2025ApJ...991...81B

We analyze the dynamics of low-mass black hole (BH) seeds in the high-redshift (z ≳ 5) Universe using a suite of [4.5 Mpc]³ and [9 Mpc]³BRAHMA cosmological hydrodynamic simulations. The simulations form seeds with mass M_seed = 2.2 × 10³M_⊙ in halos that exceed critical thresholds of dense and metal-poor gas mass (5-150M_seed) and the halo mass (1000-10,000M_seed). While the initial BRAHMA boxes pinned the BHs to the halo centers, here we implement a subgrid dynamical friction (DF) model. We also compare simulations where the BH is allowed to wander without the added DF. We investigate the spatial and velocity offsets of BHs in their host subhalos, as well as BH merger rates. We find that subgrid DF is crucial to ensure that a significant fraction of BHs effectively sink to halo centers by z ∼ 5, thereby enabling them to get gravitationally bound and merge with other BHs at separations close to the spatial resolution (∼0.2-0.4 kpc) of the simulation. For the BHs that merge, the associated merger timescales lag between ∼100 and 1000 Myr after their host halos merge. Compared to predictions using BH repositioning, the overall z ≳ 5 BH merger rates under subgrid DF decrease by a factor of ∼4-10. Under subgrid DF, the different seed models predict merger rates between ∼100 and 1000 events per year at z ≳ 5. These mergers dominate early BH growth, assembling BHs up to ∼10⁴-10⁵M_⊙ by z ∼ 5, wherein ≲2% of their mass is assembled via gas accretion. Our results highlight the promise for constraining seeding mechanisms using gravitational waves from future facilities such as the Laser Interferometer Space Antenna.