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Direct collapse black hole

A direct collapse black hole (DCBH) is a theoretical type of supermassive black hole seed that forms in the early universe through the rapid gravitational collapse of a massive, metal-poor gas cloud, bypassing the intermediate phase of star formation and supernova explosions typical of lighter black hole seeds.^[1] This mechanism produces initial black holes with masses ranging from $10^4 to $10^5 solar masses (M_\odot), providing heavy seeds capable of growing into the billion-solar-mass supermassive black holes (SMBHs) observed powering quasars at redshifts z > 6.^[2] Unlike stellar-mass black holes derived from the remnants of massive stars, DCBHs arise directly from baryonic matter in dense environments, such as atomic-cooling halos or merger remnants, under conditions that prevent gas fragmentation into stars.^[1] The concept of DCBHs was first proposed in the early 2000s to address the formation of massive seeds for high-redshift quasars. Key developments include theoretical work by Bromm and Loeb (2003) suggesting collapse in metal-free halos, and subsequent refinements incorporating Lyman-Werner radiation suppression of cooling (e.g., Spaans & Silk 2006). By the 2010s, hydrodynamical simulations explored viability, with recent models (as of 2025) integrating merger-driven scenarios.^[3]^[4] The primary motivation for the DCBH model stems from the challenge of explaining the rapid appearance and immense masses of SMBHs in the first billion years after the Big Bang, as limited accretion rates from lighter seeds (around 100 M_\odot) cannot account for their growth within the available cosmic time.^[2] Two main formation pathways have been proposed. In the classic scenario, DCBHs form in pristine, metal-free atomic-cooling halos with virial temperatures exceeding $10^4 K, where intense Lyman-Werner (LW) ultraviolet radiation from nearby star-forming galaxies suppresses molecular hydrogen (H_2) cooling, maintaining the gas at temperatures above 8000 K to inhibit fragmentation and promote monolithic collapse.^[1] An alternative merger-driven pathway, primarily proposed for z > 6, involves the collision of gas-rich, disk-dominated galaxies, which drives multi-scale inflows of gas (at rates exceeding 1000 M_\odot/yr) into compact nuclear disks, enabling direct collapse even in metal-enriched environments through efficient angular momentum transport and radiative feedback; similar merger mechanisms may also occur at lower redshifts.^[2] Both mechanisms require specific conditions, including high gas densities (\sim 10^9 g/cm³), minimal metal enrichment to avoid dust cooling, and sustained high accretion to overcome instabilities like the general relativistic radial instability threshold.^[2] Challenges to DCBH formation include the narrow window for suppressing fragmentation—estimated at less than 10 million years at z \sim 10—and the rarity of required environments, with predicted abundances of $10^{-5} to $10^{-2} per comoving Mpc³.^[1] Hydrodynamical simulations, such as those using the adaptive mesh refinement code RAMSES, demonstrate that stable, optically thick disks can form in under 10⁵ years during major mergers, supporting the feasibility of "super-Eddington" accretion phases that fuel rapid growth.^[2] Recent observations from the James Webb Space Telescope (JWST) provide tentative evidence for DCBHs. For instance, the galaxy UHZ1 at z = 10.1 hosts an overmassive black hole of approximately 10 million M_\odot in a stellar system of only 10 million M_\odot, implying a heavy seed from direct collapse rather than stellar origins.^[5] Similarly, the "Infinity" galaxy at z = 1.14, observed in the COSMOS-Web survey, features a 1-million-M_\odot black hole embedded in ionized gas between colliding nuclei, with velocity alignments suggesting in-situ formation via gas cloud collapse during the merger, demonstrating the merger mechanism at intermediate redshift.^[6] These findings, corroborated by X-ray data from Chandra, bolster the model's relevance and highlight DCBHs as potential progenitors for the SMBHs driving early cosmic reionization and structure formation.^[7]

Introduction

Definition and Characteristics

Direct collapse black holes (DCBHs) are supermassive black hole seeds with initial masses in the range of approximately $10^4 to $10^5 solar masses (M_\odot), formed through the direct gravitational collapse of massive, pristine gas clouds in the early universe, bypassing any intermediate stellar progenitor phase.^[8] These seeds arise in metal-poor environments where cooling is dominated by atomic hydrogen transitions, preventing the gas from fragmenting into stars and instead allowing a monolithic collapse.^[9] The process occurs in atomic cooling halos with virial temperatures around $10^4 K, at redshifts greater than 10 (z > 10), during the cosmic dawn era when the universe was less than 500 million years old.^[9] The absence of heavy metals (metallicity Z < 10^{-3.5} Z_\odot) is crucial, as it suppresses molecular hydrogen formation and efficient cooling below atomic levels, thereby inhibiting fragmentation and promoting the ordered infall of gas.^[9] A distinguishing feature of DCBHs is their high initial spin, resulting from the organized, low-turbulence nature of the collapsing gas cloud, which conserves angular momentum efficiently during the radial infall.^[10] Hydrodynamical simulations indicate that the dimensionless spin parameter a (where a = J c / (G M^2), with J as angular momentum, M as mass, G as the gravitational constant, and c as the speed of light) typically reaches values of approximately 0.7 to 1 for these seeds. This high spin arises because the collapse proceeds with minimal chaotic mixing, unlike more turbulent stellar collapse scenarios, leading to a Kerr-like black hole with significant rotational energy. Some models extend the seed mass range up to $10^6 M_\odot under favorable conditions, though the core range remains $10^4--$10^5 M_\odot.^[11] The event horizon of a DCBH seed is defined by the Schwarzschild radius for a non-rotating black hole, given by

r_s = \frac{2 G M}{c^2},

where higher spin reduces the effective horizon size but the formula provides a baseline scale. For a typical seed mass of $10^5 M_\odot, r_s \approx 0.002 AU (astronomical units), comparable to the diameter of the Sun's orbit around some exoplanets but vastly smaller than the host halo's extent.^[9] This compact size underscores the extreme density of the collapse, with the black hole forming at the center of a protogalactic halo containing up to $10^6 M_\odot in baryons.

Historical Development

The concept of direct collapse black holes (DCBHs) emerged in the early 2000s as a theoretical pathway to explain the existence of supermassive black holes (SMBHs) powering high-redshift quasars, which require rapid mass growth from seeds formed shortly after the Big Bang. In 2003, Volker Bromm and Abraham Loeb proposed that in primordial halos with virial temperatures exceeding 10^4 K, the absence of molecular hydrogen (H_2) cooling could lead to the gravitational collapse of atomic hydrogen gas directly into a black hole seed of approximately 10^5 solar masses, bypassing the need for intermediate stellar remnants. This mechanism addressed the challenge of assembling billion-solar-mass SMBHs by z ≈ 6, as observed in early quasars, by providing massive seeds capable of efficient accretion. Building on this, Mitchell Begelman, Marta Volonteri, and Martin Rees detailed in 2006 a multi-stage process in pre-galactic halos where runaway baryonic collapse, stabilized temporarily by electron scattering pressure, culminates in black hole formation without stellar intermediaries.^[12] Preceding these proposals, foundational work on atomic cooling in massive halos laid the groundwork. In 2002, S. Peng Oh and Zoltán Haiman analyzed the radiative cooling dynamics of halos with virial temperatures above 10^4 K, demonstrating that atomic hydrogen line emission enables efficient cooling and collapse in metal-poor environments, potentially leading to central concentrations of gas without H_2-dominated fragmentation.^[13] Their study highlighted the critical role of halo mass thresholds—around 10^7-10^8 solar masses—for initiating such atomic-line cooled collapse, influencing subsequent DCBH models by emphasizing conditions where molecular cooling is suppressed.^[13] During the 2010s, numerical simulations advanced the hypothesis by quantifying the suppression of star formation through H_2 dissociation. Hydrodynamical models showed that intense Lyman-Werner (LW) radiation from nearby star-forming regions photodissociates H_2, maintaining the gas in an atomic state and promoting monolithic collapse over fragmentation into stars; for instance, simulations of halos irradiated by LW fluxes of 10-300 J_21 units confirmed the viability of DCBH formation in rare, high-density environments at z > 10.^[14] In the 2020s, with the advent of the James Webb Space Telescope (JWST), theoretical refinements incorporated detailed radiative feedback, revealing how soft UV photons and ionizing radiation regulate collapse dynamics, enhancing the parameter space for DCBH seeds while accounting for realistic cosmic ray and metal pollution effects. Recent developments have shifted models toward hybrid scenarios, integrating direct collapse with dynamical processes. A 2025 study by Pieter van Dokkum and collaborators proposed collision-induced collapse during high-velocity protogalaxy mergers, where shocked gas compresses into a dense core that undergoes runaway gravitational instability, forming SMBH seeds of 10^6 solar masses or more in the early universe.^[7] This evolution from pure direct collapse reflects broader recognition of environmental triggers, including runaway mergers of protostars within supermassive disks, which can build intermediate-mass objects that collapse into black holes, bridging gaps in seed mass distributions observed at high redshifts.^[15]

Formation Process

Required Conditions

The formation of direct collapse black holes (DCBHs) in the early universe demands precise environmental conditions to suppress molecular hydrogen (H₂) cooling and prevent the primordial gas cloud from fragmenting into lower-mass stars. A primary requirement is an intense flux of Lyman-Werner (LW) ultraviolet radiation from nearby star-forming regions, which photodissociates H₂ molecules and sustains gas temperatures near 10⁴ K through atomic hydrogen cooling. This flux must surpass a critical threshold, J_crit ∼ 10²–10³ J₂₁ (where J₂₁ denotes the intensity in units of 10²¹ erg s⁻¹ cm⁻² Hz⁻¹ sr⁻¹ at the Lyman limit), to ensure the gas remains thermally supported against fragmentation.^[16] The host dark matter halos must exhibit specific properties conducive to monolithic collapse. These halos typically have virial masses of 10⁷–10⁸ M_⊙, enabling atomic cooling halos where hydrogen line emission sets the cooling floor without reliance on H₂. Additionally, the gas within these halos requires extremely low metallicity, Z < 10^{-3} Z_⊙, to inhibit dust grain formation and associated radiative cooling, which would otherwise promote rapid fragmentation into stellar-mass objects. DCBH formation is confined to high redshifts, primarily z ≈ 15–20, coinciding with cosmic dawn and the onset of reionization, when the universe's mean LW background is still subcritical but local sources can provide the necessary irradiation.^[16] A pivotal physical parameter is the Jeans mass, which governs the scale of gravitational instability in the gas cloud and must remain sufficiently large to enforce coherent collapse. Expressed as M_J \propto T^{3/2} \rho^{-1/2}, where T is the gas temperature and ρ is the density, this quantity scales to values comparable to the halo mass under the elevated temperatures (~10⁴ K) and moderate initial densities of these environments, thereby preserving the cloud's monolithic structure.^[17]

Mechanism and Stages

The formation of a direct collapse black hole (DCBH) begins in a pristine atomic cooling halo with a virial temperature of approximately $10^4 K and total mass around $10^8 M_\odot at redshifts z \sim 15. In this initial stage, the gas cloud contracts under its own gravity, cooling primarily through atomic hydrogen lines to a temperature of about 8000 K, where further cooling is limited. This process leads to the formation of a dense core via supersonic inflows, with the gas condensing into one or a few massive clumps on scales smaller than 1 pc, each reaching masses up to $10^6 M_\odot. Fragmentation of the collapsing cloud is suppressed by intense Lyman-Werner (LW) radiation from nearby sources, which photodissociates molecular hydrogen (H_2) and prevents efficient cooling below 8000 K. Without H_2 cooling, the collapse proceeds adiabatically at nearly constant temperature, avoiding the formation of low-mass fragments that would occur in standard molecular cooling scenarios. Angular momentum transport is crucial during this phase; gravitational torques from the host halo or instabilities in the disk, supplemented by magnetic fields in some models, efficiently remove excess angular momentum, allowing the gas to spiral inward without forming a stable centrifugal barrier.^[12]^[18]

Alternative Merger-Driven Pathway

An alternative mechanism for DCBH formation involves the merger of gas-rich, disk-dominated galaxies at z > 6. These collisions drive intense inflows of gas (rates exceeding 1000 M_\odot/yr) into compact nuclear regions, forming rotationally supported disks where angular momentum transport and radiative feedback enable direct collapse even in metal-enriched environments. Hydrodynamical simulations show that such disks can become optically thick and stable within 10^5 years, leading to supermassive seeds via hyper-accretion.^[2] In the final stages of the classic pathway, the central region undergoes runaway collapse, potentially forming a supermassive protostar of approximately $10^5 M_\odot before direct transition to a black hole seed, or collapsing straight to the black hole without a stable stellar phase. This collapse is sustained by high accretion rates, often exceeding the Eddington limit (hyper-Eddington accretion), enabling rapid growth of the seed to $10^4--$10^5 M_\odot. The Bondi-Hoyle accretion rate, adapted for the high-density conditions of the early universe, governs this inflow: \dot{M} \propto G^2 M^2 \rho / c_s^3, where M is the central mass, \rho the ambient density, and c_s the sound speed, yielding rates up to several M_\odot yr^{-1} initially, with overall infall rates reaching ~10^3 M_\odot yr^{-1} in simulations of the collapsing cloud.^[12]^[18]^[19]

Comparison to Other Formation Pathways

Versus Stellar-Mass Black Holes

Direct collapse black holes (DCBHs) differ fundamentally from stellar-mass black holes in their initial seed masses, with DCBHs forming seeds of approximately $10^4 to $10^5 solar masses (M_\odot) through the collapse of massive, pristine gas clouds, whereas stellar-mass black holes typically arise from the remnants of individual massive stars and have masses ranging from 10 to 100 M_\odot.^[20] This substantial disparity in seed mass allows DCBH progenitors to bypass the limitations of stellar evolution, enabling more efficient pathways to supermassive black hole (SMBH) masses observed in the early universe. The formation timescale for DCBHs is exceptionally rapid, occurring in less than 1 million years—often on the order of $10^3 to $10^4 years—directly from the gravitational instability of atomic-cooling halos under specific radiative conditions that suppress fragmentation. In contrast, stellar-mass black holes require the full lifecycle of massive stars, including formation, main-sequence evolution, and core-collapse supernova, which spans tens of millions of years. This accelerated timeline for DCBHs positions them as viable seeds for the rapid emergence of quasars within the first billion years after the Big Bang. DCBHs are rarer than stellar-mass black holes due to the stringent environmental requirements for their formation, such as intense Lyman-Werner radiation to maintain high gas temperatures and prevent molecular hydrogen cooling, limiting occurrences to a small fraction of primordial halos.^[20] Stellar-mass black holes, however, form more commonly from the deaths of Population III or subsequent stellar generations, but their lower masses necessitate numerous mergers or prolonged accretion episodes to contribute significantly to SMBH growth. While DCBHs offer higher efficiency in producing heavy seeds, their scarcity contrasts with the abundance of lighter stellar remnants, influencing the overall seeding budget in the early universe. The growth implications of these seed differences are profound: a DCBH seed of $10^5 M_\odot requires only a factor of about $10^4 mass increase to reach quasar-scale SMBHs of $10^9 M_\odot, achievable through super-Eddington accretion over roughly 1 gigayear at 0.1 times the Eddington rate. Stellar seeds, starting at 10–100 M_\odot, demand a much larger growth factor of $10^7 or more, often exceeding feasible accretion limits within the observed cosmic timeframe without invoking exotic mechanisms. Thus, DCBHs provide a more straightforward route to explaining high-redshift SMBHs, alleviating tensions in standard stellar-seed models.

Versus Primordial Black Holes

Direct collapse black holes (DCBHs) form through the gravitational collapse of massive, pristine gas clouds in atomic-cooling halos at redshifts [z](/page/Z) \approx 10-20, following cosmic recombination, where specific conditions such as intense Lyman-Werner radiation suppress molecular hydrogen cooling to enable monolithic collapse without fragmentation. In contrast, primordial black holes (PBHs) arise from the collapse of extreme overdensities seeded by quantum fluctuations during cosmic inflation, occurring in the radiation-dominated era prior to recombination, potentially as early as $10^{-36} seconds after the Big Bang.^[21] This fundamental difference in timing and physical drivers—astrophysical gas dynamics for DCBHs versus primordial cosmology for PBHs—highlights their distinct roles in early universe structure formation. The mass spectrum of DCBHs is narrowly tuned to heavy seeds of $10^4 to $10^5 \, M_\odot, resulting from the high accretion rates (\dot{M} \gtrsim 0.001 \, M_\odot \, \mathrm{yr}^{-1}) onto supermassive stars that subsequently collapse.^[22] PBHs, however, exhibit a potentially broad mass distribution spanning $10^{-5} to $10^5 \, M_\odot or more, determined by the scale of inflationary perturbations, with low-mass PBHs in the asteroid range (\sim 10^{-16} \, M_\odot) hypothesized to constitute a significant fraction of dark matter due to their longevity against Hawking evaporation.^[21] While both can serve as supermassive black hole seeds, the restricted mass window for DCBHs arises from the finite size of host halos ($10^7-10^8 \, M_\odot), whereas PBHs' spectrum allows for diverse populations unconstrained by baryonic physics. Detection of DCBHs benefits from their rapid post-formation growth via super-Eddington accretion, evolving into luminous quasars observable through electromagnetic signatures like broad emission lines and high-redshift luminosity functions. PBHs pose greater challenges, remaining largely inert and electromagnetically dark unless accreting in dense environments or evaporating via Hawking radiation, which produces detectable gamma rays only for PBHs below $10^{12} kg—far lighter than seed masses—while heavier ones evade direct signals.^[21] Indirect probes, such as gravitational lensing or merger rates in gravitational waves, offer limited insights for PBHs due to their diluted cosmic abundance. Recent gravitational wave detections from LIGO/Virgo have further constrained PBH contributions to mergers in the stellar-mass range, supporting their limited role as dominant seeds.^[23] Theoretically, DCBH formation is bolstered by hydrodynamical simulations validating the required pristine conditions and collapse dynamics in metal-poor environments, predicting seed number densities of $10^{-5} to $10^{-2} cMpc^{-3} at z \sim 10. PBH viability, rooted in extensions to inflation models generating non-Gaussian overdensities, faces stringent observational limits: microlensing surveys (e.g., OGLE, HSC) and CMB distortion analyses constrain the dark matter fraction in PBHs to f_\mathrm{PBH} \lesssim 0.1 for masses $10-100 \, M_\odot, ruling out dominant contributions in this range while allowing trace amounts as seeds.^[24] These bounds underscore PBHs' speculative status compared to the more astrophysically grounded DCBH pathway.

Observational Evidence

Detection Techniques

Detection of direct collapse black holes (DCBHs) relies primarily on indirect observational signatures, as these massive seeds are expected to form in the early universe at redshifts z > 10, where direct imaging is challenging. One key method involves spectroscopic identification of high-redshift quasars hosting supermassive black holes (SMBHs) that are overmassive relative to their host galaxies, indicating rapid early growth inconsistent with stellar-mass seed evolution.^[25] Another approach searches for Lyman-alpha (Lyα) emission lines from pristine gas clouds undergoing atomic cooling and collapse, which can reveal outflowing gas illuminated by central accretion or gravitational heating.^[26] These techniques probe the environmental conditions required for DCBH formation, such as low-metallicity halos with suppressed molecular hydrogen cooling. Major instruments enabling these detections include the James Webb Space Telescope's (JWST) Near-Infrared Spectrograph (NIRSpec), which provides high-resolution spectra at redshifts z > 10 to measure emission lines and continuum properties in faint, distant sources.^[27] Complementary observations come from the Atacama Large Millimeter/submillimeter Array (ALMA), which maps sub-millimeter dust continuum emission to trace rapid gas accretion onto nascent black holes in high-redshift environments.^[28] Characteristic signatures of potential DCBHs include exceptionally high bolometric luminosities exceeding L_\text{bol} > 10^{47} erg s^{-1}, driven by super-Eddington accretion rates that outpace typical stellar-seed growth.^[29] Hosts often exhibit a lack of detectable stellar light in rest-UV/optical bands, suggesting minimal star formation and dominance by direct gas collapse rather than stellar processes.^[30] JWST's NIRSpec has advanced capabilities for detecting molecular hydrogen (H_2) absorption lines in 2025 observations, potentially identifying pristine, H_2-poor clouds as precursors to collapse. Lyα profiles show asymmetric, redshifted lines with widths of hundreds of km s^{-1}, arising from low neutral hydrogen column densities (N_\text{HI} \sim 10^{19} - 10^{20} cm^{-2}) and outflows.^[31] Confirming DCBHs remains challenging due to the need to distinguish them from stellar-seed pathways, which can be addressed through measurements of merger rates via gravitational wave detections and black hole spin parameters from X-ray spectroscopy. DCBHs are predicted to exhibit near-maximal spins (a \approx 0.99) from ordered collapse, contrasting with lower spins from chaotic stellar mergers.^[12] Upcoming missions like Athena will enable precise spin estimates via X-ray reflection features in accreting systems at high redshifts.^[32]

Candidate Objects

One prominent candidate for a direct collapse black hole (DCBH) is the overmassive black hole in the galaxy UHZ1, detected by the James Webb Space Telescope (JWST) in 2023 at a redshift of z = 10.1. This supermassive black hole (SMBH) has an estimated mass of approximately $10^7 M_\odot, residing in an underluminous host galaxy with a stellar mass of about $10^7 M_\odot, which suggests rapid early growth inconsistent with standard stellar remnant seeding. The black hole's compact size, less than 100 pc, and its high Eddington accretion ratio further support the hypothesis of formation via direct collapse of a massive gas cloud in the early universe.^[30] Another intriguing prospect is the Infinity Galaxy, a rare ring-galaxy duo observed in JWST archival data and reported in 2025, where a colliding pair of galaxies forms a figure-eight structure suggestive of a direct collapse site. This system, at z = 1.14, hosts an actively accreting SMBH with quasar-like radio and X-ray luminosity, potentially formed from the direct collapse of shocked and compressed gas during the merger. Follow-up analysis by van Dokkum et al. in 2025 provides further evidence for this mechanism, highlighting the role of merger-induced gas compression in triggering runaway gravitational collapse without star formation.^[33] The quasar in GN-z11, at z = 10.6, represents an overmassive black hole candidate with a mass around $10^6 M_\odot in a young galaxy, as revealed by JWST-NIRSpec spectroscopy in 2024, where rapid accretion rates challenge conventional formation pathways and align with DCBH seeding.^[34] Similarly, CEERS 1019, an active SMBH of about 9 million solar masses in a galaxy existing just 570 million years after the Big Bang, shows properties challenging standard growth models and consistent with heavy seed origins, based on JWST observations analyzed in 2023.^[35] As of 2025, all these candidates remain tentative, with no definitive confirmation of DCBH formation, though ongoing JWST Cycle 3 observations continue to probe their properties using techniques such as NIRSpec spectroscopy for emission line analysis.

Theoretical and Cosmological Implications

Role in Supermassive Black Hole Growth

Direct collapse black holes (DCBHs), with initial masses typically ranging from $10^4 to $10^5 M_\odot, are proposed as massive seeds that can rapidly evolve into supermassive black holes (SMBHs) through sustained super-Eddington accretion in the early universe.^[36] In the first 500 million years after the Big Bang, these seeds can accrete gas at rates 10 to 100 times the Eddington limit, leveraging the dense gas reservoirs in primordial galaxies to grow to masses exceeding $10^9 M_\odot.^[37] This rapid growth is facilitated by the high-density environments of atomic cooling halos, where radiative feedback is insufficient to halt inflow, allowing continuous mass buildup.^[36] Observations of high-redshift quasars, such as J0313-1806 at z = 7.642 hosting a black hole of approximately $1.6 \times 10^9 M_\odot, challenge standard seed models from stellar remnants, which struggle to reach such masses within the available cosmic time.^[38] DCBHs address this "overmassive" black hole problem by providing heavier initial seeds that require fewer e-folds of growth compared to lighter stellar-mass alternatives, aligning better with the observed SMBH populations at z > 6.^[39] Recent analyses favor direct collapse scenarios over lighter seeds to explain the rapid assembly of these early quasars.^[39] Hierarchical mergers further contribute to DCBH evolution, particularly in protocluster environments where multiple seeds coalesce. Simulations indicate that runaway merger chains among DCBH seeds can build intermediate-mass cores up to $10^8 M_\odot within dense early structures, enhancing overall SMBH growth before significant accretion dominates.^[40] The black hole growth timescale is approximated by

t_\text{growth} \approx \frac{t_\text{Edd}}{f_\text{Edd}} \ln\left(\frac{M_\text{final}}{M_\text{seed}}\right),

where t_\text{Edd} = \frac{\epsilon \sigma_T c}{4\pi G m_p} \approx 4.5 \times 10^8 \, \text{yr} for radiative efficiency \epsilon = 0.1, \dot{M}_\text{Edd} = L_\text{Edd} / (\epsilon c^2), and f_\text{Edd} representing the accretion rate factor (often exceeding 1 for super-Eddington phases).^[41] This formulation highlights how super-Eddington conditions (f_\text{Edd} \gg 1) drastically shorten the required time for DCBHs to reach observed SMBH masses.^[41]

Challenges and Open Questions

One major theoretical challenge in direct collapse black hole (DCBH) formation lies in achieving complete suppression of gas cloud fragmentation, which is essential for monolithic collapse into a massive seed of ~10^4–10^5 M_⊙. Strong far-ultraviolet (FUV) radiation in the Lyman-Werner band is required to photodissociate molecular hydrogen (H₂), preventing efficient cooling that would otherwise lead to fragmentation into smaller clumps. However, soft X-ray irradiation (≲1 keV) from nearby star-forming regions enhances H₂ formation through electron-catalyzed reactions, necessitating a higher critical FUV flux (J_crit) by factors of 3–10 compared to X-ray-free estimates, thereby reducing viable DCBH sites at z > 10 by orders of magnitude.^[42] Magnetic fields introduce additional complexity by influencing angular momentum transport during the collapse phase. In primordial atomic-cooling halos, amplified magnetic fields via the small-scale dynamo can efficiently remove angular momentum from the central gas reservoir, promoting rapid infall and potentially enabling DCBH formation even in marginally rotating systems. Simulations indicate that seed field strengths of ~10^{-15} G (comoving) lead to magnetic braking that stabilizes the disk against fragmentation, though stronger fields (>10^{-12} G) may instead disrupt the collapse by launching outflows.^[43] Hybrid models that blend direct collapse with remnants from Population III (Pop III) stars address some limitations of pure DCBH scenarios but remain theoretically contentious. In these frameworks, initial Pop III star formation in metal-poor halos leaves behind intermediate-mass black hole seeds (~10^2–10^3 M_⊙), which then merge or accrete in dense environments to mimic DCBH masses, potentially easing the stringent radiation requirements for H₂ suppression. Such models suggest that dynamical interactions in minihalos could bridge stellar and direct collapse pathways, though they predict lower overall seed masses unless super-Eddington accretion is invoked. Simulational efforts face significant limits in resolving the ab initio hydrodynamics of DCBH formation, particularly in capturing the multi-scale physics from kiloparsec halos to sub-parsec accretion disks. High-resolution simulations require at least 10^6–10^9 particles to adequately model radiative transfer, turbulence, and fragmentation, but current cosmological runs often rely on subgrid prescriptions that underestimate H₂ self-shielding or metal enrichment, leading to artificial suppression rates. Recent updates from the IllustrisTNG suite, incorporating refined seeding prescriptions, yield low DCBH fractions of ~10^{-5} in atomic-cooling halos at z ~ 15–20, highlighting the rarity of conditions for successful collapse amid competing Pop III star formation.^[44] Observationally, a key gap is the absence of direct detection of H₂ emission signatures, which would confirm the photodissociation necessary for DCBH-prone atomic halos; instead, current surveys probe indirect tracers like Lyα emission, but the suppressed H₂ lines remain elusive due to the exact conditions required. James Webb Space Telescope (JWST) data on z ~ 7–10 active galactic nuclei reveal overmassive black holes, yet contamination from rapidly growing stellar-mass seeds (~10^2 M_⊙) complicates attribution to DCBHs, as spectral features like Balmer breaks are masked by host galaxy stellar emission and limited resolution fails to isolate accretion physics. Recent JWST observations as of 2025, such as the "Infinity" galaxy candidate, offer further tentative support for DCBHs but still face challenges in distinguishing from stellar seeds due to resolution limits.^[6] Future facilities like the Extremely Large Telescope (ELT) are essential for spectroscopic follow-up at z > 15, offering the sensitivity to resolve faint UV lines and distinguish DCBH environments from stellar-dominated systems.^[45] Open questions persist regarding the demographic frequency of DCBHs, with estimates suggesting one seed per 10^4–10^6 galaxies at z > 10, contingent on the prevalence of pristine halos irradiated by J_LW > 10^3 J_21; semi-analytic models tied to cosmological simulations predict number densities of ~10^{-6}–10^{-8} Mpc^{-3}, but these vary widely with assumptions on merger rates. The influence of Pop III stars on ultraviolet (UV) feedback further complicates this, as their ionizing photons can preheat surrounding gas, either inhibiting H₂ formation to favor DCBHs or triggering fragmentation if feedback is insufficient, with recent models indicating that isolation distances >75 kpc are needed to preserve collapse conditions.^[46]^[47]

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Jan 20, 2020 · In this work we argue that the critical condition for suppressing the H 2 cooling in the collapsing gas could be described in a more general way.
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Primordial black holes as supermassive black holes seeds
### Summary: Primordial Black Holes (PBH) vs. Direct Collapse Black Holes (DCBH) as SMBH Seeds
[21]
[2002.12778] Constraints on Primordial Black Holes - arXiv
Feb 27, 2020 · We update the constraints on the fraction of the Universe that may have gone into primordial black holes (PBHs) over the mass range 10^{-5}\text{--}10^{50} g.
[22]
First Detection of an Over-Massive Black Hole Galaxy UHZ1 - arXiv
Aug 4, 2023 · As the first OBG candidate, UHZ1 provides compelling evidence for the formation of heavy initial seeds from direct collapse in the early Universe.Missing: >10^ | Show results with:>10^
[23]
Lyman Alpha Signatures from Direct Collapse Black Holes - arXiv
Feb 24, 2016 · We study Lyman Alpha (Lya) radiative transfer through simplified representations of the DCBH-scenario.Missing: accretion emission
[24]
[2205.14163] Finding lensed direct-collapse black holes and ... - arXiv
May 27, 2022 · Direct-collapse black holes (DCBHs) may be the seeds of the first quasars, over 200 of which have now been detected at z > 6 . The James Webb ...
[25]
ALMA 300 pc Resolution Imaging of a z = 6.79 Quasar - IOP Science
Oct 16, 2023 · We present Atacama Large Millimeter/submillimeter Array (ALMA) [C ii] 158 μm and dust continuum observations of the z = 6.79 quasar J0109–3047 at a resolution ...
[26]
The growth of the gargantuan black holes powering high-redshift ...
High-redshift quasars (z ≳ 6), powered by black holes (BHs) with large inferred masses, imply rapid BH growth in the early Universe.<|control11|><|separator|>
[27]
First Detection of an Overmassive Black Hole Galaxy UHZ1
Dec 29, 2023 · First detection of an overmassive black hole galaxy UHZ1: evidence for heavy black hole seed formation from direct collapse.
[28]
LYα SIGNATURES FROM DIRECT COLLAPSE BLACK HOLES
The shape of the Lyα spectrum indicates that this gas is outflowing. These requirements imply that if CR7 harbors a DCBH, then the physical conditions that ...Missing: H2 | Show results with:H2
[29]
[PDF] The Athena+ Co-ordination Group - NASA
first detailed X-ray spectra of accreting black holes at the peak of galaxy ... Reverberation signals also allow measurement of the black hole spin. In ...
[30]
[2506.15619] Further Evidence for a Direct-Collapse Origin of ... - arXiv
Jun 18, 2025 · The system is likely the result of a nearly face-on collision between two disk galaxies with massive bulges. In van Dokkum et al. (2025) we ...
[31]
A small and vigorous black hole in the early Universe | Nature
Jan 17, 2024 · Here we present an extensive analysis of the JWST-NIRSpec spectrum of GN-z11, an exceptionally luminous galaxy at z = 10.6.
[32]
The Mass Function of Supermassive Black Holes in the Direct ...
... Bromm & Loeb 2003). The idea is ... A lognormal distribution for the birth mass function of DCBH seeds is consistent with the results of Ferrara et al.
[33]
How the super-Eddington regime regulates black hole growth in ...
Super-Eddington accretion is one scenario that may explain the rapid assembly of ∼109 M⊙ supermassive black holes (BHs) within the first billion year of the ...
[34]
[2101.03179] A Luminous Quasar at Redshift 7.642 - arXiv
Jan 8, 2021 · We report the discovery of a luminous quasar at z=7.642, J0313-1806, the most distant quasar yet known. This quasar has a bolometric luminosity of 3.6\times10^ ...
[35]
Tracing the rise of supermassive black holes
We report the identification of 64 new candidates of compact galaxies, potentially hosting faint quasars with bolometric luminosities of Lbol = 1043–. 1046 erg ...
[36]
Supermassive black holes from runaway mergers and accretion in ...
ABSTRACT. Rapid formation of supermassive black holes occurs in dense nuclear star clusters that are initially gas-dominated. Stellar-mass black hole remna.
[37]
Accretion, growth of supermassive black holes, and feedback in ...
In this paper, we perform an analytical investigation of the growth of supermassive black holes via joint sub- and super-Eddington accretion in the framework of ...
[38]
The suppression of direct collapse black hole formation by soft X-ray ...
Our results suggest that Jcrit should be even higher than previously estimated, and pose an additional challenge for the direct collapse scenario via strong FUV ...
[39]
Role of magnetic fields in the formation of direct collapse black holes
Oct 7, 2022 · Abstract:Direct collapse black holes (DCBHs) are the leading candidates for the origin of the first supermassive black holes.Missing: angular momentum
[40]
Building a Semi-analytic Black Hole Seeding Model using ...
A major open question in astrophysics is the mechanisms by which massive black holes (BHs) form in the early Universe, which pose constraints on seeding models.
[41]
Physical Pathways for JWST-observed Supermassive Black Holes in ...
We use the cosmological simulation code GIZMO to study the formation and growth of DCBH seeds in the early Universe. To grow the DCBHs, we implement a gas ...Missing: review | Show results with:review
[42]
On the number density of 'direct collapse' black hole seeds
According to the direct collapse scenario, metal-free/metal-poor haloes with mass >107–108 M⊙ may host DCBHs under specific conditions. An inflow rate (higher ...
[43]
emergence and ionizing feedback of Pop III.1 stars as progenitors ...
The 'direct collapse' (DC) mechanism has been proposed to produce heavy-seed black holes via the accretion of atomically cooling, metal-free gas onto a ...