A monster black hole appeared first, then its galaxy began to grow around it

Using observations gathered by the James Webb Space Telescope (JWST), an international team of astronomers have revealed that one supermassive black hole in the early universe must have formed before a galaxy developed around it. Publishing their results in Monthly Notices of the Royal Astronomical Society, a team led by Roberto Maiolino at the University of Cambridge hope their results could lead to a better understanding of the origins of these immense objects.

Mysteriously supermassive

Supermassive black holes (SMBH) are known to lurk at the centers of most galaxies, including our own Milky Way. Carrying up to billions of times the mass of the sun, they have presented a long-standing conundrum to astronomers.

According to our latest models, black holes form from the remnants of supernova explosions, which most often occur when massive stars reach the ends of their lives. Afterwards, they can grow by consuming gas from surrounding accretion disks—but their growth rate is restricted by a brightness threshold called the “Eddington limit.” Beyond this point, the outward pressure from radiation exceeds the gravitational pull, and material is ejected into space.

The problem for astronomers is that SMBHs have been observed just a few hundred million years after the Big Bang—incredibly early on cosmological timescales, and far too soon for such immense objects to form under the restrictions imposed by the Eddington limit.

“Several alternative scenarios have been proposed,” Maiolino explains. “Small ‘seed’ black holes might have undergone brief phases of extremely rapid growth; intermediate-mass black holes could have formed through runaway mergers in dense stellar systems; or black holes may have formed already massive—so-called ‘heavy seeds.'”

In this third scenario, SMBHs either form through the direct collapse of vast clouds of material—only possible under the extreme conditions present in the very early universe—or from “primordial” black holes: extremely dense concentrations of matter formed shortly after the Big Bang, first proposed by Stephen Hawking.

Pristine environment

To explore these possibilities, Maiolino’s team carried out an in-depth analysis of QSO1: a SMBH present when the universe was around 700 million years old, visible via light emitted by its accretion disk. QSO1 is part of a class of objects known as Little Red Dots—enigmatic sources first observed by JWST that are believed by some astronomers to be primordial galaxies containing nascent SMBHs, though they lack the X-ray signatures usually associated with accreting black holes.

The team selected this object in part because it is gravitationally lensed by a foreground galaxy cluster, which bends more of its emitted light into our line of sight, effectively magnifying it.

Reconstructing the chemical surroundings

To achieve the highest possible resolution, Maiolino’s team used an integral field observation mode, capturing spectra at every point across a small patch of sky. “The combination of high spatial and spectral resolution allowed us to resolve the black hole’s ‘sphere of influence,” where gas motions are dominated by its gravity,” Maiolino describes. “This made it possible to directly measure the black hole’s mass.”

The same data enabled precise measurements of emission from ionized hydrogen and oxygen, revealing the chemical composition of the gas surrounding the black hole.

When the first atoms formed after the Big Bang, they consisted entirely of hydrogen, helium, and traces of lithium. Heavier elements can only form through nuclear fusion inside stars, enriching the surrounding environment when supernova explosions scatter them into space.

In this case, “we found that QSO1 is embedded in an environment with extremely low chemical enrichment,” Maiolino illustrates. “In particular, the abundance of oxygen relative to hydrogen is less than 1% of the value measured in the sun, indicating a near-pristine composition.”

Heavy seed scenario

This result suggests that very few stars had formed in QSO1’s surroundings—implying the black hole is likely far more massive than the system around it. “Together, these findings point toward a scenario in which the black hole formed before the bulk of its host galaxy, rather than growing within a pre-existing galaxy as traditionally assumed,” Maiolino continues.

Of the formation scenarios considered, this finding most closely aligns with the “heavy seed” scenario, in which black holes are already very massive at birth. Based on this result, Maiolino’s team are hopeful that their result could pave the way for a long-awaited breakthrough in our understanding of how SMBHs first formed.

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