It’s the astronomical equivalent of the chicken or the egg debate: which comes first – the galaxy or the black hole?
The answer has evaded researchers for decades – but new findings, described as “remarkable” have shone a light on the darkest reaches of the universe.
Image of Abell 2744 (Pandora’s Cluster) and Little Red Dot Abell2744-QSO1, captured by Webb’s NIRCam. Picture: NASA, ESA, CSA, Lukas Furtak (Ben-Gurion University); Image Processing: Alyssa Pagan (STScI)
We know that when large stars within existing galaxies consume their fuel, they collapse to form black holes, which gobble up surrounding material and then merge with one another over time to form supermassive black holes.
But thousands of black holes millions to billions of times the mass of the Sun have been detected in the early universe, leaving astronomers baffled as to how they formed from such small seeds.
Now an international team of researchers led by the University of Cambridge have used the James Webb Space Telescope to detect clear observational evidence that some supermassive black holes were enormous from the beginning.
They formed without going through a stellar collapse phase – and without a significantly more massive host galaxy to feed them.
“This is a remarkable finding,” said Prof Roberto Maiolino from Cambridge’s Cavendish Laboratory and Kavli Institute for Cosmology, co-author of two studies published in Nature and the Monthly Notices of the Royal Astronomical Society. “It’s a total revisiting of the classical scenarios of how black holes form and grow.”
They discovered this using detailed observations of a prototypical Little Red Dot – crimson dots that appear in images of the early universe a few hundred million years after the Big Bang.
This one – Abell2744-QSO1 (QSO1) – existed just 700 million years after the Big Bang and is more than 13 billion light years away and only 1,300 light years across.
But it is magnified and appears in three different locations in the sky because it is ‘gravitationally lensed’ by a galaxy cluster called Pandora’s Cluster, making it easier to study.
QSO1 was believed to be a cloud of glowing hydrogen and helium gas, circling a supermassive black hole, although there was doubt over the black hole’s size.
The researchers used instruments on the James Webb Space Telescope to trace the effects of the black hole’s gravity on the gas swirling around it, while also mapping the distribution of various elements in the gas.
They found that the gas has Keplerian rotation, meaning it orbits a central point in the same way that planets in our solar system orbit the Sun.
“This is important because it tells us that most of the mass of QSO1 is concentrated in the black hole at the centre. If the mass were more distributed, as it would be if there were a lot of stars, the gas would not have this perfect Keplerian rotation,“ explained Cambridge PhD student Ignas Juodžbalis, who was co-lead author on one of the studies.
Keplerian motion is governed by simple laws of gravity so the researchers were able to use the gas velocity measurements to calculate the black hole mass directly – something not previously possible.
This confirmed the black hole immense – roughly 50 million solar masses – and makes up two-thirds of QSO1’s total mass.
The James Webb Space Telescope. Picture: NASA
This is thousands of times greater than in nearby galaxies, where supermassive black holes make up only a tiny fraction of the host galaxy’s total mass.
“This is a phenomenal result,” said Cosimo Maiolino, of the University of Florence, co-lead author. “It is the first direct measurement of a black hole mass within the first billion years after the Big Bang, and it is consistent with the previous measurements.”
It suggests that assumptions used for indirect mass measurements are valid and that the masses of other black holes in the early universe have not been overestimated.
And the outsized mass of QSO1 relative to its host galaxy suggests it cannot have formed gradually from much smaller, stellar-mass black holes merging and feeding.
“It seems that we have found a black hole that does not have a substantial host galaxy and that has predated stellar processes,” said Ignas. “This is very exciting because it is evidence for primordial black holes or direct collapse black holes, which have been theorised but have not been confirmed.”
QSO1’s black hole may have evolved from a ‘heavy seed’ that formed within the first second of the big bang or later from the collapse of a giant cloud of gas. But it was almost certainly born big – and may be in the early stages of building a galaxy around it.
The researchers believe Little Red Dots like QSO1 cannot have been rare in the early universe.
They are now analysing similar objects to find out whether supermassive black holes do predate the galaxies where they are currently found.