Detailed computer simulations of conditions in the early universe suggest that the mysterious compact red light sources recently observed by the James Webb Space Telescope could be young, rapidly growing black holes. The work, led by researchers at the Max Planck Institute for Astrophysics in Germany, may resolve a long-standing puzzle about how the first supermassive black holes formed and grew so quickly.
Astronomers know that supermassive black holes already existed when the universe was less than one billion years old. What remains unclear is how the seeds of these black holes formed and how they grew into the cosmic giants observed today. The James Webb Space Telescope has allowed astronomers to explore the universe at an exceptionally early stage of its development, and its images have revealed a multitude of compact red sources of light. These so-called little red dots are galaxies with a redshift greater than 4 to 6, indicating they formed between 1 and 1.5 billion years after the Big Bang, which occurred about 13.8 billion years ago.
To explore possible links between these dots and supermassive black holes, the 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. They then zoomed in on individual gas clouds within those galaxies. The simulations showed that at redshifts greater than 4 to 6, intense far-ultraviolet radiation from the 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.
That supermassive star subsequently collapses into a black hole seed with about one million times the mass of the Sun, an order of magnitude more massive than theory predicts. These seeds are surrounded by dense, thick discs of gas that trap radiation, allowing the black holes to grow through a process known as super-Eddington accretion. By a redshift of about 8, the black holes can reach masses of roughly 30 million solar masses. Most notably, the simulations showed that electron scattering within the growth-promoting gas clouds produces the same type of hydrogen-alpha 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 said. «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 added. 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.
Chon noted 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 explained.
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 evolved. «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 said. The work is detailed in the journal Nature.
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