Assembly of Intermediate-Mass Black Holes Along Star Formation

Citation

Shi, Yanlong (2023) Assembly of Intermediate-Mass Black Holes Along Star Formation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/thmn-7h07. https://resolver.caltech.edu/CaltechTHESIS:06072023-051240339

Abstract

Intermediate-mass black holes (IMBHs) are poorly observed and not as well understood as stellar-mass black holes (BHs) and supermassive black holes (SMBHs). However, they can be important to complement the formation scenario of massive BHs other than stellar-mass ones and reconcile the existence of some high-energy sources in the Universe. The thesis studies the assembly of IMBHs in star-forming giant molecular clouds (GMCs) of ∼ 5 – 500 pc and ∼ 10⁴ – 10¹⁰ M_⊙, which are realistic environments for some scenarios of IMBH formation, including runaway collisions in dense star clusters and super-Eddington accretion onto ∼ 100 M_⊙ BH seeds like remnants of massive stars.

We first inspect the runaway-collision scenario where IMBHs form as remnants of “quasi-stars” after stellar collisions. Density profiles of young massive clusters can be important for this scenario but are missing observational hints. We measure density profiles of cluster populations in star-formation simulations in GMCs and conduct both analytic derivations and Monte-Carlo simulations to estimate the mass of the quasi-star in different clusters. The analytic expression is in approximate agreement with observations.

The following three chapters are about the super-Eddington accretion scenario. The first question to solve is the availability of super-Eddington accretion in turbulent and star-forming environments. We run simulations of BH accretion in GMCs with star formation based on FIRE-2 physics. We find that dense clumps generated by stellar feedback and turbulence can feed BHs at high accretion rates. We also conclude that GMCs with high surface densities are favored for super- or hyper-Eddington accretion, in which self-gravity dominates over stellar feedback.

After convincing the availability of super-Eddington accretion in dense GMCs, we study the self-regulation of BH accretion through its feedback. We construct a sub-grid model of BH accretion and feedback, including radiation, winds/jets, and relativistic diffusive cosmic rays. We find that super-Eddington accretion is still achievable with proper radiative feedback models but is challenged by BH mechanical feedback. We also quantify BH feedback effects and find that they can be analytically explained with momentum-driven arguments. Moreover, we study the effects of multiple sub-grid parameters and BH feedback’s impact on star formation in GMCs.

Finally, we study another mode of accretion due to steady gas inflow towards BHs. This complements the missing interaction between stars and BHs in previous studies. Along with star formation, star clusters form and merge hierarchically, creating deep potential wells to capture BHs. At the late stage of the simulation, a ∼ 10 pc disk structures form. The gas inflow rate can be ∼ 10 M_⊙/yr. We find a non-trivial strong toroidal magnetic field in the disk, which is thermally heated and ionized by feedback from stars.

Thesis Files