An official website of the United States government
ABSTRACT Formation of supermassive black holes (BHs) remains a theoretical challenge. In many models, especially beginning from stellar relic ‘seeds,’ this requires sustained super-Eddington accretion. While studies have shown BHs can violate the Eddington limit on accretion disc scales given sufficient ‘fuelling’ from larger scales, what remains unclear is whether or not BHs can actually capture sufficient gas from their surrounding interstellar medium (ISM). We explore this in a suite of multiphysics high-resolution simulations of BH growth in magnetized, star-forming dense gas complexes including dynamical stellar feedback from radiation, stellar mass-loss, and supernovae, exploring populations of seeds with masses $$\sim 1\!-\!10^{4}\, \mathrm{M}_{\odot }$$. In this initial study, we neglect feedback from the BHs: so this sets a strong upper limit to the accretion rates seeds can sustain. We show that stellar feedback plays a key role. Complexes with gravitational pressure/surface density below $$\sim 10^{3}\, \mathrm{M}_{\odot }\, {\rm pc^{-2}}$$ are disrupted with low star formation efficiencies so provide poor environments for BH growth. But in denser cloud complexes, early stellar feedback does not rapidly destroy the clouds but does generate strong shocks and dense clumps, allowing $$\sim 1{{\ \rm per\ cent}}$$ of randomly initialized seeds to encounter a dense clump with low relative velocity and produce runaway, hyper-Eddington accretion (growing by orders of magnitude). Remarkably, mass growth under these conditions is almost independent of initial BH mass, allowing rapid intermediate-mass black hole (IMBH) formation even for stellar-mass seeds. This defines a necessary (but perhaps not sufficient) set of criteria for runaway BH growth: we provide analytic estimates for the probability of runaway growth under different ISM conditions.
more »
« less
Simulations of black hole fueling in isolated and merging galaxies with an explicit, multiphase ISM
ABSTRACT We study gas inflows on to supermassive black holes using hydrodynamics simulations of isolated galaxies and idealized galaxy mergers with an explicit, multiphase interstellar medium (ISM). Our simulations use the recently developed ISM and stellar evolution model called Stars and MUltiphase Gas in GaLaxiEs (SMUGGLE). We implement a novel super-Lagrangian refinement scheme that increases the gas mass resolution in the immediate neighbourhood of the black holes (BHs) to accurately resolve gas accretion. We do not include black hole feedback in our simulations. We find that the complex and turbulent nature of the SMUGGLE ISM leads to highly variable BH accretion. BH growth in SMUGGLE converges at gas mass resolutions ≲3 × 103 M⊙. We show that the low resolution simulations combined with the super-Lagrangian refinement scheme are able to produce central gas dynamics and BH accretion rates very similar to that of the uniform high resolution simulations. We further explore BH fueling by simulating galaxy mergers. The interaction between the galaxies causes an inflow of gas towards the galactic centres and results in elevated and bursty star formation. The peak gas densities near the BHs increase by orders of magnitude resulting in enhanced accretion. Our results support the idea that galaxy mergers can trigger AGN activity, although the instantaneous accretion rate depends strongly on the local ISM. We also show that the level of merger-induced enhancement of BH fueling predicted by the SMUGGLE model is much smaller compared to the predictions by simulations using an effective equation of state model of the ISM.
more »
« less
- PAR ID:
- 10378835
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 517
- Issue:
- 4
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 4752-4767
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)The existence of ∼10 9 M ⊙ supermassive black holes (SMBHs) within the first billion years of the Universe has stimulated numerous ideas for the prompt formation and rapid growth of black holes (BHs) in the early Universe. Here, we review ways in which the seeds of massive BHs may have first assembled, how they may have subsequently grown as massive as ∼10 9 M ⊙ , and how multimessenger observations could distinguish between different SMBH assembly scenarios. We conclude the following: ▪ The ultrarare ∼10 9 M ⊙ SMBHs represent only the tip of the iceberg. Early BHs likely fill a continuum from the stellar-mass (∼10M ⊙ ) to the supermassive (∼10 9 ) regimes, reflecting a range of initial masses and growth histories. ▪ Stellar-mass BHs were likely left behind by the first generation of stars at redshifts as high as ∼30, but their initial growth typically was stunted due to the shallow potential wells of their host galaxies. ▪ Conditions in some larger, metal-poor galaxies soon became conducive to the rapid formation and growth of massive seed holes, via gas accretion and by mergers in dense stellar clusters. ▪ BH masses depend on the environment (such as the number and properties of nearby radiation sources and the local baryonic streaming velocity) and on the metal enrichment and assembly history of the host galaxy. ▪ Distinguishing between assembly mechanisms will be difficult, but a combination of observations by the Laser Interferometer Space Antenna (probing massive BH growth via mergers) and by deep multiwavelength electromagnetic observations (probing growth via gas accretion) is particularly promising.more » « less
-
ABSTRACT Recent systematic searches for massive black holes (BHs) in local dwarf galaxies led to the discovery of a population of faint active galactic nuclei (AGNs). We investigate the agreement of the BH and AGN populations in the Illustris, TNG, Horizon-AGN, EAGLE, and SIMBA simulations with current observational constraints in low-mass galaxies. We find that some of these simulations produce BHs that are too massive, and that the BH occupation fraction (OF) at z = 0 is not inherited from the simulation seeding modelling. The ability of BHs and their host galaxies to power an AGN depends on BH and galaxy subgrid modelling. The fraction of AGN in low-mass galaxies is not used to calibrate the simulations, and thus can be used to differentiate galaxy formation models. AGN fractions at z = 0 span two orders of magnitude at fixed galaxy stellar mass in simulations, similarly to observational constraints, but uncertainties and degeneracies affect both observations and simulations. The agreement is difficult to interpret due to differences in the masses of simulated and observed BHs, BH OF affected by numerical choices, and an unknown fraction of obscured AGN. Our work advocates for more thorough comparisons with observations to improve the modelling of cosmological simulations, and our understanding of BH and galaxy physics in the low-mass regime. The mass of BHs, their ability to efficiently accrete gas, and the AGN fraction in low-mass galaxies have important implications for the build-up of the entire BH and galaxy populations with time.more » « less
-
ABSTRACT Possible formation scenarios of supermassive black holes (BHs) in the early universe include rapid growth from less massive seed BHs via super-Eddington accretion or runaway mergers, yet both of these scenarios would require seed BHs to efficiently sink to and be trapped in the Galactic Centre via dynamical friction. This may not be true for their complicated dynamics in clumpy high-z galaxies. In this work, we study this ‘sinking problem’ with state-of-the-art high-resolution cosmological simulations, combined with both direct N-body integration of seed BH trajectories and post-processing of randomly generated test particles with a newly developed dynamical friction estimator. We find that seed BHs less massive than $$10^8\, \mathrm{M}_\odot$$ (i.e. all but the already-supermassive seeds) cannot efficiently sink in typical high-z galaxies. We also discuss two possible solutions: dramatically increasing the number of seeds such that one seed can end up trapped in the Galactic Centre by chance, or seed BHs being embedded in dense structures (e.g. star clusters) with effective masses above the mass threshold. We discuss the limitations of both solutions.more » « less
-
Aims.The scenario of feedback-free starbursts (FFB), which predicts excessively bright galaxies at cosmic dawn as observed using JWST, may provide a natural setting for black hole (BH) growth. This involves the formation of intermediate-mass seed BHs and their runaway mergers into super-massive BHs with high BH-to-stellar mass ratios and low Active Galactic Nucleus (AGN) luminosities. Methods.We present a scenario of merger-driven BH growth in FFB galaxies and study its feasibility. Results.Black hole seeds form within the building blocks of the FFB galaxies, namely, thousands of compact star clusters, each starbursting in a free-fall time of a few million years before the onset of stellar and supernova feedback. The BH seeds form by rapid core collapse in the FFB clusters, in a few free-fall times, which is sped up by the migration of massive stars due to the young, broad stellar mass function and stimulated by a “gravo-gyro” instability due to internal cluster rotation and flattening. BHs of ∼104 M⊙are expected in ∼106 M⊙FFB clusters within sub-kiloparsec galactic disks atz ∼ 10. The BHs then migrate to the galaxy center by dynamical friction, hastened by the compact FFB stellar galactic disk configuration. Efficient mergers of the BH seeds will produce ∼106 − 8 M⊙BHs with a BH-to-stellar mass ratio ∼0.01 byz ∼ 4 − 7, as observed. The growth of the central BH by mergers can overcome the bottleneck introduced by gravitational wave recoils if the BHs inspiral within a relatively cold disk or if the escape velocity from the galaxy is boosted by a wet compaction event. Such events, common in massive galaxies at high redshifts, can also help by speeding up the inward BH migration and by providing central gas to assist with the final parsec problem. Conclusions.The cold disk version of the FFB scenario provides a feasible route for the formation of supermassive BHs.more » « less
