Researchers from the University of Cambridge have discovered a massive black hole in the early universe, that is “napping” after overfeeding itself. The study was published in the journal Nature.
Artist’s impression of a black hole during one of its short periods of rapid growth. Image Credit: Jiarong Gu
This black hole has overfed to the point where it is dormant in its host galaxy, much like a bear overindulging in salmon before hibernating for the winter.
Just 800 million years after the Big Bang, a global team of astronomers led by the University of Cambridge discovered this black hole in the early universe using the NASA/ESA/CSA James Webb Space Telescope.
The black hole is among the biggest that Webb has found, with a mass 400 million times that of the Sun. Since most black holes in the local universe are only about 0.1% of their host galaxy's mass, the black hole is so big that it accounts for about 40% of the host galaxy's total mass.
Despite its enormous size, this black hole is essentially dormant because it is accreting, or eating, the gas it needs to grow at a very slow rate, roughly 100 times below its theoretical maximum limit.
Existing theories about how black holes form are called into question by the existence of such an over-massive black hole so early in the universe that it is not expanding. According to the researchers, the most likely scenario is that black holes experience extended periods of dormancy after brief bursts of extremely rapid growth.
Even with extremely sensitive telescopes like Webb, black holes are harder to detect when “napping” because they are less bright. Since black holes cannot be seen directly, they are identified by the distinctive glow of a whirling accretion disc that forms close to the black hole's edges. When black holes are actively expanding, the accretion disc's gas gets very hot and begins to glow and emit ultraviolet light.
Even though this black hole is dormant, its enormous size made it possible for us to detect. Its dormant state allowed us to learn about the mass of the host galaxy as well. The early universe managed to produce some absolute monsters, even in relatively tiny galaxies.
Ignas Juodžbalis, Study Lead Author, Kavli Institute for Cosmology, University of Cambridge
Standard models predict that up to a certain point, called the Eddington limit, where the radiation pressure on matter surpasses the black hole's gravitational pull, black holes are formed from the collapsed remnants of dead stars and accrete matter.
The enormous size of this black hole, however, raises the possibility that conventional models may not sufficiently explain the formation and growth of these monsters.
It is possible that black holes are ‘born big’, which could explain why Webb has spotted huge black holes in the early universe. But another possibility is they go through periods of hyperactivity, followed by long periods of dormancy.
Roberto Maiolino, Study Co-Author and Professor, University of Cambridge
The Cambridge researchers collaborated with their Italian counterparts to run various computer simulations to model how this dormant black hole could have grown to such a massive size so early in the universe.
According to the researchers, the most likely scenario is that black holes can briefly surpass the Eddington limit, grow extremely quickly, and then go into extended periods of inactivity. They estimate that black holes like this one likely eat for five to ten million years and sleep for roughly 100 million years.
“It sounds counterintuitive to explain a dormant black hole with periods of hyperactivity, but these short bursts allow it to grow quickly while spending most of its time napping,” said Maiolino.
Astronomers will most likely find black holes during these times because the dormant periods are significantly longer than the ultra-fast growth periods.
This was the first result I had as part of my PhD, and it took me a little while to appreciate just how remarkable it was. It was not until I started speaking with my colleagues on the theoretical side of astronomy that I was able to see the true significance of this black hole.
Ignas Juodžbalis, Study Lead Author, Kavli Institute for Cosmology, University of Cambridge
If black holes in the early universe spent the majority of their time in a dormant state, the researchers say, this black hole is most likely the tip of a much larger iceberg. Because of their low luminosities, dormant black holes are harder for astronomers to detect.
“It is likely that the vast majority of black holes out there are in this dormant state – I am surprised we found this one, but I am excited to think that there are so many more we could find,” said Maiolino.
The observations were made as part of the JWST Advanced Deep Extragalactic Survey (JADES). The European Research Council and the Science and Technology Facilities Council (STFC), a division of UK Research and Innovation (UKRI), provided partial funding for the study.
Journal Reference:
Juodžbalis, I., et al. (2024) A dormant overmassive black hole in the early Universe. Nature. doi.org/10.1038/s41586-024-08210-5.