How do supermassive black holes get so massive? New simulations show how black holes might have grown rapidly in the early universe.

A deep-sky image from JWST. The inset shows the galaxy JADES-GS-z14-0, which is currently the most distant known galaxy. This image shows how the galaxy looked less than 300 million years after the Big Bang. [NASA, ESA, CSA, STScI, B. Robertson (UC Santa Cruz), B. Johnson (CfA), S. Tacchella (Cambridge), P. Cargile (CfA)]
Black Holes as Far as Our Telescopes Can See
When JWST first examined galaxies in the early universe, it discovered something extraordinary: galaxies just a few hundred million years after the Big Bang are home to black holes, and these black holes are massive.
How these black holes grew to millions or billions of times the mass of the Sun so quickly is an open question, but most theories fall into two general categories: these early black holes either sprouted from stellar-mass pips that bulked up at a prodigious rate, or they got their start as more massive black holes that grew at a more modest rate. Other, more speculative origin stories invoke exotic forms of dark matter or supermassive stars. All options face serious challenges, and many researchers turn to simulations to find a way forward.
Captured by Clusters
Yanlong Shi (California Institute of Technology and University of Toronto) and collaborators used simulations to test a way for young black holes to become supermassive in short order. The simulations began with 100 million solar masses of gas collected in a cloud about 330 light-years across, representative of a dense star-forming clump in an early galaxy.

Gas density (colored areas), star locations (cyan dots), and selected black hole locations (black stars) during three stages of the simulated evolution. These images show how the growing black holes are captured by tight star clusters and transported to the center of the cloud. From there, an accretion disk forms and channels gas to the black holes. Click to enlarge. [Shi et al. 2024]
Once the black holes reached the center of the cloud, their growth kicked into a whole new gear. Now attracted by a deep gravitational potential well, gas from throughout the cloud sank toward the central black holes and swirled together in an accretion disk. The magnetic field threaded throughout the disk prevented the disk from fragmenting and forming new stars. In about a million years, the two fastest-growing black holes had grown to more than 2 million solar masses.
Connecting the Little Red Dots

Another illustration of the movement and growth of the black holes. The leftmost image shows the capture of the black holes by the star clusters and the motion of these captured black holes to the center of the cloud. The images on the right show the masses and mass ratios of selected star clusters and black holes over time. Click to enlarge. [Shi et al. 2024]
The team’s simulations may yield a clue to the solution of another cosmic mystery. Near the end of the simulation, the assemblage of young stars and massive black holes has similar properties to the unusual “little red dot” galaxies spotted by JWST just 700 million years after the Big Bang.
While Shi’s team hopes to explore the observational implications of their model results further, the predictions will likely be difficult to test, requiring telescopes to pierce the dense, dusty gas that obscures the inner galactic regions where black holes grow.
Citation
“From Seeds to Supermassive Black Holes: Capture, Growth, Migration, and Pairing in Dense Protobulge Environments,” Yanlong Shi et al 2024 ApJL 969 L31. doi:10.3847/2041-8213/ad5a95