Signatures of Black Hole Seeding on the <i>M</i> _• – <i>σ</i> Relation: Predictions from the <tt>BRAHMA</tt> Simulations

Abstract The James Webb Space Telescope has identified a large population of supermassive (10 6 –10 8 M ⊙ ) black holes (BHs) in the early Universe ( z ∼ 4–7). Current measurements suggest that many of these BHs exhibit higher BH-to-stellar mass ratios than local populations, opening a new window into the earliest stages of BH–galaxy coevolution and offering the potential to place tight constraints on BH seeding and growth in the early Universe. In this work, we use the BRAHMA simulations to investigate the impact of BH seeding on the M • – σ relation. These simulations adopt heavy ∼10 5 M ⊙ seeds and systematically varied BH seeding models, resulting in distinct predictions for seed abundances. We find that different seed models lead to different normalizations of the M • – σ relation at higher redshifts ( z &gt; 2) across all σ , and at low redshift for systems with low σ (50 km s −1 ≲ σ ≲ 80 km s −1 ). The most lenient seed model also shows negligible evolution in the M • – σ relation across redshift, while more restrictive models have substantially lower normalization on the M • – σ relation for high σ (∼100 km s −1 ) at high redshifts, and evolve upward toward the local relation. We demonstrate that, while an evolving M * – σ relation mitigates changes in the M • – σ relation, any M • – σ evolution is a direct consequence of merger-dominated BH growth in low mass galaxies (≲10 9 M ⊙ ) and accretion-dominated BH growth in high-mass (≳10 9 M ⊙ ) galaxies. Furthermore, the scatter in the M • – σ relation is larger for the more restrictive models due to the inability of many BHs to grow significantly beyond their seed mass.