Where Do Supermassive Black Holes Come From? express
A field sown with seeds. Some seeds are large and sprout at the slightest rain; others are finicky, requiring special soil and rare conditions. Which seeds fall into the ground determines what kind of forest will grow years later. Astrophysicists think about the birth of black holes in the early universe in much the same way.
The James Webb telescope looked back 12–13 billion years and saw black holes with masses of billions of suns. Such giants could not have grown from small seeds in that time: there simply wasn't enough time. So we need "heavy seeds": gas clouds that collapse directly into a hole with a mass of tens or hundreds of thousands of suns, bypassing the star stage. But how often did that happen?
The authors ran three scenarios. In two "generous" versions, seeds appeared in almost every suitable cloud of cold, metal-poor gas. In the "strict" scenario, they formed only under rare circumstances, like direct collapse. By the time the universe was about a billion years old, all models produced roughly the correct ratio of black hole mass to stellar mass in large galaxies. But only the generous scenarios produced many "overgrown" systems where the hole rivals the stars in mass. These are exactly the ones that shine as brightly as those seen by Webb.
The difference becomes crystal clear when you look at mergers. In the generous models, black holes collide frequently — more than a hundred events per year across the observable universe, and almost every galaxy, even dwarf ones, harbors a hole at its center. In the strict model, mergers are rare, and small galaxies almost never host holes. Future gravitational wave detectors will be able to count these mergers and tell us which "seed" was the right one. Thus, the forest of today will tell us about the soil of the past.
Original abstract
as written by the authors arXiv:2606.12851
The JWST discoveries of supermassive black holes (BHs) at $$z \gtrsim 5$$ may provide key insights into their seeding origins. Using new $$[18{-}72~\rm Mpc]^3$$ BRAHMA cosmological simulations, we investigate how variations in heavy-seed prescriptions, coupled with a subgrid dynamical friction model, shape BH populations at $$z \sim 5$$ and $$z \sim 0$$. We consider two "lenient'' seed models, in which all halos containing sufficient dense & metal-poor gas form $$\sim10^4$$ and $$\sim10^5~M_{\odot}$$ seeds, and a "strict'' seed model, in which $$\sim10^5 M_{\odot}$$ seeds form only under additional constraints motivated by direct collapse black hole formation. By $$z \sim 5$$, all models produce $$M_*-M_{\rm BH}$$ relations broadly consistent with the observed local Universe for $$M_*\gtrsim10^9~M_{\odot}$$ galaxies, but only the lenient scenarios generate systems near the upper envelope of the observed local scatter. In galaxies hosting $$M_{\rm BH} \sim 10^8$$-$$10^9~M_{\odot}$$ BHs, lenient production of $$\sim10^5~M_{\odot}$$ seeds also produces multiple overmassive systems with $$M_{\rm BH}/M_* \gtrsim 0.01$$. Although their growth is dominated by seeding and mergers, these systems reach luminosities of $$\sim10^{43}$$-$$10^{45}\mathrm{erg s^{-1}}$$, comparable to those inferred for JWST-detected BHs. As a key observational signature, the lenient seed models yield merger rates of $$\gtrsim100\mathrm{yr^{-1}}$$ and near-unity local BH occupation fractions even in galaxies with $$M_* \lesssim 10^7~M_{\odot}$$. In contrast, the strict seed model produces merger rates of only $$\sim1\mathrm{yr^{-1}}$$ and local occupation fractions of $$\lesssim10\%$$ for galaxies with $$M_* \lesssim 10^8~M_{\odot}$$. Future gravitational-wave event rates and measurements of local BH occupation fractions will therefore provide strong constraints on the dominant pathways responsible for high-redshift BH assembly.
In the generous scenarios, black holes exist in almost every galaxy, even those with only a few million stars, like having a skyscraper in every village.
In Terry Pratchett's novel "The Pillars of Creation," the world rests on elephants and a turtle; here, the fate of galaxies is decided by tiny seeds in the first moments.