A supermassive black hole in the far reaches of the Universe has been found guzzling down material at one of the fastest rates ever seen.
At the heart of a quasar galaxy called RACS J0320-35, just 920 million years after the Big Bang, the black hole appears to be devouring matter at 2.4 times the Eddington limit – the theoretical maximum rate, according to a team led by astrophysicist Luca Ighina of the Harvard & Smithsonian Center for Astrophysics.
This super-Eddington accretion, as the phenomenon is known, may help explain how supermassive black holes grew to masses billions of times that of the Sun, before the Universe was even a billion years old.
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"How did the Universe create the first generation of black holes?" says astrophysicist Thomas Connor of the Harvard & Smithsonian Center for Astrophysics. "This remains one of the biggest questions in astrophysics and this one object is helping us chase down the answer."
Supermassive black holes are major players in the Universe. Matter arranges itself in galaxies that swirl around the gravitational hub these black holes provide, the glue that holds the galaxy in orbit.
But they're also a mystery. Supermassive black holes, millions to billions of times the mass of the Sun, lurk in the first billion years of cosmic history – far too early to have formed from gradually devouring material over time.
This is because there's only so much material at a time a black hole can swallow. The maximum sustainable rate at which a black hole can feed is the Eddington limit.
When a black hole actively accretes large amounts of material, it doesn't fall straight down. Instead, the material swirls like water circling a drain, with only material at the disk's inner edge crossing the horizon into the black hole. Meanwhile, the incredible amount of friction and gravity in the disk heats the material to extreme temperatures, causing it to blaze with light.
But the thing about light is that it exerts a form of pressure. A single photon isn't going to do much, but the blaze of an active supermassive black hole accretion disk is another matter. At a certain point, the outward pressure of radiation matches the inward gravitational pull of the black hole, preventing material from moving closer. That's the Eddington limit.
However, for short periods of time, a black hole's accretion rate can break the Eddington limit, absolutely guzzling down material before radiation pressure can push it away. This super-Eddington accretion is one of the ways scientists think that black holes can get so big in such a short space of time after the Big Bang.
For the theory to have validity, it helps to acquire observational evidence. That's not easy; the beginning of the Universe is very far away across spacetime.
RACS J0320-35 could be one piece of that evidence. In 2023, the incredibly bright object was discovered in X-ray data obtained using NASA's Chandra X-ray Observatory, brighter in X-rays than any other object in the first billion years of the Universe.
Follow-up radio observations were then obtained using the Giant Metrewave Radio Telescope, the Australia Telescope Compact Array, and the Australian Large Baseline Array. Analysis of this data revealed how the galaxy's light is distributed across the electromagnetic spectrum.
The researchers then compared this against electromagnetic distribution models for super-Eddington accretion. They found the light from RACS J0320-35 is a close match, suggesting that the supermassive black hole at the heart of the galaxy is indulging in super-Eddington gluttony.
This needs to be validated, but the researchers make a strong case – which means RACS J0320-35 could become a tool for modeling how supermassive black holes formed and grew at the beginning of everything.
"By knowing the mass of the black hole and working out how quickly it's growing, we're able to work backward to estimate how massive it could have been at birth," says co-author Alberto Moretti of INAF-Osservatorio Astronomico di Brera in Italy. "With this calculation, we can now test different ideas on how black holes are born."
The research has been published in The Astrophysical Journal Letters.