Supermassive black holes ‘born big’

The Webb Space Telescope has spotted a galaxy at a very early stage in the universe. The light from this system, called J1120+0641, took about the same amount of time to reach Earth as it took the universe to become as large as it is today. But somehow, the supermassive black hole at its center had a mass of more than a billion solar masses.

Galaxies have changed tremendously over the past 13.8 billion years (the age of the universe). They grew larger and gained more mass – either by attracting gas from their environment or sometimes by merging.

It has long been assumed that the supermassive black holes at the heart of galaxies grew gradually with the galaxies. But black holes can’t grow at arbitrary speeds. Matter flowing toward the black hole forms a hot “accretion disk” that swirls around it. If this happens to a supermassive black hole, it results in what is called an active galactic nucleus. The brightest of these objects, called quasars, are among the brightest in the universe. But this brightness limits the amount of matter that can fall on the black hole: light exerts pressure that slows the flow of matter.

That’s why astronomers were surprised when, while observing distant quasars, they discovered very small black holes with masses of 10 billion solar masses. All sorts of explanations have been proposed for this, for example that the first black holes were more efficient at attracting gas than their modern counterparts. Or that the presence of dust around the quasar distorts its mass.

To determine which of these explanations is correct, a team led by Sarah Bosmann of the Max Planck Institute for Astronomy (MPIA, Germany) observed J1120+0641 for two and a half hours with the Webb Space Telescope in January 2023—not to take a snapshot of the distant galaxy, but to record its infrared spectrum.

“feeding mechanism”

Observations show that the supermassive black hole in J1120+0641 is not growing exceptionally quickly, suggesting that it uses the same “feeding mechanism” as its current counterparts. The only difference is that the dust surrounding the distant quasar is about a hundred degrees warmer than the dust surrounding the closer quasar. But nothing has been found to indicate that the quasar is surrounded by an exceptional amount of dust.

“Overall, the new observations add to the mystery: early quasars look surprisingly normal,” Bosman concludes. “No matter what wavelengths we observe, they look pretty much the same across all cosmic epochs.”

According to Bosman, the early supermassive black holes appear to have simply been “born big.” They did not form from the remains of the first stars and then grow very rapidly, but their initial mass was at least a hundred thousand solar masses—perhaps due to the contraction of large gas clouds.