Supermassive Black Hole Assembly from Heavy Seeds with Dynamical Friction in the BRAHMA Simulations: Implications for JWST, LISA, and the Local Universe

The JWST discoveries of supermassive black holes (BHs) at $z\gtrsim 5$ may provide key insights into their seeding origins. Using new $[18-72\mathrm{M}\mathrm{p}\mathrm{c}{]}^3$ 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 $\sim {10}^4$ and $\sim {10}^5{M}_{\odot }$ seeds, and a "strict’’ seed model, in which $\sim {10}^5{M}_{\odot }$ seeds form only under additional constraints motivated by direct collapse black hole formation. By $z\sim 5$, all models produce $M_{\ast }-M_{\mathrm{B}\mathrm{H}}$ relations broadly consistent with the observed local Universe for $M_{\ast }\gtrsim {10}^9{M}_{\odot }$ galaxies, but only the lenient scenarios generate systems near the upper envelope of the observed local scatter. In galaxies hosting $M_{\mathrm{B}\mathrm{H}}\sim {10}^8$-${10}^9{M}_{\odot }$ BHs, lenient production of $\sim {10}^5{M}_{\odot }$ seeds also produces multiple overmassive systems with $M_{\mathrm{B}\mathrm{H}}\mathrm{/}{M}_{\ast }\gtrsim 0.01$. Although their growth is dominated by seeding and mergers, these systems reach luminosities of $\sim {10}^{43}$-${10}^{45}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$, comparable to those inferred for JWST-detected BHs. As a key observational signature, the lenient seed models yield merger rates of $\gtrsim 100\mathrm{y}{\mathrm{r}}^{-1}$ and near-unity local BH occupation fractions even in galaxies with $M_{\ast }\lesssim {10}^7{M}_{\odot }$. In contrast, the strict seed model produces merger rates of only $\sim 1\mathrm{y}{\mathrm{r}}^{-1}$ and local occupation fractions of $\lesssim 10\mathrm{\%}$ for galaxies with $M_{\ast }\lesssim {10}^8{M}_{\odot }$. 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.