Active Galactic Nuclei in the Young Universe
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 02 / Article 09
Active galactic nuclei in the young universe
Some early galaxies shone from their centers with extraordinary power. Their light reveals growing black holes—and the gas that fed, concealed, and responded to them.
A galactic center can outshine its stars.
A young galaxy may contain millions or billions of stars, yet much of its light can come from a much smaller region at its center.
The power comes from accretion: matter moving inward and releasing energy near a black hole. The radiation we detect is produced outside the event horizon, before material crosses the boundary from which light cannot escape.[1], [2]
Studying these sources brings several questions together. How quickly did early black holes grow? What did their activity do to their galaxies? And what can their light tell us about space when the universe was still becoming ionized?
What makes a galactic nucleus active?
An active galactic nucleus, or AGN, is a compact central source powered by an accreting massive black hole. “Active” describes the energy-producing state. A galaxy can retain its central black hole after that bright phase fades.
Quasars occupy the luminous end of the AGN population. Their central emission can overwhelm the host’s starlight, making them appear almost point-like in images.[1]
Disk and corona
Orbiting gas heats as it moves inward through an accretion disk. A hotter region called the corona can boost photons to X-ray energies.[2]
Emission-line regions
Illuminated gas produces bright spectral lines. Gas closer to the black hole often moves faster, giving broader lines; more distant gas usually produces narrower ones.[20]
Obscuring gas and dust
Surrounding material can absorb central light and reradiate it, including in the infrared. Viewing angle and obscuration help explain why AGN can look so different.[1]
The familiar disk-and-surroundings picture is a guide. The structures visible in an individual source depend on its feeding state, orientation, and environment.
How early did luminous quasars appear?
Quasar spectra show that some enormous black holes were already active hundreds of millions of years after the Big Bang. Two well-studied discoveries illustrate the timing:
ULAS J1120+0641
z = 7.085Its light comes from about 770 million years after the Big Bang. The discovery analysis inferred a black hole mass of roughly two billion Suns.[3]
J0313−1806
z = 7.642Seen at about 670 million years, this quasar’s discovery analysis estimated a black hole mass near 1.6 billion Suns and identified evidence for a fast outflow.[4]
These masses are estimates from the emitted light and gas dynamics, with systematic uncertainties. Even allowing for those uncertainties, the sources set demanding growth deadlines.
Euclid expands the early-quasar census
Yang and colleagues reported 31 newly discovered quasars spanning redshifts 6.6–
What does a redshift of 7 actually mean?
Cosmic expansion stretches a photon’s wavelength during its journey. At z = 7, the observed wavelength is eight times the wavelength at emission, because the stretching factor is 1 + z.
This moves much of a distant galaxy’s emitted ultraviolet and visible light into the infrared. Converting redshift into an age requires an expansion model; the ages here are approximate values in standard cosmology.
How does a black hole grow so quickly?
Gas must shed enough angular momentum—the motion that keeps it orbiting—to reach the central accretion flow. Gravitational disturbances, disk instabilities, and galaxy encounters can help transport it inward.
A merger is one possible trigger, but gas-rich galaxies can also feed their nuclei without a major collision. Heating and outflows may interrupt the supply, making activity episodic.[6]
Growth depends on the starting black hole seed, how early it formed, how often it accreted, and how efficiently the inflowing material produced radiation. Early quasars constrain these factors together.[6]
How fast is growth at the Eddington luminosity?
With continuous accretion and a radiative efficiency of 10%—one tenth of the incoming rest-mass energy escaping as radiation—the black hole’s mass doubles in roughly 35 million years.
At that rate, growth from 100 to a billion solar masses takes about 800 million years. A heavier seed starts closer to the target; interruptions lengthen the time. This illustrates why seed age, efficiency, and feeding history matter.[7]
In some models, rapid inflow traps radiation and permits faster mass growth. “Super-Eddington accretion” therefore does not necessarily mean that the escaping luminosity exceeds the benchmark by the same factor.[8]
How do astronomers recognize an early AGN?
At these distances, telescopes usually cannot resolve the central accretion disk. Astronomers combine images, spectra, and changes in brightness to distinguish accretion-powered light from starlight.[1]
| Observation | What it reveals | What needs care |
|---|---|---|
| Infrared images and spectra | Redshifted ultraviolet and visible emission, gas lines, and sometimes host-galaxy starlight. | A compact red source needs further evidence to establish what powers it. |
| X-rays | High-energy emission that can reveal accretion close to a black hole. | Absorption, intrinsic weakness, and sensitivity limits can hide an AGN. |
| Radio emission | Jets and energetic particles in suitable sources. | Many AGN have weak radio emission; star formation can also emit radio waves. |
| Broad spectral lines | Gas dynamics and, with suitable assumptions, a black hole mass estimate. | Winds, scattering, and viewing geometry can affect the line width. |
No single detection method finds every active nucleus. Combining measurements reduces the ambiguity.[2], [9], [10], [20]
How is a distant black hole “weighed”?
A common method combines a broad line’s width with the source’s luminosity, which estimates the size of the line-emitting region through an empirical calibration. If gravity dominates the gas motions, these quantities yield a virial mass estimate.
The result depends on calibration and geometry. A wind-distorted line is a less reliable tracer of orbital speed; the brightest point in a spectrum is not a direct measurement of mass.[20]
Why can different lines give different redshifts?
Gas moving toward us can shift a line toward shorter wavelengths relative to the galaxy as a whole. High-ionization lines such as C IV can show strong wind-related shifts.
Researchers therefore compare lines and, where available, use host-galaxy tracers such as far-infrared emission from ionized carbon to establish the galaxy’s systemic redshift.[4]
What are Webb’s little red dots?
Webb revealed numerous compact sources with very red optical emission in their own rest frame. Many display broad hydrogen lines. Nicknamed little red dots, they opened a new window on possible black hole growth below the luminosities of the brightest quasars.[9]
The name describes observed appearance, rather than one proven physical structure. Light from stars, accretion, and surrounding gas can combine in ways that are difficult to separate.
Broad lines need interpretation
Rusakov and colleagues reported in 2026 that electron scattering explains much of the broad line wings in many objects in their selected sample. Their model places accreting black holes inside dense gas cocoons.
When scattering supplies part of the broadening, treating the entire width as orbital motion can overestimate the black hole’s mass.[10]
Abell 2744-QSO1
A separate 2026 study used gas rotation in this strongly lensed source at z = 7.04 to infer a central mass near 50 million Suns.
That estimate agreed with its spectroscopic virial mass. It shows why a correction inferred for one sample should not automatically be applied to every little red dot.[11]
The emerging picture includes strong evidence for accretion in individual sources. Determining their masses, gas structure, and diversity remains essential to understanding how much early black hole growth these objects represent.
What does quasar light reveal about reionization?
During cosmic reionization, ultraviolet radiation gradually stripped electrons from intergalactic hydrogen. A distant quasar acts as a backlight: hydrogen between it and us absorbs parts of its spectrum.
A dark absorption trough
The Gunn–Peterson trough is a region of very strong absorption associated with hydrogen’s Lyman-alpha transition.
It can become nearly black even when only a small fraction of the hydrogen remains neutral. A black trough alone therefore cannot tell us that most of the surrounding universe was neutral.
A broader absorption wing
A damping wing can extend to wavelengths redward of the quasar’s Lyman-alpha emission and provide stronger constraints on substantial neutral gas.
Interpreting it requires a model of the quasar’s original spectrum, nearby absorbers, patchy intergalactic gas, and the quasar’s own ionizing influence.[12]
Did AGN reionize the universe?
Luminous quasars are too rare to explain hydrogen reionization by themselves. The larger population of faint AGN is harder to count, and only ionizing photons that escape their surroundings contribute to the wider process.[13], [18]
A 2025 JWST analysis by Jiang and colleagues placed an upper bound of about one third on the required ionizing-photon budget from AGN near z ≈ 7.5 under its assumptions. The result supports a major role for star-forming galaxies; the precise AGN share remains dependent on the population and radiation models.[13]
A quasar ionizes nearby gas. Its observable proximity zone depends partly on how long it has been shining and on conditions around it. The zone’s edge is not simply a direct outline of an ionized bubble.[12]
How does an AGN change its host galaxy?
AGN feedback occurs when energy or momentum from the nucleus affects surrounding material. Radiation can heat gas or push on it; winds and jets can shock gas and drive larger-scale motions.
These effects may remove fuel, keep it too hot to form stars, or interrupt further accretion. Their strength depends on how efficiently the energy reaches the gas and how quickly that gas cools.[14]
An outflow is one part of the evidence
Detecting an outflow establishes that material is moving outward. Establishing quenching—a sustained reduction in star formation—also requires knowing how much fuel is affected, whether it escapes, and whether fresh gas replaces it.
Outflows can carry heavy elements into the surrounding medium. Those elements were made through stellar evolution and explosions; the wind redistributes them. See Feedback Effects: Radiation and Winds for the wider cycle.
Do galaxies and black holes grow together?
Nearby observations show relationships between black hole mass and properties of a galaxy’s central stellar bulge. The MBH–σ relation concerns stellar velocity dispersion—the spread of stellar velocities. A relation with bulge mass is a separate comparison.[15]
These patterns suggest connections among gas supply, galaxy assembly, and black hole growth. They do not establish that every black hole and galaxy grow in step. At high redshift, bright-nucleus selection and uncertain host measurements can distort comparisons with nearby samples.[15], [16]
Where do early AGN fit in cosmic history?
Quasars also help explore the cosmic web. Some have crowded neighborhoods, but a luminous nucleus does not guarantee the richest surrounding structure.
JWST’s EIGER observations found a wide range of nearby galaxy densities around four early quasars, from a strong concentration to a field consistent with average density. Environment must be measured, rather than assigned from quasar brightness alone.[17]
Luminous quasar activity became more common later, reaching a broad peak around z ≈ 2–3, during the era often called cosmic noon. Its timing depends on which luminosities and populations are counted. The first-billion-year sources reveal the earlier stages of this history.[18]
How much growth is concealed?
Infrared, X-ray, and radio measurements together can test which accreting sources individual surveys miss.
How do the two grow?
Better spectra and host-galaxy measurements can connect accretion, gas motion, and star formation.
What can mergers reveal?
The planned LISA observatory aims to detect gravitational waves from massive black hole mergers, providing a complementary test of their assembly.[19]
The central challenge is to explain a population: its luminosities, masses, surroundings, and changes through time. More complete samples turn remarkable individual discoveries into tests of how early galaxies and black holes formed.
Ancient light reveals both growth and change.
An active galactic nucleus shows gas releasing energy as a black hole grows. Its spectrum also records moving material, obscuring gas, and hydrogen far beyond its host.
These sources connect the smallest central regions of galaxies to the history of the wider universe. Next, Observing the First Billion Years brings together the techniques used to reconstruct that early history.
Sources and further reading
Observations and mission descriptions reviewed in September 2026. Discovery-era mass estimates are identified as such; interpretations of little red dots remain dependent on the objects and methods studied.
- NASA — What Are Active Galactic Nuclei?The central energy source and observations of active galaxies.
- NASA — Black Hole AnatomyAccretion disks, hot coronae, event horizons, and jets.
- Mortlock et al. (2011) — A luminous quasar at a redshift of z = 7.085The discovery of ULAS J1120+0641 and its early-universe setting.
- Wang et al. (2021) — A Luminous Quasar at Redshift 7.642J0313−1806, its inferred black hole mass, and evidence for an outflow.
- Yang et al. (2026) — Euclid: Discovery of 31 new quasars at 6.6 < z < 7.8The July 2026 sample, including a quasar at redshift 7.77.
- Inayoshi, Visbal & Haiman (2020) — The Assembly of the First Massive Black HolesSeed formation, fuel delivery, and early growth scenarios.
- Shen (2013) — The Mass of QuasarsAccretion timescales and the calibration and limitations of spectroscopic masses.
- Madau, Haardt & Dotti (2014) — Super-Critical Growth of Massive Black Holes from Stellar-Mass SeedsHow high inflow rates and low radiative efficiency can accelerate growth.
- Matthee et al. (2024) — Little Red Dots: an abundant population of faint AGN at z ∼ 5 revealed by the EIGER and FRESCO JWST surveysEarly observations of compact, red sources with broad hydrogen emission.
- Rusakov et al. (2026) — Little red dots as young supermassive black holes in dense ionized cocoonsElectron scattering as a source of line broadening in the objects studied.
- Juodžbalis et al. (2026) — A direct black-hole mass measurement in a little red dot at high redshiftGas dynamics in the strongly lensed source Abell 2744-QSO1.
- Davies et al. (2018) — Quantitative Constraints on the Reionization History from the IGM Damping Wing Signature in Two Quasars at z > 7Absorption signatures and the assumptions required to infer neutral hydrogen.
- Jiang et al. (2025) — AGN ruled out as the dominant source of cosmic reionizationA JWST-based upper bound on the AGN contribution near redshift 7.5.
- Faucher-Giguère & Quataert (2012) — The physics of galactic winds driven by active galactic nucleiHow nuclear winds transfer energy and momentum to galactic gas.
- Kormendy & Ho (2013) — Coevolution (Or Not) of Supermassive Black Holes and Host GalaxiesBlack hole relationships with bulges, velocity dispersion, and galaxy components.
- Lauer et al. (2007) — Selection Bias in Observing the Cosmological Evolution of the Mbh–sigma and Mbh–L RelationshipsWhy differently selected samples can give misleading evolutionary trends.
- Eilers et al. (2024) — EIGER VI: The Correlation Function, Host Halo Mass and Duty Cycle of Luminous Quasars at z ≳ 6The observed diversity of galaxy environments around early quasars.
- Shen et al. (2020) — The Bolometric Quasar Luminosity Function at z = 0–7Quasar demographics across time and their estimated ionizing contribution.
- ESA — LISA factsheetThe planned space mission and its massive black hole merger targets.
- Vestergaard & Peterson (2006) — Determining Central Black Hole Masses in Distant Active Galaxies and Quasars IICalibration of mass estimates from emission-line widths and luminosities.
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
- 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 — you are here
- Observing the First Billion Years