Active Galactic Nuclei and Quasars

An active galactic nucleus, or AGN, is a compact central source powered by accretion onto a massive black hole. The term covers a wide range of luminosities and appearances. A galaxy can retain its central black hole long after a bright episode fades.

Many familiar AGN host black holes with millions to billions of solar masses, but black hole activity also occurs in lower-mass systems. The presence of a large bulge is not a requirement.[16]

The flow changes with the feeding conditions. Weakly supplied nuclei can contain hot, tenuous gas that radiates inefficiently. At very high inflow rates, thicker flows, strong winds, and trapped radiation can become important.[1], [3]

A black hole’s mass therefore does not determine its brightness on its own. How much matter reaches the inner flow—and how that flow releases energy—also matters.

Gas arriving near a black hole usually has angular momentum, the quantity associated with its orbital motion. It cannot simply fall straight inward. Stresses within the flow transfer angular momentum between regions, allowing some material to move closer to the hole.

Magnetic fields and turbulence play a central role in this transport. As the gas moves inward, orbital energy can become heat, radiation, or energy carried by outflows.[2]

An efficiency of 10% is a useful example, not a constant shared by all black holes. A large-scale supply of gas also need not all reach the radiating inner flow.[1]

These names are not successive ages of a galaxy. They combine observational properties, and their categories can overlap.[5], [7]

Type 1 and type 2: what is visible in the spectrum?

Illuminated gas close to the nucleus often moves rapidly, producing broad emission lines. More extended gas generally produces narrower lines. In the standard obscuration picture, surrounding dusty material determines whether we see the central broad-line region directly.

The purple ovals are cut faces of the surrounding dusty material. Small clouds near the disk represent broad-line gas; clouds farther along the polar opening represent narrow-line gas. Arrows show the observer’s viewing direction. Both diagrams use the same simplified geometry, with sizes not to scale; the open region is not a jet.

Orientation explains many type differences, but real AGN also vary in their structures, accretion states, and amount of obscuration.[5], [8]

The familiar dusty “torus” need not be a smooth, rigid doughnut. Observations motivate clumpy distributions and combinations of dusty disks and outflows. Dust absorbs some central radiation and reradiates it in the infrared.[6]

Gas must move from galactic scales down to the much smaller accretion flow. Several processes can contribute, and each stage can interrupt or redirect the supply.

Reaching a central gas reservoir is therefore only part of the journey. There is no fixed rule that mergers make quasars while bars make Seyferts.[10]

Studies of AGN hosts find different merger associations depending on how nuclei and disturbances are selected. Some populations show strong connections; many active galaxies show no obvious recent major merger. Luminosity alone does not identify the trigger.[11]

Activity has several timescales

Rapid brightness variations trace changes in compact emitting regions. Longer episodes of enhanced accretion contribute to black hole growth, while extended gas and radio structures may preserve evidence of previous activity. These are different measures of an AGN’s history.[12], [13]

Changing-look AGN can gain or lose conspicuous broad emission lines while our viewing direction stays effectively fixed. Changes in the accretion state or obscuration help explain such behavior, showing why orientation alone is incomplete.[8]

Combining bands helps distinguish the AGN from its surroundings and estimate its total output. No single wavelength finds every active nucleus.[4], [5], [6], [7], [14]

Reverberation mapping: measure a light echo

When the central radiation changes, nearby gas or dust can respond after a delay. The rest-frame delay—corrected for cosmic time dilation—provides a characteristic light-travel distance to the responding material.

Combining the broad-line response time with the line’s velocity width yields a black hole mass estimate. The result depends on the geometry, viewing angle, and motions of the emitting gas; it is not simply a direct weighing of the hole.[13]

Spatially resolved spectroscopy adds information about gas and stars farther from the nucleus. Tracking both the central source and the surrounding galaxy helps connect rapid nuclear changes with effects on larger scales.

Powerful jets are not present in every AGN. A large-scale outflow can also contain gas swept up along the way, rather than consist entirely of material launched beside the accretion disk.[10], [14]

In the Blandford–Znajek mechanism, magnetic fields threading a rotating black hole allow rotational energy to contribute to jet power. This describes energy extraction through the surrounding electromagnetic field, not matter escaping from inside the event horizon.[3]

Jets and winds can operate together. Terms such as “radio mode” and “quasar mode” are useful shorthand in some models, but real sources need not fit two mutually exclusive states.[14]

AGN feedback means that energy and momentum from the active nucleus influence the surrounding material. It can change the gas available for star formation by heating it, moving it, or altering its ability to cool and collapse.

Detecting disturbed or outflowing gas does not establish permanent, galaxy-wide quenching. Gas can remain bound or return, and local enhancement can coexist with broader suppression. The long-term outcome depends on how the outflow couples to the different gas phases.[14]

A strong observational case: cavities in hot atmospheres

In many central galaxies of groups and clusters, radio jets inflate cavities in the surrounding X-ray-emitting gas. Their mechanical energy helps offset radiative cooling. Some gas still cools, and some star formation continues: the observations point toward regulation through repeated activity.[15]

The black hole–bulge connection

Black hole mass correlates with properties of classical bulges and elliptical galaxies, including the spread of stellar velocities, written σ. This is the host’s stellar velocity dispersion, a different measurement from the width of a nuclear gas emission line.

These relationships support connections between black hole growth and galaxy assembly, but they do not prove one feedback mechanism. Other galaxy components show different relationships, and shared growth histories or repeated mergers can also contribute.[16]

Galaxy simulations use feedback prescriptions to connect unresolved central physics with larger-scale gas. Their success is best judged against several observations together, including gas distributions, cooling, star formation, and black hole populations.[10], [14]

Quasar surveys reconstruct how the abundance of active black holes changed with time. Combining infrared, optical, and X-ray samples helps account for sources missed through obscuration or selection. The inferred history depends on luminosity as well as cosmic epoch.[19]

Some billion-solar-mass black holes were already active within the universe’s first billion years. Their growth depends jointly on the starting seed, its formation time, the continuity of fueling, radiative efficiency, and possible periods of very rapid accretion.[18]

For that early chapter, continue with Active Galactic Nuclei in the Young Universe and Supermassive Black Hole Seeds.

A backlight for intervening gas

As quasar light crosses space, intervening gas absorbs particular wavelengths. The resulting absorption features reveal matter between us and the source. At high redshift, they help researchers investigate the changing ionization state of the early universe.[18]

Multiple images, different arrival times

A foreground galaxy, or sometimes a group or cluster, can gravitationally lens a quasar into several images. A brightness fluctuation appears in those images at different times because the light follows different paths through the gravitational field.

These delays, combined with a model of the lens and matter along the line of sight, constrain a time-delay distance: a particular combination of cosmological distances that is sensitive to the expansion rate. Precision depends on the mass modeling; magnification alone does not provide an automatically precise luminosity distance.[20]

The delays used in lensing arise along separate paths across cosmic space. Reverberation delays arise when gas or dust near the quasar responds to changes in its own central radiation. Each method turns variability into a different kind of measurement.[13], [20]