The Assembly of the First Massive Black Holes (1911.05791v1)

Abstract: The existence of $\approx$10^9 Msun supermassive black holes (SMBHs) within the first billion year of the universe has stimulated numerous ideas for the prompt formation and rapid growth of 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 $\approx$10^9 Msun, and how multi-messenger observations could distinguish between different SMBH assembly scenarios. We conclude the following: (1) The ultra-rare $\approx$10^9 Msun SMBHs represent only the tip of the iceberg. Early BHs likely fill a continuum from stellar-mass (approx. 10 Msun) to the super-massive ($\approx$10^9 Msun) regime, reflecting a range of initial masses and growth histories. (2) Stellar-mass BHs were likely left behind by the first generation of stars at redshifts as high as z=30, but their initial growth was typically stunted due to the shallow potential wells of their host galaxies. (3) 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. (4) 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. (5) Distinguishing between assembly mechanisms will be difficult, but a combination of observations by LISA (probing massive BH growth via mergers) and by deep multi-wavelength electromagnetic observations (probing growth via gas accretion) is particularly promising.

• The paper presents detailed formation mechanisms for massive black holes in the early universe, including direct collapse and runaway collisions.
• The paper utilizes radiation hydrodynamical simulations and analytical models to capture hyper-Eddington accretion and feedback processes.
• The paper outlines observational strategies with JWST and gravitational wave detectors to test its predictions on black hole assembly.

An Expert Analysis of "The Assembly of the First Massive Black Holes"

The paper by Inayoshi et al. titled "The Assembly of the First Massive Black Holes" delves deeply into the formation mechanisms, early growth processes, and observational implications of supermassive black holes (SMBHs) that emerged within the first billion years after the Big Bang. This comprehensive review articulates the intricate details surrounding the initial conditions conducive to massive black hole (BH) formation, while also examining subsequent growth scenarios tethered to specific cosmological environments.

Key Observations and Theoretical Framework

The document begins with astronomical observations revealing that SMBHs at $z > 6$, with masses exceeding $10^9~M_\odot$, are exceedingly rare yet pivotal in understanding early universe dynamics. These quasars were initially discovered in large-scale optical and infrared surveys. The studies prompt fundamental hypotheses about their formation, as their masses require understanding beyond conventional Population III stellar remnants due to the limited time available for Eddington-limited growth.

Formation Pathways

The authors note multiple scenarios for the formation of early massive BH seeds:

1. Light Seeds from Pop III Remnants: Standard formation via remnants of the first generation stars, which typically yields BHs with masses around $10-100~M_\odot$. However, early star-formation feedback mechanisms and gravitational wave-induced BH ejections impede consistent early growth.
2. Massive Seeds from Direct Collapse: Critical conditions in atomic cooling halos (ACHs) enable gas direct collapse, bypassing fragmentation. These scenarios necessitate particular environmental traits, such as high Lyman-Werner (LW) radiation inhibiting molecular hydrogen formation, to maintain the requisite high temperatures for isothermal collapse.
3. Runaway Collisions in Stellar Clusters: In metal-poor environments, clusters of stars could undergo coalescence aided by dense gas environments, offering another channel for intermediate mass BH formation.

Numerical Simulations and Analytical Models

Through radiation hydrodynamical simulations and revised analytical models, the research encapsulates the plausible hyper-Eddington accretion phases that could account for rapid BH growth. This includes adapting the slim-disk model to incorporate photon trapping effects and feedback mechanisms within dense stellar environments.

Observational Diagnostics and Future Prospects

The paper underlines future methodologies to alleviate current observational constraints:

• Observations via Upcoming Telescopes: Projects like the James Webb Space Telescope (JWST) and other next-gen telescopes are expected to probe higher redshifts, potentially unraveling lower mass SMBHs that can illuminate early seed formation.
• Gravitational Wave Astronomy: Instruments such as LISA may help detect and paper mergers of massive BHs beyond $z > 10$, providing insights into the merger-driven growth pathways of SMBHs.
• Multi-Messenger Approaches: Combining GW detections with electromagnetic observational data could discriminate among different formation and growth scenarios.

Implications and Speculation

The theoretical and numerical explorations in this paper have ramifications not only for our understanding of SMBH formation but also for galaxy formation and evolution paradigms. Furthermore, these insights encourage a reevaluation of the role of AGN feedback in early cosmic environments and open discussions on the exotic channels of BH assembly, such as SMBH seeds from primordial star processes or direct collapse driven by dark matter interaction dynamics.

Conclusion

Inayoshi et al.’s work provides a foundational framework for deciphering the ancestry of high-redshift quasars. Although the road to unpacking the assembly of SMBHs is paved with theoretical uncertainties and observational challenges, this paper contributes significant clarity and direction for future research in cosmology and high-energy astrophysics.