The initial supermassive black holes were surprisingly average

The early Universe is a puzzling and, in many ways, still unknown place. The first billion years of cosmic history saw the explosive creation of stars and the growth of the first galaxies. It’s also a time when the earliest known black holes appeared to grow very massive quickly. Astronomers want to know how they grew and why they feed more like “normal” recent supermassive black holes (SMBH).

Today we see SMBH in galaxies that can have upwards of millions or billions of solar masses sequestered away. Astronomers naturally assumed that it took a long time for such monsters to build up, like billions of years. So, when JWST observed the most distant quasar J1120+0641, they expected to see an active galactic nucleus as it looked some 770 million years after the Big Bang. That is, they expected a still-growing central supermassive black hole. They were intrigued to find that it had a mass of at least a billion suns.

Theory vs Observation: How Do Supermassive Black Holes Form?

The growth of SMBH in the early Universe is a hot topic these days. The standard idea for a long time was that they grew slowly through mergers and acquisitions during galaxy formation. Since those mergers take a long time—millions of years at least—it seemed that the black holes would go along for the long, slow ride. And, you can’t speed up black hole growth too much once one forms. As matter swirls into the black hole, it does so through the accretion disk that feeds it. The disk, the active galactic nucleus, is very bright due to the radiation emitted as the matter heats up through friction and magnetic field interactions. The light pressure pushes stuff away, limiting how quickly the black hole can eat. Still, astronomers found these early SMBH sporting 10 billion solar masses when, by conventional wisdom, they should have been less massive.

For J1120+0641, astronomers considered different scenarios for its growth, including a so-called “ultra-effective feeding mode”. That implies early SMBH had some very efficient way of accreting gas, dust, and other material. So, astronomers looked at these active galactic nuclei at the hearts of distant quasars in more detail using JWST. It has the MIRI spectrograph that looks at the light from those quasars in great detail. The MIRI spectra of J1120+0641 revealed the presence of a large dust torus (a donut-shaped ring) surrounding the accretion disk of the SMBH. That disk is feeding the SMBH at a very “normal” rate, similar to SMBH in the “modern” Universe. The quasar’s broad-line region, where clumps of gas orbit the black hole at speeds near the speed of light, looks normal too.

In the Final Analysis

By almost all the properties that can be deduced from the spectrum, J1120+0641 turns out to be feeding no differently than quasars at later times. So, what does that mean for theories of SMBH formation in the early Universe? According to Sarah Bosman, who headed up the team that used JWST to study this and other quasars, the observations rule out fast feeding and other explanations for why the SMBH is so massive. “Overall, the new observations only add to the mystery: early quasars were shockingly normal. No matter in which wavelengths we observe them, quasars are nearly identical at all epochs of the Universe,” she said.

If you extrapolate these observations to other ideas about early SMBH, it means the process of black hole growth was pretty much set early in cosmic history. They didn’t start as stellar-mass black holes that got big. Instead, they formed from the collapse of very massive early clouds of gas to become massive primordial seeds. From there, not only did they feed from their accretion disks, but probably did grow even more massive through those mergers and acquisitions. Thanks to JWST, astronomers now know that the early feeding mechanisms were already in place very early in cosmic time. Now they just need to figure out when the primordial seeds of SMBH first appeared in the infant Universe.

For More Information

A Black Hole of Inexplicable Mass
A Mature Quasar at Cosmic Dawn Revealed by JWST Rest-frame Infrared Spectroscopy
First rest-frame Infrared Spectrum of a Z > 7 Quasar: JWST/MRS Observations of J1120+0641