Supermassive Black Hole “Seeds”
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 02 / Article 04
Supermassive black hole seeds
Some black holes became cosmic giants while the universe was still young. Did they begin with a large head start, grow at exceptional rates, or combine both?
A huge black hole in a very young universe.
Looking far into space also means looking far into the past. Some of the light arriving today comes from galaxies whose central black holes were already extraordinarily massive.
That creates a question of timing. Before a black hole could become a giant, something had to form its initial mass and then keep supplying material for growth.
A black hole seed is that starting object. “Seed” describes its role in a possible growth history, rather than a separate kind of black hole. A seed might remain small, wander away from a galaxy’s center, or eventually become supermassive.[3]
Why is there a race against time?
A quasar is an exceptionally luminous active galactic nucleus, powered by matter accreting onto a central black hole. The bright emission comes from hot material and related activity outside the event horizon. Light is not escaping from inside the black hole.[1]
Quasars let astronomers investigate black holes at enormous distances. Their spectra provide redshifts and allow estimates of black hole mass, although those estimates depend on models of the emitting gas.
The challenge is to explain the combination of a very large inferred mass and a very young cosmic age.
Could the first stars have supplied the seeds?
The first stellar generation, called Population III, formed from essentially pristine gas. Some massive members could collapse into black holes, leaving seeds with masses of tens to hundreds of Suns in commonly studied models.
There is no single rule mapping every star’s birth mass to its remnant. Core evolution, rotation, mass loss, and companions affect the outcome. Some stars leave neutron stars; some make black holes; a complete pair-instability explosion leaves no remnant.[3]
This route connects black hole seeding to the first star-forming clouds. Its main difficulty is often what happens next: the newborn black hole must find enough dense gas to grow.
The host environments are explored in Early Mini-Halos and Protogalaxies.
What does “direct collapse” actually mean?
Heavy-seed scenarios aim to put a much larger amount of mass into the initial black hole. In one family of models, rapid gas inflow builds a supermassive protostar, which later collapses. The gas avoids distributing most of its mass among an ordinary population of smaller stars.[5]
“Direct collapse” therefore does not always mean a cloud instantly becomes a black hole, or that no star-like stage occurs. The collapse history can include a rapidly growing central star, some fragmentation, and continuing gas supply.
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Maintain a rapid inflow
A sufficiently deep halo gathers gas. The material must lose enough angular momentum to reach the center instead of remaining on wide orbits.[3]
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Concentrate the mass
Under suitable thermal and chemical conditions, inflow can build a supermassive central object. Rapidly accreting models can develop extended, relatively cool envelopes.[5]
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Collapse into a seed
The central object can become unstable and collapse. The seed mass depends on the object’s growth and evolution; it need not equal the mass of the original gas cloud.[5]
What keeps the gas from forming ordinary stars first?
In a classic scenario, low enrichment limits cooling by heavy elements and dust, while ultraviolet Lyman–Werner radiation suppresses molecular hydrogen. Atomic hydrogen can still cool the gas, helping maintain a warm flow around several thousand to 10,000 kelvins.[3], [5]
There may be more than one way
A strong external ultraviolet field is not required by every heavy-seed model. Simulations also explore rapidly assembling halos where heating and dynamical conditions delay ordinary star formation while gas accumulates.[6]
Could clusters or the early universe itself make seeds?
Runaway collisions in a dense cluster
In an extremely compact young cluster, repeated stellar collisions could build a very massive star before the massive stars finish their lives. Its later collapse could produce a seed larger than many ordinary stellar remnants.
Some models produce seeds around a thousand solar masses. The outcome depends on density, cluster dynamics, gas, stellar mass loss, and chemical composition; this is not a universal upper limit.[7]
Repeated mergers between black holes offer a related possibility. They involve a different process from collisions between stars and require the growing remnants to remain in the cluster.
How much time does growth take?
Accretion adds mass while releasing energy. Escaping radiation can push on the surrounding gas, so luminosity and growth influence one another.
The Eddington luminosity is a reference value where outward radiation forces balance gravity under specified assumptions, commonly electron scattering in ionized gas. It is not an absolute ceiling on every possible flow. Turning a luminosity into a mass-growth rate also requires knowing how efficiently accretion produces escaping radiation.[9], [10]
A controlled comparison
Assume continuous accretion at the Eddington luminosity, a radiative efficiency of 10%, and no mergers. Accounting for the fraction of incoming rest-mass energy radiated away, the black hole grows by a factor of about 2.72 every 50 million years.[9]
| Initial seed mass | Required mass increase | Elapsed growth time |
|---|---|---|
| 100 solar masses | 10 million times | ~810 million years |
| 100,000 solar masses | 10,000 times | ~460 million years |
Illustrative calculations from the stated model, rounded to two significant figures. The clock starts at seed formation. These are not measured histories of particular quasars.
The heavier seed saves roughly 350 million years in this comparison. Pauses in feeding lengthen the calendar time. Changing the radiative efficiency or the accretion regime changes the result.
See the growth equation and assumptions
For a constant Eddington ratio and radiative efficiency, the idealized growth is exponential:
M(t) = Mseed exp(t / te)
te ≈ 450 million years × η / [(1 − η) λ]
Here η is the fraction of accreted rest-mass energy emitted as radiation, and λ is luminosity divided by Eddington luminosity. With η = 0.1 and λ = 1, te is about 50 million years; the doubling time is about 35 million years. The reference calculation assumes electron scattering in fully ionized hydrogen and uninterrupted accretion.[9]
The commonly quoted estimate near 45 million years neglects the correction for the radiated fraction at this efficiency. Both conventions occur in introductory discussions, so numerical comparisons need consistent assumptions.
Can a seed grow faster?
In some dense accretion flows, photons can be carried inward faster than they escape. Such photon trapping reduces the escaping radiation per unit mass supplied, permitting faster growth than the simple model above. Geometry and outflows also matter.[10]
Super-Eddington mass inflow therefore does not mean that the escaping luminosity rises by the same factor. Establishing how often these episodes occur, and how long they last, is part of the seed puzzle.
What controls whether a seed keeps growing?
Transport angular momentum
Gas usually has orbital motion. Gravitational torques, turbulence, and stresses within an accretion flow can redistribute angular momentum, allowing some material to move inward.[3]
Survive feedback
Stars can leave a seed in diffuse gas, while radiation and outflows from accretion can disrupt later feeding. Gas supply and feedback must be followed together.[4], [10]
Retain the remnant
A black hole displaced from dense central gas can struggle to accrete. Shallow hosts are particularly vulnerable to losing a merger remnant through recoil.[11]
Mergers contribute, but they are not instant shortcuts
When galaxies merge, their black holes may eventually meet. First they must approach each other and lose orbital energy; a galaxy merger does not guarantee immediate black hole coalescence.
A black hole merger combines much of the pair’s mass, radiates some energy as gravitational waves, and can kick the remnant in a new direction. Depending on the kick and the host’s gravity, it may stay central, wander, or escape. Merger-driven growth therefore also has environmental requirements.[3], [11]
For the larger setting, see Merging and Hierarchical Growth and Feedback Effects: Radiation and Winds.
How can astronomers distinguish the pathways?
An observed black hole usually carries the effects of both its birth and its later growth. Researchers look for combinations of mass, age, host properties, abundance, and merger history that competing models can or cannot reproduce.
Spectra and multiple wavelengths
Infrared spectra can reveal redshifted emission from distant active nuclei. X-rays can provide complementary evidence for accretion. Together, these observations help constrain the nature of a source and its surrounding gas.
Mass estimates still require assumptions. Broad lines, brightness, obscuration, and lensing must be interpreted carefully.[2], [13], [14]
Masses and stellar motions
Nearby samples show relationships between black hole mass and properties of certain galaxy components. The MBH–σ relation uses the spread of stellar velocities in a bulge; the relation with bulge stellar mass is a separate correlation.
These patterns constrain shared growth histories. They do not, by themselves, establish which object formed first.[12]
Gravitational waves
The planned LISA space observatory is designed to measure gravitational waves in a frequency range that includes mergers of massive black holes. Event masses and rates could help distinguish different assembly histories.
Such measurements would complement light-based searches, with interpretation depending on the population and growth models.[3], [15]
The wider population
Rare luminous quasars sample especially successful black hole growth. Smaller or weakly accreting objects can be harder to find, yet are needed to learn how common different beginnings were.
A model must account for a population as well as its most spectacular members.[3]
UHZ1: why an interpretation needs continued testing
In 2023, researchers interpreted X-ray and infrared observations of the distant galaxy UHZ1 as evidence for a black hole unusually massive relative to its host. A subsequent paper argued that its properties supported a heavy-seed origin.[13]
A 2026 reanalysis, using additional Chandra data and JWST mid-infrared constraints, found that the claimed X-ray detection was not robustly reproduced and argued against a luminous obscured active nucleus. UHZ1 should therefore not be presented as settled proof of direct collapse.[14]
The general lesson is practical: researchers must establish the source, estimate its properties, and then test its possible origin. A challenge to one candidate does not settle the broader question of whether heavy seeds formed.
What remains unresolved?
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Which seeds were common?
A route that explains rare early quasars need not account for most black holes. Researchers need the abundance of seeds, including those that remained small.
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How often was growth rapid?
The duration and frequency of strong feeding episodes determine whether a light seed can catch up with one that began heavier.
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Which environments were decisive?
Gas inflow, enrichment, nearby stars, and merger history influence both how seeds form and what happens afterward.
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Which evidence preserves the beginning?
Later accretion can conceal the initial mass. The most useful tests combine young systems, less-grown black holes, host properties, and merger populations.
Several pathways could operate in the same universe. The aim is to learn which combinations of formation and growth explain the observed diversity, and under what conditions.[3], [10]
A seed’s mass is only the beginning of its history.
The first quasars require successful growth as well as a starting object. A heavy seed gains time; a light seed can gain ground through favorable accretion. Both depend on what happens in the surrounding galaxy.
Black holes were one possible legacy of early stellar evolution. The next article turns to another: primordial supernovae and element synthesis, which began changing the chemical ingredients available to later generations.
Sources and further reading
Observational examples reviewed in September 2026. Numerical growth times are illustrative calculations, with assumptions stated above.
- NASA — Black Hole AnatomyAccretion disks, event horizons, and the origin of light around black holes.
- Wang et al. (2021) — A Luminous Quasar at Redshift 7.642The discovery and estimated black hole mass of the early quasar J0313−1806.
- Inayoshi, Visbal & Haiman (2020) — The Assembly of the First Massive Black HolesResearch review of seed formation, gas supply, growth, and possible observational tests.
- Johnson & Bromm (2007) — The aftermath of the first stars: massive black holesHow radiation from a first star can affect the gas available to its black hole remnant.
- Hosokawa et al. (2013) — Formation of Primordial Supermassive Stars by Rapid Mass AccretionModels of rapidly growing supermassive protostars and their evolution toward collapse.
- Wise et al. (2019) — Formation of massive black holes in rapidly growing pre-galactic gas cloudsRapid halo growth and heating as contributors to conditions favorable for massive seeds.
- Devecchi & Volonteri (2009) — Formation of the first nuclear clusters and massive black holes at high redshiftA proposed route through compact star clusters and runaway stellar collisions.
- Carr & Kühnel (2020) — Primordial Black Holes as Dark Matter: Recent DevelopmentsPrimordial formation hypotheses, observational constraints, and possible seed applications.
- Shen (2013) — The Mass of Quasars: IntroductionThe relation between radiative efficiency, Eddington ratio, and exponential mass growth.
- Madau, Haardt & Dotti (2014) — Super-Critical Growth of Massive Black Holes from Stellar-Mass SeedsHow rapid accretion with low radiative efficiency could shorten growth times.
- Merritt et al. (2004) — Consequences of gravitational radiation recoilHow merger recoil can displace black holes or eject them from shallow gravitational wells.
- Kormendy & Ho (2013) — Coevolution (Or Not) of Supermassive Black Holes and Host GalaxiesBlack hole relationships with stellar velocity dispersion, bulge mass, and different galaxy components.
- Natarajan et al. (2024) — First Detection of an Over-Massive Black Hole Galaxy UHZ1: Evidence for Heavy Black Hole Seed Formation from Direct CollapseThe original heavy-seed interpretation, reported in 2023 and subsequently challenged.
- Zou et al. (2026) — Revisiting the Claim for a Direct-Collapse Black Hole in UHZ1 at z = 10.05A reanalysis of Chandra and JWST data challenging the luminous active-nucleus interpretation.
- ESA — LISAThe planned space observatory for gravitational waves, including signals from black hole mergers.
All articles in this chapter
- Gravitational Clumping and Density Fluctuations
- Population III Stars: The Universe’s First Generation
- Early Mini-Halos and Protogalaxies
- Supermassive Black Hole Seeds — you are here
- Primordial Supernovae: Element Synthesis
- Feedback Effects: Radiation and Winds
- Merging and Hierarchical Growth
- Galaxy Clusters and the Cosmic Web
- Active Galactic Nuclei in the Young Universe
- Observing the First Billion Years