Detailed simulations of conditions in the early universe suggest that the mysterious “little red dots” recently observed by the James Webb Space Telescope (JWST) could be young, fast-growing black holes. These simulations, which come from researchers at the Max Planck Institute for Astrophysics in Germany, may resolve a long-standing puzzle concerning how the first supermassive black holes in the universe formed and grew.
Astronomers know that supermassive black holes already existed when the universe was less than one billion years old. What they don’t know is how the seeds of these black holes formed, and how they then grew into the cosmic monsters we observe today.
Thanks to the JWST, astronomers are now able to explore the universe at an exceptionally early stage in its development. Recently, the telescope captured images containing a multitude of compact, red sources of light. These “little red dots”, as astronomers have dubbed them, are galaxies with a red shift z of more than 4 to 6, indicating that they must have formed between 1 and 1.5 billion years after the Big Bang some 13.8 billion years ago.
To explore possible links between these dots and supermassive black holes, researchers led by Sunmyon Chon used the National Astronomical Observatory of Japan’s ATERUI III supercomputer to simulate the hydrodynamic conditions surrounding galaxies during this period of the universe’s history. They then zoomed in on individual gas clouds in these galaxies.
Black holes grow at extremely fast rates
They found that at z > 4‒6, intense far-ultraviolet radiation from these galaxies suppresses the cooling and fragmentation of molecules in the gas clouds. As a result, a single supermassive star forms rather than many smaller ones. This supermassive star subsequently collapses into a black hole “seed” with 106 times the mass of our own Sun (M⊙) – an order of magnitude more massive than theory predicts.
These black hole seeds are surrounded by dense, thick discs of gas that trap radiation, allowing the black holes to grow via a process known as super-Eddington accretion. By z ≈ 8, the black holes can reach masses of about 3 x 107 M⊙. Most notably, the simulations showed that electron scattering within the growth-promoting gas clouds produces the same type of Hα emission as the patterns astronomers see coming from the little red dots
“To our knowledge, this is the first time that the little red dots have been connected to a short-lived, enshrouded phase of heavy black hole seed formation,” Chon says. “Our simulations suggest that the dots may be associated with a population of very young, rapidly growing black holes.”
The result is particularly important because these dots appear to be abundant in the early universe, he adds. “If many of them are indeed associated with growing black holes, they could represent an important phase of early black-hole and galaxy evolution and may also influence star formation in their host galaxies.”
Zoom-in simulation technique
Chon notes that resolving the environment in which massive black holes form while simultaneously following how their cosmological surroundings evolved was no easy task. One major challenge was that the distance scales involved differ by more than 10 orders of magnitude. “Our zoom-in simulation technique allows us to bridge this enormous range of scales and self-consistently track black-holes forming within their environment,” he explains.
‘Little red dots’ could herald the birth of supermassive black holes
Read more
The researchers now plan to increase the number of simulated black-hole formation events in their dataset so they can make additional statistical comparisons with observations. This, they say, will help them understand the relative importance of different black-hole formation pathways and how these early black holes subsequently evolve. “Ultimately, we would like to understand the origin of the supermassive black holes that reside at the centres of galaxies, including the one at the centre of our own Milky Way,” Chon tells Physics World.
The present work is detailed in Nature.