Supermassive black holes ‘born big’

The Webb Space Telescope has observed a galaxy at a very early stage in the universe. Light from this system, called J1120+0641, took about the same 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 supermassive black holes at the cores of galaxies gradually grew along with the galaxies. But black holes cannot grow at arbitrary speeds. The material flowing toward the black hole forms a hot “accretion disk” swirling around it. If this happened to a supermassive black hole, it would lead to what is called an active galactic nucleus. The brightest of these objects, called quasars, are among the brightest objects in the universe. But this brightness limits the amount of matter that can fall on the black: 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 a mass 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 the presence of dust around the quasar distorts its mass.

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To determine which of these interpretations is correct, a team led by Sarah Bosman of the Max Planck Institute for Astronomy (MPIA, Germany) observed J1120+0641 for two and a half hours using the Webb Space Telescope in January 2023 — not taking a snapshot. An image of a distant galaxy, but recording the 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 very similar at 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.

Megan Vasquez

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