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; the resulting radiation, winds and jets transfer energy and momentum to that gas. The interaction can heat or expel cold fuel, prevent hot halo gas from cooling back into the galaxy, and sharply reduce star formation. In other conditions, jet pressure can compress clouds and trigger small, localized bursts of stars. “Feedback” therefore means regulation, not an automatic shutdown.
What an active galactic nucleus is—and what “feedback” adds
A supermassive black hole is normally dark except for the effects of its gravity. When gas forms a rapidly rotating accretion disk and spirals inward, friction and magnetic processes convert part of its gravitational energy into an extremely bright central source: the active galactic nucleus (AGN). The black hole does not need to consume all of the inflowing material. Radiation can push on surrounding gas, while magnetic fields can launch broad winds or narrow, relativistic jets.
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AGN feedback begins when that output couples to gas outside the immediate accretion disk. It may act on the interstellar medium (the gas inside the galaxy), the circumgalactic medium (the extended halo around it), or the very hot atmosphere of a galaxy group or cluster. The important question is not simply whether a black hole is growing, but where its energy goes, which phase of gas it reaches, and whether the affected gas can cool and return.
Feedback is broader than quenching. Quenching means strong, sustained suppression or cessation of a galaxy’s star formation. Regulation includes any change—negative or positive—in the rate, location or duration of star formation.
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The two main pathways: ejective and preventative feedback
Astronomers often use “quasar mode” or “radiative mode” for luminous, wind-rich activity and “radio mode” or “kinetic mode” for jet-dominated activity at lower accretion rates. These are useful descriptions of dominant behavior, not compulsory, mutually exclusive stages that every galaxy follows in a fixed order. A single episode can involve more than one carrier of energy.
| Broad mode | Primary carrier | Gas most directly affected | Typical timescale | How star formation changes | Best-established setting |
|---|---|---|---|---|---|
| Radiative or quasar/wind | Radiation pressure and fast winds | Cold and warm interstellar gas; ionized, neutral and sometimes molecular phases | Rapid: can deplete or heat a reservoir during an active episode | Immediate suppression when fuel is heated or expelled; the lasting effect depends on replenishment | Luminous AGN and rapidly growing black holes |
| Radio or kinetic | Mechanical energy in jets and inflated bubbles | Hot circumgalactic or intracluster gas | Long-term maintenance over repeated or continuing episodes | Slows cooling and reduces the supply of gas that would otherwise feed the disk | Massive ellipticals, groups and cool-core clusters |
| Jet-triggered (a possible outcome of kinetic activity) | Shock or pressure compression | Dense atomic and molecular clouds near a jet | Localized and comparatively short-lived | Can increase star formation locally, often less efficiently than in an undisturbed disk | Individual jet–cloud interactions |
How quasar and wind feedback removes star-forming fuel
Radiation heats and pushes the interstellar medium
In a high-luminosity AGN, ultraviolet and X-ray radiation ionizes gas and transfers momentum when photons scatter from electrons or are absorbed by dust. The gas can become too warm to collapse into molecular clouds. Radiation can also help drive a larger-scale outflow when it acts together with a wind launched close to the black hole.
Fast winds carry energy beyond the nucleus
X-ray spectra of luminous AGN show highly ionized winds moving at approximately 0.1–0.2 times the speed of light, according to the 2015 review by Andrew King and Ken Pounds. As such a wind plows into the surrounding interstellar medium, it can form a shocked shell. Energy-driven expansion heats ambient gas and can accelerate cooler material; momentum-driven interaction is more effective when radiative losses remove much of the shock energy.
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The observable outflow may therefore contain several phases moving at different speeds. The 2026 observational review of galaxy quenching describes ionized outflows identified through kinematically offset emission lines, including [O III], H-alpha, H-beta, [S II] and [N II]. Neutral outflows are traced by blueshifted absorption features such as Na I D, Mg II and Fe II. Ionized gas is often faster but less massive than neutral gas. Molecular outflows are also plausible quenching agents at very high redshift, although a detected flow does not prove that the host will remain quenched.
Why one outflow does not automatically equal permanent quenching
Removing gas from the central few kiloparsecs can interrupt star formation quickly, but the galaxy may later draw on a larger halo reservoir. Whether the gas escapes the halo, falls back in a fountain, or is replaced by fresh accretion determines the duration of suppression. Outflow rates and kinetic powers are inferred from density, geometry, covering factor and velocity; each introduces uncertainty. King and Pounds also note that powerful winds may be visible for only a few percent of their lifetimes, so observed samples capture brief episodes rather than a complete feedback cycle.
How radio jets prevent the halo from refilling the galaxy
X-ray cavities are a direct view of mechanical coupling
Jets launched by a low-accretion-rate AGN can inflate enormous bubbles in the surrounding hot plasma. In X-ray images of cool-core groups and clusters, these bubbles appear as cavities coincident with radio emission. The jet has done work against the hot atmosphere, driving shocks, sound waves and turbulence and distributing mechanical energy over a large volume.
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Maintenance heating interrupts the cooling supply line
Hot halo gas radiates energy and would otherwise cool, condense, fall inward and replenish the galaxy’s star-forming reservoir. Repeated jet heating reduces that radiative cooling. A.C. Fabian’s 2012 observational review describes a roughly continuous heating/cooling balance in cluster environments and reports an order-of-magnitude reduction in radiative cooling and subsequent star formation in the context it examined. That factor belongs to those cool-core cluster systems; it is not a universal multiplier for every galaxy.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteThis is called preventative feedback because it prevents new fuel from arriving rather than merely ejecting fuel that is already in the disk. The 2026 theoretical review argues that one-time removal is insufficient for long-lived quiescence if the surrounding halo can cool and refill the galaxy. Stabilizing the circumgalactic medium is consequently central to durable quenching in massive systems.
Environment and halo mass set the confidence level
The clearest maintenance-feedback evidence comes from massive ellipticals, galaxy groups and clusters, where hot atmospheres are bright in X-rays and cavities can be measured. The same review finds the case less conclusive for relativistic jets in lower-mass haloes: their atmospheres are fainter, their gas supply is more easily disturbed, and the balance between escape, cooling and reaccretion differs. A claim established for a cluster should not be generalized automatically to a Milky-Way-sized halo.
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Feedback can also trigger star formation
Jet compression is not always destructive. When a jet encounters a dense cloud, its pressure can compress the cloud, raise its density and promote conversion from atomic to molecular gas. Françoise Combes’s 2017 review discusses star formation near jet-associated structures produced this way. The resulting activity is usually localized and less efficient than star formation in normal galaxy disks: only part of the compressed material collapses, while other gas is heated or shredded.
The positive and negative outcomes can occur in the same object. A jet may trigger a few young stellar clusters along its path while simultaneously heating the wider halo and reducing the galaxy’s total fuel supply. The sign of the net effect depends on cloud density, jet power, geometry, metallicity, cooling time and the scale over which star formation is measured.
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What observations actually demonstrate
Different tracers answer different causal questions. A spectrum showing an outflow establishes that gas is moving, not how much future star formation it prevented. A cavity establishes that a jet deposited energy in hot gas, but translating that energy into a galaxy-wide star-formation history still requires models of cooling and replenishment.
| Observation | What it measures | What it supports | What remains uncertain |
|---|---|---|---|
| Offset ionized emission lines | Velocity and excitation of warm ionized gas | An AGN-associated ionized outflow | Total mass, duration and whether cold fuel is permanently lost |
| Blueshifted Na I D, Mg II or Fe II absorption | Neutral gas moving toward the observer | A neutral component of an outflow | Three-dimensional geometry and eventual fate of the gas |
| Molecular-line emission or absorption | Cold, dense gas participating in a flow | Potential removal of the phase most directly used to form stars | Conversion between line signal and mass; true coupling to long-term quenching |
| X-ray cavities and hot-gas disturbances | Mechanical work by radio jets | Direct energy transfer to a hot halo | How efficiently heating offsets cooling at lower halo mass |
| Star-formation history after an AGN episode | Timing of stellar growth relative to activity | A possible population-level consequence | Causation, because mergers, environment and internal processes can act at the same time |
The 2018 review by Harrison and colleagues emphasizes substantial observational and theoretical uncertainty when kinetic coupling efficiencies are derived from outflow measurements. Geometry, clumping, inclination and uncertain gas densities can change an inferred mass or energy rate by large factors. This is why an impressive velocity or mass-loading estimate should be presented with its assumptions rather than as a universal property of AGN.
Numbers that need their context
- 0.1–0.2 c: the approximate velocity of highly ionized winds seen in X-ray spectra of luminous AGN, as summarized by King and Pounds (2015); it is not the speed of every outflow phase.
- A few percent: the estimated fraction of an episodic wind’s lifetime during which it may be detectable under the observability constraints discussed by King and Pounds. A survey therefore samples an intermittent phenomenon.
- An order of magnitude: Fabian’s (2012) reported reduction in cooling and subsequent star formation for the continuously heated cool-core cluster context he reviewed, not a prediction for all host galaxies.
- 1 to 10: the range Combes (2017) summarizes for molecular-outflow loading factors in nearby galaxies, where the loading factor is the outflow rate divided by the star-formation rate. The range varies with how each rate is measured.
Why the same black hole can regulate different galaxies differently
- Gas phase: Ionized, neutral, molecular and hot plasma respond on different timescales and may not share the same velocity.
- Coupling and geometry: A jet pointed into a low-density channel can miss most of the disk, while a wind intersecting dense gas can deposit energy efficiently.
- Host structure: A deep stellar potential can retain gas that would escape a small galaxy; a compact disk can also shield part of its cold reservoir.
- Halo mass and environment: Groups and clusters possess hot atmospheres that can be heated for maintenance, whereas lower-mass haloes have less certain jet-coupling evidence.
- Timing: Star formation and AGN luminosity vary over different cycles. A galaxy observed during an outflow may still form stars from gas that was already present, or may show a delayed decline later.
- Competing processes: Stellar winds, supernovae, mergers and environmental stripping can remove or heat gas at the same time, complicating attribution to the AGN.
A practical way to interpret an AGN-feedback claim
- Identify the carrier. Is the evidence radiation, a fast wind, a radio jet, or a combination?
- Name the gas phase and tracer. Distinguish an optical emission-line outflow from a neutral absorption feature, a molecular flow or an X-ray cavity.
- Set the scale. Decide whether the reported effect is nuclear, disk-wide, halo-wide or cluster-wide.
- Check the timescale. Ejection can be rapid; preventative heating requires continued or repeated energy input and concerns future replenishment.
- Separate observation from inference. “Gas is moving” is an observation. “The galaxy will stay quenched” is a model-dependent conclusion.
- Specify the environment. A result in a cool-core cluster cannot automatically stand in for a low-mass isolated galaxy.
- Allow both signs. Look for evidence of suppressed global star formation and for localized, jet-triggered star formation rather than assuming one outcome.
AGN feedback versus black-hole growth
Black-hole accretion and feedback are related but not synonymous. Accretion describes matter reaching the black hole and the luminosity produced near it. Feedback describes the fraction of that output that couples to gas on larger scales. Two AGN with similar luminosity can have different galactic effects if their winds, jets or orientations differ. Conversely, a low-luminosity jet can have a major cumulative effect in a dense cluster atmosphere even when it contributes little visible light.
For the same reason, the presence of an AGN does not establish that star formation has stopped. Some hosts continue forming stars during an active episode; others experience only a temporary decline. Strong, durable quenching requires both a reduction in the cold reservoir and a mechanism—often halo maintenance in massive systems—that prevents rapid replacement.
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AGN feedback regulates star formation through two complementary routes. Radiative output and winds can heat or expel the interstellar gas that stars need immediately. Jets can heat a surrounding halo, slow cooling and prevent fresh fuel from returning over much longer periods. In dense clouds, however, the same jet pressure can trigger localized star formation. The most secure direct evidence comes from observed outflows and X-ray cavities; the size and duration of the resulting change in a galaxy’s star-formation history remain dependent on gas phase, geometry, halo mass, environment and time.
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