Using the James Webb Space Telescope (JWST), astronomers have discovered a supermassive black hole at “cosmic dawn” that appears impossibly massive. The confusion stems from the fact that it doesn’t appear that this gigantic void was feasting on much surrounding matter at the time – but to reach its immense size you would expect it to have been ravenously hungry when time began.
The feeding supermassive black hole, which powers a quasar at the heart of the galaxy J1120+0641, was seen as it was when the universe was only about 5% of its current age. It also has a mass more than a billion times that of the Sun.
While it is relatively easy to explain how denser, and therefore more recently, supermassive black holes have grown to billions of solar masses, the fusion and feeding processes that enable such growth are expected to take about a billion years. That means finding such supermassive black holes that existed before the 13.8 billion-year-old universe was a billion years old is a real dilemma.
Since entering service in the summer of 2022, the JWST has proven particularly efficient at detecting such challenging black holes at cosmic dawn.
One theory surrounding the early growth of these cavities is that they were engaged in a feeding frenzy called an “ultra-effective feeding mode.” However, JWST observations of the supermassive black hole in J1120+0641 did not reveal a particularly efficient feeding mechanism in the material in its immediate vicinity. This finding casts doubt on the ultrafast feeding growth mechanism of supermassive black holes and means scientists may know even less about the early evolution of the cosmos than they realized.
Related: How did supermassive black holes become so big so quickly just after the Big Bang?
“Overall, the new observations only add to the mystery: early quasars were shockingly normal,” team leader and postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA), Sarah Bosman, said in a statement. “No matter what wavelength we observe them at, quasars are virtually identical across all epochs of the universe.”
Supermassive black holes control their own diet
Over the past 13.8 billion years of cosmic history, galaxies have grown in size by acquiring mass, either by absorbing surrounding gas and dust, by cannibalizing smaller galaxies, or by merging with larger galaxies.
About twenty years ago, before the JWST and other telescopes began finding troubling supermassive black holes in the early universe, astronomers had assumed that the supermassive black holes at the hearts of galaxies were gradually growing in step with the processes that led to galactic growth.
In fact, there are limits to how fast a black hole can grow – limits that these cosmic titans are helping to set for themselves.
Due to the conservation of angular momentum, matter cannot fall directly into a black hole. Instead, an oblate cloud of matter called an accretion disk is formed around the black hole. In addition, the immense gravity of the central black hole creates powerful tidal forces that create turbulent conditions in the accretion disk, heating it and emitting light across the entire electromagnetic spectrum. These emissions are so bright that they often exceed the combined light of every star in the surrounding galaxy. The regions where all this happens are called quasars and represent some of the brightest celestial bodies.
This brightness has another function. Even though it has no mass, light does exert pressure. This means that the light emitted by quasars pushes on surrounding matter. The faster the black hole feeding the quasar, the greater the radiation pressure and the more likely the black hole will cut off its own food supply and stop growing. The point at which black holes, or any other accretor, starve themselves by pushing away surrounding matter is known as the ‘Eddington limit’.
That means supermassive black holes can’t simply feed and grow as fast as they want. So finding supermassive black holes with masses as large as 10 billion suns in the early cosmos, especially less than a billion years after the Big Bang, is a real problem.
Astronomers need to know more about early quasars to determine whether early supermassive black holes were able to exceed the Eddington limit and become so-called “super-Eddington accretors.”
To do this, in January 2023, the team focused the JWST’s Mid-Infrared Instrument (MIRI) on the quasar at the heart of J1120+0641, which is 13 billion light-years away and is seen to appear only 770 million years after the Big Bang. The research represents the first mid-infrared study of a quasar that existed during the cosmic dawn.
The light spectrum from this early supermassive black hole revealed the properties of the large, ring-shaped ‘torus’ of gas and dust that encircles the accretion disk. This torus helps guide matter to the accretion disk, from where it is gradually fed to the supermassive black hole.
MIRI observations of this quasar showed that the cosmic supply chain functions similarly to that of ‘modern’ quasars closer to Earth, which therefore exist in later eras of the universe. That’s bad news for proponents of the theory that an improved feeding mechanism led to the rapid growth of early black holes.
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In addition, measurements of the region around the supermassive black hole, where matter swirls at nearly the speed of light, matched observations of the same regions of modern quasars.
The JWST observations of this quasar revealed one major difference between it and its modern counterparts. The dust in the torus surrounding the accretion disk had a temperature of about 2,060 degrees Fahrenheit (1,130 degrees Celsius), which is about 100 degrees hotter than the dust rings around supermassive black hole-powered quasars seen closer to Earth.
The study favors a different method of early supermassive black hole growth, suggesting that these cosmic titans got a head start in the early universe by forming from “seeds” of black holes that were already huge. These heavy seeds would have had masses at least a hundred thousand times that of the Sun, which is formed directly by the collapse of early and enormous gas clouds.
The team’s research was published June 17 in the journal Nature Astronomy.