How did black holes behave in the early days of the universe?

An international team of astrophysicists has used the James Webb Space Telescope (JWST) to study one of the most massive and distant black holes in the universe, at a distance of about 13 billion light-years, or as seen 13 billion years ago, when the universe was only about 800 million years old.

What the study found reveals, surprisingly, is that the black hole feeds in the same way as current black holes in the nearby universe.

The study is the work of an international team with significant participation from scientists from the Centre for Astrobiology (CAB), a joint centre of the Spanish National Research Council (CSIC) and the National Institute of Aerospace Technology (INTA).

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 become so massive so quickly? The observations in the new study provide strong evidence against some proposed explanations, in particular against an extremely effective feeding mode for increasing the mass of the first massive black holes in the universe.

The limits of growth of a supermassive black hole

Stars and galaxies have changed enormously over the past 13.8 billion years — the age of the universe. Galaxies have grown and gained more mass, either by consuming surrounding gas or (occasionally) merging with one another. 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 black hole growth can’t be arbitrarily fast. Matter falling onto a black hole forms a bright, hot, spinning accretion disk. When this happens around a supermassive black hole, the result is an active galactic nucleus. The brightest 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 become so massive so quickly?

That’s why astronomers were surprised when, over the past twenty years, observations of distant quasars revealed very young black holes that had nonetheless reached masses of up to 10 billion solar masses. Light needs 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 “cosmic dawn” — less than a billion years after the Big Bang, when the first stars and galaxies formed. Explaining those first massive black holes is a considerable challenge for current models of galaxy evolution. Could it be that the first 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 caused researchers to overestimate the masses of the first black holes? There are numerous explanations proposed at the moment, but none are widely accepted.

A closer look at the early growth of black holes

Deciding which explanation is correct requires a more comprehensive study of quasars than has been available until now. With the arrival of the JWST space telescope, specifically the MIRI mid-infrared instrument, astronomers’ ability to study distant quasars took a giant leap forward. The MIRI instrument was built by an international consortium involving scientists and engineers from the Spanish National Research Council (CSIC) and the National Institute of Aerospace Technology (INTA). In exchange for building the instrument, the consortium received a certain amount of observing time. In 2019, years before the launch of JWST, the European MIRI Consortium decided to use some of this time to observe what was then the most distant quasar known, an object bearing the designation J1120+0641.

Artist’s impression of the black hole J1120+0641 and its surroundings, including a quasar-like envelope, as it appeared 770 million years after the creation of the Universe. (Image: ESO/M. Kornmesser. CC BY 4.0)

Observing one of the first black holes

Luis Colina and Javier Álvarez Márquez of CAB were in charge of designing the data collection of the quasar and its subsequent calibration, correcting for instrumental effects. The analysis of the observations fell to Sarah Bosman, a postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) in Germany in close collaboration with CAB scientists. 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 in the period of cosmic dawn, just 770 million years after the Big Bang (redshift z=7). The information does not come from an image, but from a spectrum: the rainbow-shaped decomposition of the object’s light into components of different wavelengths.

Tracking fast-moving dust and gas

The general shape of the mid-infrared (“continuum”) spectrum encodes the properties of a large torus (a toroid) of dust surrounding the accretion disk in typical quasars. This torus helps guide matter into the accretion disk, feeding the black hole. The bad news for those whose preferred solution for the first massive black holes lies in alternative modes of rapid growth is that the torus, 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 somewhat higher dust temperature, about a hundred degrees Celsius warmer than the roughly 1,000 degrees found for the hottest dust in less 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 quasar light is not dimmed by any more dust than usual. Arguments that we may be overestimating the masses of the first black holes due to extra dust are also not a solution.

The first quasars are “surprisingly normal”

The quasar’s broad line region, where clumps of gas orbit the black hole at near-light speeds, allowing deductions about the black hole’s mass and the density and ionization of the surrounding matter, also appears normal.

In almost all properties that can be deduced from the spectrum, J1120+0641 is indistinguishable from quasars of later epochs.

“Overall, the new observations only add to the mystery: the first quasars are surprisingly normal. No matter what wavelengths we observe them in, quasars are almost identical at 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 just 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 considerable masses from the start – in astronomy jargon: that they are primordial. Supermassive black holes did not form from the remains of early stars and then become massive very quickly. They must have formed early with initial masses of at least a hundred thousand solar masses, presumably through the collapse of huge early gas clouds.

The study is titled “JWST rest-frame infrared spectroscopy reveals a mature quasar at cosmic dawn.” It has been published in the academic journal Nature Astronomy. (Source: CAB)