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AGN feedback is the effect an active galactic nucleus has on gas in and around its host galaxy. An active nucleus is powered by matter falling toward a supermassive black hole, and it can transfer energy and momentum through radiation, winds and relativistic jets. Those channels may heat or expel cold gas, prevent hot halo gas from cooling, or, in some locations, compress clouds and trigger new stars.
The result is regulation rather than an automatic shutdown. Feedback can suppress galaxy-wide star formation, maintain a long-lived quiet state in massive systems, or stimulate localized star formation. Which outcome occurs depends on the energy channel, gas phase, host mass, environment and timing.
What an active galactic nucleus does
A supermassive black hole is normally invisible because the black hole itself emits no light. When gas, dust or a disrupted star spirals inward, gravitational energy is converted into intense radiation and mechanical energy before the material crosses the event horizon. This luminous, compact region is the active galactic nucleus, or AGN.
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Stars need relatively cold, dense molecular gas. Feedback regulates star formation by changing whether that reservoir exists, whether it can collapse, and whether fresh gas can cool from the halo and replenish it. A black hole growing through accretion is therefore not itself feedback: feedback is the physical interaction between the active nucleus and nearby or distant gas.
The two broad feedback pathways
| Pathway | Main energy carrier | Gas it affects most directly | Typical regulation mechanism | Most secure observational setting |
|---|---|---|---|---|
| Ejective, often called quasar or radiative/wind feedback | Radiation pressure and fast multiphase winds | Interstellar gas, including ionized, neutral and sometimes molecular phases | Heating, accelerating or expelling gas so less cold material remains for stars | Observed outflows in luminous AGN and high-redshift galaxies, with causal impact still requiring interpretation |
| Preventative, often called radio or kinetic feedback | Mechanical power in radio-emitting jets | Hot circumgalactic and intracluster gas | Inflating cavities and offsetting radiative cooling, so less gas condenses back onto the galaxy | X-ray cavities and heated gas in massive galaxies, groups and cool-core clusters |
| Locally positive feedback | Jet-driven shocks and pressure fronts | Dense clouds near jet paths or radio structures | Compressing clouds, sometimes helping atomic gas become molecular and form stars | Localized star formation associated with jet structures; generally less efficient than ordinary disk star formation |
“Quasar mode” and “radio mode” are useful shorthand for these behaviors, not guaranteed, mutually exclusive stages. An AGN can change accretion state, and more than one channel can operate during the same episode.
How ejective quasar or wind feedback works
Radiation and winds couple to the interstellar medium
At high accretion luminosity, radiation pressure pushes on dust and electrons, while gas close to the nucleus can be launched as a wind. Shocks form when the wind runs into the surrounding interstellar medium. Those shocks heat gas, stir it turbulently and can accelerate it to speeds that exceed the host galaxy’s ability to retain it.
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If cold or warm gas is removed faster than it is replenished, the molecular clouds that feed star formation are depleted. Heating can also keep gas from reaching the densities needed for gravitational collapse. The immediate effect can be a decline in star formation even before the entire gas supply has escaped.
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What observers measure
Ionized outflows are identified through emission lines whose velocity components are offset or broadened relative to the host galaxy. In the early Universe, reviews discuss lines including [O III], H-alpha, H-beta, [S II] and [N II]. Neutral outflows can be traced through blueshifted absorption from Na I D, Mg II and Fe II. Ionized components are often faster but contain less mass than neutral components; both may be parts of the same multiphase flow.
Highly ionized winds seen in X-ray spectra of luminous AGN have reported velocities of approximately 0.1–0.2 times the speed of light, according to the 2015 review by Andrew King and Ken Pounds. That figure describes the detected wind component in those observations, not every AGN outflow.
Nearby-galaxy studies summarized by Françoise Combes report molecular-outflow loading factors of roughly 1 to 10. A loading factor is the outflow mass-loss rate divided by the host’s star-formation rate; it is a ratio for the systems analyzed, not a universal property of AGN.
Why one observed outflow is not proof of permanent quenching
An outflow snapshot does not reveal automatically how much gas will escape, how long the flow will last or whether gas farther out will later cool and return. Geometry matters: a narrow cone can clear one direction while leaving a large disk reservoir untouched. Estimates of mass, energy and kinetic coupling also depend on density, ionization, clumping and the assumed distance from the nucleus.
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King and Pounds note that episodic winds may be detectable for only a few percent of their lifetimes. The winds easiest to observe are therefore not necessarily representative of the complete feedback cycle. A measured outflow is evidence that energy is coupling to gas; it is not, by itself, a measurement of the star formation that was prevented.
How preventative radio or kinetic feedback maintains a quiet galaxy
Jets inflate cavities in hot halos
At lower accretion rates, a larger share of the AGN’s power can emerge mechanically through collimated, radio-emitting jets. When a jet enters hot halo or intracluster gas, it inflates a bubble and displaces the X-ray-emitting atmosphere. X-ray images can show these cavities, while surrounding temperature and density measurements reveal the response of the gas.
The jet does not need to eject all halo gas to reduce star formation. By supplying heat and stirring the atmosphere, it can offset radiative losses. Gas then cools more slowly, so less of it falls toward the central galaxy, condenses into a cold phase or replenishes the molecular reservoir.
The cooling-and-refill problem
Removing gas once is often insufficient for long-term quiescence. A massive galaxy sits in a reservoir of circumgalactic gas that can cool after an outburst fades. Preventative feedback addresses this refill route by stabilizing the halo over repeated episodes.
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A.C. Fabian’s cluster review describes a roughly continuous heating/cooling balance and reports an order-of-magnitude reduction in radiative cooling and subsequent star formation in the cool-core cluster context he examined. That factor belongs to that cluster setting; it is not a universal reduction for every galaxy.
The 2026 theoretical review of quenching likewise identifies maintenance of the circumgalactic cooling balance as central to lasting quiescence in massive groups and clusters. Its evidence assessment is strongest for high-mass halos and explicitly more limited for relativistic jets in lower-mass halos. A jet detected in a small galaxy therefore cannot automatically be assigned the same long-term effect seen in a cool-core cluster.
When AGN feedback promotes star formation
Feedback is not defined by a negative outcome. A jet-driven shock can compress a dense cloud, raise its pressure and help convert atomic gas into molecular gas. Combes reviews examples of stars forming near jet-associated structures. This is sometimes called positive feedback, although the resulting star formation is generally less efficient than in an undisturbed galactic disk.
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A step-by-step picture of regulation
- Accretion switches the nucleus on. Gas moving toward the supermassive black hole releases radiation and mechanical energy.
- Energy selects a coupling route. Radiation and winds interact first with the interstellar medium; jets carry power into the halo and, in groups or clusters, the surrounding atmosphere.
- Gas changes phase or motion. It may be ionized, heated, shocked, accelerated, mixed or compressed. Some material becomes unbound, while some remains in the halo.
- The cold reservoir changes. Ejection removes fuel directly; heating delays the conversion of hot halo gas into cold clouds; compression can create a localized molecular reservoir.
- Star formation responds on multiple scales. The host-wide rate may fall, remain steady because cooling is balanced, or rise in small regions near a jet.
- The outcome depends on replenishment. If the halo cools again after the outburst, suppression may be temporary. Repeated maintenance heating is more important for durable quiescence in massive systems.
What counts as convincing evidence?
Different observations answer different parts of the causal question. A spectrum can establish that gas is moving, but not necessarily that the galaxy will remain quenched. An X-ray cavity directly demonstrates that a jet has done work on hot gas, but translating that heating into a precise star-formation reduction still requires a model of cooling and replenishment.
| Observation | What it establishes | What remains uncertain |
|---|---|---|
| Broad or shifted ionized emission lines | Ionized gas is moving at unusual velocities relative to the host | How much mass is involved, whether it escapes and how long star formation is affected |
| Blueshifted neutral absorption | Neutral material lies between the observer and the continuum source and is moving toward the observer | The total three-dimensional mass and the fate of the flow |
| Molecular outflow measurements | Cold, potentially star-forming gas is participating in an outflow | Conversion factors, geometry, duration and the fraction that actually leaves the galaxy |
| X-ray cavities around a radio AGN | A jet has displaced hot gas and deposited mechanical energy | The exact reduction in future cooling and the response of the cold disk |
| Young stars along a jet or shock front | Compression and local star formation occur near the AGN structure | Whether the local increase outweighs suppression elsewhere in the host or halo |
The clearest direct examples of energy coupling are X-ray cavities and heated gas in cool-core clusters. Outflows are observed in many other environments, including the early Universe, but connecting a short-lived observation to a durable, galaxy-wide change in star-formation history is harder.
Quenching versus regulation
Quenching means strong suppression or cessation of star formation. Regulation is broader: it includes any AGN-driven change in the rate, timing or location of star formation, whether the net change is negative, neutral or positive.
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Why the effect varies from galaxy to galaxy
- Host mass: Deep potential wells can retain wind material, while lower-mass galaxies may lose gas more readily. Evidence for jet-driven maintenance is strongest in massive groups and clusters.
- Gas phase: Ionized, neutral, molecular and hot X-ray gas trace different parts of the flow and cannot be substituted for one another.
- Geometry: Biconical winds and narrow jets can affect a small fraction of a disk while leaving other regions untouched.
- Accretion state: High-luminosity episodes tend to emphasize radiation and winds; lower-luminosity episodes often emphasize jets, but the labels overlap.
- Timing: A rapid ejection, a cooling delay and a repeated maintenance cycle operate on different timescales.
- Environment: A cluster provides a large, hot atmosphere that can be heated repeatedly; an isolated low-mass galaxy may have no comparable reservoir.
The practical takeaway
AGN feedback regulates star formation by controlling the supply and state of gas. Winds and radiation can heat or expel interstellar fuel, producing rapid suppression when the coupling is strong. Jets can act more preventatively by heating a halo so it does not cool and refill the galaxy, a mechanism best established in massive groups and clusters. Jet pressure can also trigger limited star formation where it compresses clouds.
The scientifically careful conclusion is therefore conditional: AGN feedback is a major route by which galaxies change their star-formation histories, but an outflow detection is not a guaranteed quenching measurement, and no single feedback mode operates identically in every host.
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