Surprisingly inconspicuous: despite their average appetite, black holes already had masses more than a billion times that of the Sun in the early universe.
A peek into the early stages of the 13.8 billion year old universe James Webb Space Telescope We discovered a galaxy 700 million years in the future. big bang. Why Black Hole When the universe was still in its infancy, the center of a black hole could have been as much as a billion times the mass of the Sun. James Webb’s observations were designed to take a closer look at the feeding mechanism of black holes, but they found nothing unusual. It seems that black holes were already growing in a way similar to how they do today. But the results are even more significant: they could indicate that astronomers don’t know as much about the formation of galaxies as they thought. Still, the measurements are not disappointing. On the contrary.
The mystery of early black holes
The first billion years of the universe’s history pose a conundrum: the oldest black holes, at the centers of galaxies, have astonishingly large masses. How could they have become so massive so quickly? New observations described here provide strong evidence against several proposed explanations, in particular a “super-efficient feeding mode” of the oldest black holes.
Limits on the growth of supermassive black holes
In the 13.8 billion years since the beginning of the Universe, stars and galaxies have undergone significant changes: galaxies have grown larger and more massive by consuming the gas around them and sometimes merging with each other. Astronomers have long assumed that the supermassive black holes at the centers of galaxies have grown gradually along with the galaxies themselves.
But black holes cannot grow at an arbitrary rate. The material that falls into a black hole forms a swirling, hot, and bright “accretion disk.” When this happens around a supermassive black hole, an active galactic nucleus is formed. The most luminous such objects are called quasars, and are among the most luminous objects in the universe. But their brightness limits the amount of material that can fall into the black hole: light exerts pressure and prevents more material from falling in.
How did a black hole get so massive so quickly?
That’s why astronomers were surprised over the past 20 years when observations of distant quasars revealed a very young black hole with a mass 10 billion times that of the Sun. Because it takes time for light to travel from a distant object to us, observing distant objects means observing the distant past. The most distant quasars we can observe date back to less than a billion years after the Big Bang, a time known as the “cosmic dawn,” when the first stars and galaxies were forming.
Explaining early massive black holes poses a considerable challenge to current models of galaxy evolution. Were early black holes much more efficient at accreting gas than modern black holes? Or did the presence of dust affect quasar mass estimates, causing researchers to overestimate the mass of early black holes? Numerous explanations have been proposed to date, but none are universally accepted.
A closer look at early black hole growth
To determine which one is correct, and if so, which one, we need a more complete picture of quasars than ever before. The arrival of the JWST space telescope, and especially its mid-infrared instrument MIRI, has dramatically improved astronomers’ ability to study distant quasars. MIRI is 4,000 times more sensitive than any previous instrument when it comes to measuring the spectra of distant quasars.
Instruments like MIRI are built by international consortia, with scientists, engineers and technicians working closely together. Naturally, the consortium is very interested in testing whether its instrument works as planned. In return for building the instrument, the consortium is usually given a certain amount of observing time. In 2019, many years before the launch of JWST, the MIRI European consortium decided to use some of this time to observe an object designated J1120+0641, which was then the most distant known quasar.
One of the earliest black holes observed
The observations were analyzed by Dr. Sarah Bosman, a postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) and member of the MIRI European consortium. MPIA is contributing to the MIRI instrument by manufacturing some of its key internal components. Dr. Bosman was asked to join the MIRI collaboration to bring her expertise on how to most effectively use the instrument to study the early universe, and in particular the first supermassive black holes.
The observations took place in January 2023 during the JWST’s first observing cycle and lasted about two and a half hours. It will be the first mid-infrared study of a quasar at the dawn of the universe, just 770 million years after the Big Bang (at redshift z=7). The information comes not from an image, but from a spectrum – a rainbow-like breakdown of an object’s light into its components of different wavelengths.
Tracking dust and fast-moving gases
The overall shape of the mid-infrared spectrum (“continuum”) is characteristic of a large dusty torus surrounding the accretion disk of a typical quasar. This torus directs material into the accretion disk and “feeds” the black hole. Bad news for those who favor a different rapid growth mode as a solution for early massive black holes, but the torus, and thus the feeding mechanism, of this very early quasar appears to be the same as that of more modern quasars. The only difference is one not predicted by models of the rapid growth of early quasars: the dust temperature is about 100 K higher than the 1300 K found in the hottest dust of more distant quasars.
The short-wavelength part of the spectrum is dominated by radiation from the accretion disk itself, suggesting that the quasar’s light isn’t being dimmer to us from afar because of more dust than usual. The argument that we might just be overestimating the mass of the early black hole because of the increased dust doesn’t help either.
Early quasars are ‘surprisingly normal’
The broadline regions of the quasar, where clumps of gas orbit the black hole at close to the speed of light and from which we can infer the mass of the black hole and the density and ionization of the surrounding matter, also appear normal. Nearly every property that can be inferred from the spectrum makes J1120+0641 no different from later quasars.
“Overall, the new observations only deepen the mystery. Early quasars were surprisingly normal. Whatever wavelength we observe, quasars are roughly the same at every age in the universe,” Bosman said. When the universe was only 5% of its current age, it seems that not only the supermassive black holes themselves, but also their feeding mechanisms were already fully “mature.” By ruling out several alternatives, the results strongly support the idea that supermassive black holes started out with substantial masses from the very beginning, “primitive” or “massively generated,” in astronomical terms. Supermassive black holes did not form from the remnants of early stars and then rapidly grow to massive sizes. They must have formed early on, with initial masses at least 100,000 times that of the Sun, by the collapse of a giant early gas cloud.
Reference: “Mature quasars at cosmic dawn revealed by JWST rest-frame infrared spectroscopy” 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, Ewaine F. van Dischoek, Bart van den Busche, June 17, 2024, Natural Astronomy.
Publication date: 10.1038/s41550-024-02273-0