The In Situ Growth of Stellar-mass “Light” Seed Black Holes in Nuclear Star Clusters

Abstract Remnant black holes (BHs) of massive stars (“light seeds”) are a potential origin for supermassive black holes. We use magnetohydrodynamic simulations to study the formation and growth of light seeds in star-forming giant molecular clouds (GMCs) with masses 10 5 –10 9 M ⊙ , which evolve for ∼10–30 Myr and form compact star clusters, akin to high-redshift nuclear star clusters. In particular, the simulations resolve very massive stars (VMSs; 100–300 M ⊙ ), including their radiative and mechanical feedback, and model feedback-regulated accretion onto remnant BHs. We find that, even in compact GMCs capable of forming deep potential wells, the gas reservoir is expelled by sustained stellar feedback and is rapidly dispersed after supernova explosions. Remnant BH populations emerge ∼3 Myr after the starburst and concentrate at the cluster center (where ρ BH ∼ 10 4 –10 6 M ⊙ pc −3 ). With our fiducial subgrid BH accretion/feedback model, in situ BH accretion is inefficient for forming heavy seeds: some direct-collapse BHs briefly accrete at ∼(1–10) times the Eddington rate, but they reach only ∼400–500 M ⊙ . A top-heavy initial mass function or natal kicks do not change this conclusion. Runaway accretion is only possible if the subgrid BH model allows a high fraction of Bondi inflow to reach the BH, in which case a few seeds can grow to ∼10 6 M ⊙ . We also discuss multiple-generation star formation that may be intrinsically correlated with remnant BH accretion.