How do supermassive black holes get so massive? New simulations show how black holes might have grown rapidly in the early universe.
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.
Into this cloud the authors randomly scattered black hole seeds with masses between 100 and 10,000 solar masses. At first, the black hole seeds grew randomly as they encountered and devoured dense clumps of gas. But at some point, the black hole seeds were captured by nearby massive star clusters. From there, the black holes went along for the ride, trapped within the gravitational wells of the star clusters that migrated toward the center of the cloud. The black holes reached the center of the cloud much faster than if they were not shepherded by star clusters.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
This scenario appears to provide a path forward for the formation of supermassive black holes in the early universe. In particular, it shows how the capture of a black hole by a star cluster rapidly puts the black hole into prime position to accrete large amounts of gas from its surroundings. The magnetically stabilized accretion disk then provides a way for the black holes to accrete faster than the Eddington limit — the theoretical limit beyond which the outward pressure of radiation generated by accretion overpowers the inward pull of gravity.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