Intermediate massive black hole accretion and feedback in giant molecular clouds

We study the accretion and feedback of intermediate massive black holes (IMBHs) in star-forming and turbulent giant molecular clouds (GMCs), especially the possibility of super/hyper-Eddington accretion towards SMBHs. The problem can be divided into two aspects/scales: i. how much mass can be captured by the IMBH with the Bondi-Hoyle accretion; ii. the impact of sub-grid IMBH mechanical/radiative feedback. We run numerical MHD simulations of GMCs and IMBHs with FIRE-2 physics, including star formation and stellar feedback, to check both aspects of the problem.

In the first part (Paper I) of the study, we check how much mass can be accreted on IMBHs due to the Bondi-Hoyle accretion by resoluting the Bondi-Hoyle accretion length in the simulation. We randomly sample IMBH seeds weighing M_bh = 10²–10⁴ M_⊙ inside GMCs with low/medium/high initial mean surface densities (Σ₀ ~ 10², 10³, 10⁴ M_⊙/pc²) and radii. We found that for high surface density clouds the gas is gravitationally bound against mechanical/radiative stellar feedback, and the Bondi-Hoyle accretion is significant for some IMBHs with shallow dependence on their initial mass/position/velocity. Microscopically, the accretion is amplified in case that IMBHs are crossing dense clumps with low relative velocity. The study shows that for certain IMBHs, Bondi-Hoyle accretion is possible to reach the super-Eddington regime.

In the second part (Paper II), we consider IMBH feedback effects by embedding BH sub-grid models in simulations of high surface density clouds (Σ₀ ~ 10⁴ M_⊙/pc²). The sub-model includes an α-disk-like gas reservoir which is gravitationally bound to the IMBH and transfer the mass following t_dep ∝ 1/α. We also vary the strength of radiative feedback and collimated mechanical outflow. Under moderate feedback strength, significant IMBH mass growth is still possible. The final IMBH mass also depends on the reservoir mass, which is dominated by Bondi-Hoyle accretion when feedback is weak, while largely suppressed when gas is expelled by strong feedback. Future studies bridging both the large and small scales of IMBH accretion would constrain the parameter space to a better level.