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

The James Webb Space Telescope (JWST) discoveries of supermassive black holes (BHs) at z ≳ 5 may provide key insights into their seeding origins. Using new [18–72 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 ∼ 5 and ∼ 0. We consider two “lenient” seed models, in which all halos containing sufficiently dense and metal-poor gas form ∼10 4 and ∼10 5 M ⊙ seeds, and a “strict” seed model, in which ∼10 5 M ⊙ seeds form only under additional constraints motivated by direct collapse BH formation. By z ∼ 5, all models produce M * – M BH relations broadly consistent with the observed local Universe for M * ≳ 10 9 M ⊙ galaxies, but only the lenient scenarios generate systems near the upper envelope of the observed local scatter. In galaxies hosting M BH ∼ 10 8 –10 9 M ⊙ BHs, lenient production of ∼10 5 M ⊙ seeds also produces multiple overmassive systems with M BH / M * ≳ 0.01. Although their growth is dominated by seeding and mergers, these systems reach luminosities of ∼10 43 –10 45 erg s −1 , comparable to those inferred for JWST-detected BHs. As a key observational signature, the lenient seed models yield merger rates of ≳100 yr −1 and near-unity local BH occupation fractions even in galaxies with M * ≲ 10 7 M ⊙ . In contrast, the strict seed model produces merger rates of only ∼1 yr −1 and local occupation fractions of ≲10% for galaxies with M * ≲ 10 8 M ⊙ . 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.