A black hole discovered lurking in the Cosmic Dawn is just way too big to easily explain. Sitting at the center of a galaxy called J1120+0641, it tips the scales at well over a billion Suns’ worth of mass.
Bigger black holes exist all around us today. The problem is the when of J1120+0641’s existence. At less than 770 million years after the Big Bang, it’s hard to figure out how the black hole had the time to gain that much mass.
We’ve known about the galaxy and its overstuffed black hole for more than a decade, and scientists had ideas about how it came to be. Now, observations using the JWST have nixed one of those notions. By all measures taken, J1120+0641 appears “shockingly normal”, leaving open more exotic explanations for the black hole’s weight-gain.
The discovery of J1120+0641 was announced back in 2011, and for a few years it remained the most distant quasar galaxy known. It was a good few years, actually. As far as we knew, J1120+0641 was an outlier, with one possible explanation for its size still on the table.
Quasar galaxies are galaxies that have a central supermassive black hole feeding at a tremendous rate. They’re surrounded by a vast cloud of gas and dust, which they slurp down as fast as they can. The friction and gravity around the black hole heat the material, causing it to shine brightly.
But the speed at which a black hole can feed is not limitless. The maximum stable rate is determined by its Eddington limit, beyond which the heated material shines so brightly that radiation pressure would exceed gravitational pull, pushing the material away and leaving nothing for the black hole to feed upon.
Now, black holes can briefly enter super-Eddington accretion, where they push through this limit and guzzle as much material as they can before the radiation pressure kicks in. This is one of the possible explanations for the black hole at the center of J1120+0641 and, as we find them in greater numbers, other large black holes lurking at the beginning of the Universe.
To look for the signs of super-Eddington accretion, astronomers needed data of a sufficient resolution to perform a detailed analysis of the galaxy’s light, looking for signatures associated with extreme processes. And for this, we needed JWST, the most powerful space telescope ever built, optimized for peering into those distant reaches of space and time.
JWST observed the galaxy in early 2023, and a team led by astronomer Sarah Bosman of the Max Planck Institute for Astronomy in Germany teased apart the light it collected to catalog the properties of the material around the black hole: a huge torus of dust on the outskirts, and a glowing disk swirling around and feeding into the black hole.
This analysis reveals that the black hole is actually feeding pretty normally – there’s nothing about its accretion that appears significantly different from other, more recent quasar galaxies.
One possible explanation for these giant black holes is that extra dust was leading astronomers to overestimate their masses. And yet there’s no sign of additional dust, either.
That means that J1120+0641 is what it appears to be: a pretty normal quasar galaxy, with a black hole that is not guzzling down material at a super-high rate. The black hole, and the way it feeds, were already relatively mature by the time we observed it, within a few hundred million years of the Big Bang
“Overall, the new observations only add to the mystery: Early quasars were shockingly normal,” Bosman says. “No matter in which wavelengths we observe them, quasars are nearly identical at all epochs of the Universe.”
This means that super-Eddington accretion isn’t the solution to the growth of puzzlingly massive black holes at the dawn of time.
The other leading explanation is that the black holes formed from pretty large ‘seeds’ to start with. Rather than a slow, gradual process from something the size of a star, this theory proposes that the black holes formed from the collapse of clumps of matter or even extremely huge stars up to hundreds of thousands of times the mass of the Sun, giving their growth a head start.
As we find more and more of these behemoths lurking in the fog at the beginning of the Universe, this notion seems less bizarre, and more like the best possible explanation we have for this mysterious epoch in our Universe’s history.
The research has been published in Nature Astronomy.
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