Kavli Affiliate: Mark Vogelsberger
| Summary:
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 $sim10^43$-$10^45mathrmerg s^-1$, comparable to those inferred for JWST-detected BHs. As a key observational signature, the lenient seed models yield merger rates of $gtrsim100mathrmyr^-1$ 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 $sim1mathrmyr^-1$ 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.
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