertheless had 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 were in an era known as the “cosmic dawn,” less than a billion years after the Big Bang, when the first stars and galaxies were forming.
Explaining those 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 pulling in gas than their modern counterparts? Or could the presence of dust skew quasar mass estimates in a way that leads researchers to overestimate the masses of early black holes? There are currently numerous proposed explanations, but none is widely accepted.
A closer look at the early growth of black holes
To determine which, if any, explanations are correct, a more complete picture of quasars than previously available is needed. With the advent of the JWST space telescope, and in particular its mid-infrared instrument MIRI, astronomers’ ability to study distant quasars took a giant leap. MIRI is 4,000 times more sensitive than any previous instrument for measuring distant quasar spectra.
Instruments like MIRI are built by international consortia, with scientists, engineers and technicians working closely together. A consortium is naturally very interested in testing whether their instrument performs as well as planned. In return for building the instrument, consortia are typically given a certain amount of observing time. In 2019, years before JWST was launched, the MIRI European Consortium decided to use some of this time to observe the then most distant known quasar, an object known as J1120+0641.
Observation of one of the earliest black holes
Analyzing the observations fell to Dr. Sarah Bosman, a postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) and a member of the European MIRI consortium. MPIA’s contributions to the MIRI instrument included building a number of key internal components. Bosman was asked to join the MIRI collaboration specifically to bring expertise on how the instrument could best be used to study the early Universe, in particular the first supermassive black holes.
The observations were performed 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 at the period of cosmic dawn, 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 object’s light into components at different wavelengths.
Tracing dust and fast moving gas
The overall shape of the mid-infrared (“continuum”) spectrum encodes the properties of a large torus of dust surrounding the accretion disk in typical quasars. This torus helps funnel matter into the accretion disk, and “feeds” the black hole. The bad news for those who seek their preferred solution for the enormous early black holes in alternative rapid growth modes: the torus, and by extension the feeding mechanism in this very early quasar, appear to be the same as for 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 more distant quasars.
The shorter wavelength part of the spectrum, dominated by 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 may simply be overestimating the masses of early black holes because of extra dust are also not the answer.
Early quasars “shockingly normal”
The broad line region of the quasar, where clumps of gas orbit the black hole at velocities close to the speed of light – allowing inferences about the mass of the black hole and the density and ionization of the surrounding matter – also looks normal. According to 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. No matter what wavelength we observe them in, quasars are virtually identical at all ages 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. 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 beginning, in astronomical jargon: that they are “primordial” or “seeded large.” Supermassive black holes did not form from the remains of early stars and then grow enormous very quickly. They must have formed early with an initial mass of at least a hundred thousand solar masses, presumably via the collapse of enormous early gas clouds.
Reference: “A mature quasar at cosmic dawn revealed by JWST rest-frame infrared spectroscopy” by Sarah EI Bosman, Javier Álvarez-Márquez, Luis Colina, Fabian Walter, Almudena Alonso-Herrero, Martin J. Ward, Göran Östlin, Thomas R. Greve, Gillian Wright, Arjan Bik, Leindert Boogaard, Karina Caputi, Luca Costantin, Andreas Eckart, Macarena García-Marín, Steven Gillman, Jens Hjorth, Edoardo Iani, Olivier Ilbert, Iris Jermann, Alvaro Labiano, Danial Langeroodi, Florian Peißker, Pierluigi Rinaldi, Martin Topinka, Paul van der Werf, Manuel Güdel, Thomas Henning, Pierre-Olivier Lagage, Tom P. Ray, Ewine F. van Dishoeck and Bart Vandenbussche, June 17, 2024, Nature Astronomy.
DOI file: 10.1038/s41550-024-02273-0