Supermassive black holes can influence a galaxy when gas falling toward them makes the central region active. Radiation, winds and jets from this active galactic nucleus (AGN) can heat gas, move it away from star-forming regions or drive some of it out. Because stars form from cool gas, these changes can reduce star formation and affect how the galaxy grows. The process is important, but it is not a universal switch: its impact depends on the galaxy, its surroundings and the black hole’s activity.
What a black hole has to do with star formation
A supermassive black hole sits at the center of many large galaxies. Its gravity is intense, but the black hole itself is tiny compared with its host galaxy. Its strongest influence on galaxy-wide gas usually comes during an active phase, when gas is moving inward and accumulating around it. This bright, energetic central region is called an active galactic nucleus, or AGN.
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The distinction matters: the energy that affects surrounding gas does not come from material escaping from inside the event horizon. It is released by matter in the environment around the black hole as that matter accretes, and by associated outflows. An active nucleus can send energy into its surroundings as radiation, winds and jets. Those channels can heat gas, push it into different regions or, in some cases, carry it out of the galaxy.
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Stars form from gas that is sufficiently cool and concentrated to collapse under gravity. If gas is heated, dispersed or removed from star-forming regions, less may be available to make new stars. This can lower a galaxy’s rate of star formation or help it settle into a quieter state. It does not mean that every black hole turns star formation off, or that an entire galaxy’s gas supply is always expelled.
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How the feedback loop works
A useful way to understand the connection is as a cycle rather than a one-way event. Gas can feed both the black hole and star formation, while activity near the black hole can change the conditions for later star formation and accretion.
- Gas moves inward. Some gas in a galaxy reaches its central region. A portion may accrete toward the supermassive black hole, making the nucleus active.
- The active nucleus transfers energy. Accretion powers radiation, and the active region can launch winds or narrow, powerful jets. These channels carry energy and matter into gas around the nucleus and, in the case of jets, farther into the surrounding environment.
- The gas changes. Energy input can heat gas so it cools less readily, push gas away from dense star-forming areas or contribute to its removal. In galaxy clusters, jets can inflate bubbles in the hot gas between galaxies and help offset some of that gas’s radiative cooling.
- Star formation responds. If less cool gas is able to gather and collapse, the supply for forming stars can diminish. The strength and timing of the response depend on where the gas is and how the energy couples to it.
- Fuel becomes scarcer. A changed gas supply can also restrict later black-hole accretion. The activity may weaken as its fuel changes, while the galaxy’s future growth is affected by the same supply.
This cycle is called feedback because the output of the active nucleus affects the gas conditions that help determine its own future fuel supply, as well as the host galaxy’s star formation. It need not be a smooth or permanent shutdown: activity and gas conditions can change over time.
Radiation, winds and jets: different ways to affect gas
Researchers distinguish several feedback channels. They are useful categories, not mutually exclusive switches: a real system may show overlapping effects, and the importance of each depends on the black hole’s activity, the host and its environment.
| Feedback channel | How energy is carried | Possible effect on gas | What the observations establish |
|---|---|---|---|
| Radiative or quasar mode | Radiation from luminous accretion, often accompanied by wide-angle winds. | Can heat gas or help drive it outward. | A 2012 review described direct observations of this mode as more difficult to obtain than evidence for kinetic or radio-mode feedback, though observations of exceptional cases were accumulating. |
| Kinetic or radio mode | Mechanical energy carried by jets, often associated with lower-level AGN activity. | Jets can inflate bubbles and heat hot surrounding gas, reducing its cooling. | X-ray observations of massive elliptical galaxies and central galaxies in cool-core clusters provide evidence for this process. |
| Outflow-driven effects | Winds or fast streams of gas associated with an active black hole. | Can move gas through or out of a host, potentially regulating star formation and further accretion. | NASA has described ultra-fast outflows as a proposed connection between active black holes and galaxy-wide regulation; their effects should not be assumed to be identical in every system. |
The table’s labels describe routes by which energy and matter affect gas, not three independent explanations that can be ranked for every galaxy. To compare systems, astronomers also need to consider which gas phase is affected, how far from the nucleus the effect reaches, the source’s luminosity and activity history, and the surrounding environment. A visible outflow, an X-ray bubble and an indicator of young stars are different kinds of evidence; none alone gives a complete account of the whole feedback cycle.
What observations show
Different observations capture different stages of galaxy growth. Ultraviolet light can reveal young stars, X-rays can trace hot gas and bubbles, and submillimeter observations can identify vigorous star formation obscured by dust. These methods provide evidence for relationships between AGN activity and star formation, but the results apply to the systems and measurements studied rather than proving a universal rule.
Nearby elliptical galaxies: a GALEX survey
A 2006 NASA/JPL account of a GALEX ultraviolet survey reported results for more than 800 nearby elliptical galaxies. Larger galaxies were less likely to show evidence of young stars. The report described a pattern in which black holes appeared to reach a critical size relative to their host before star formation stopped. That is evidence consistent with feedback in the surveyed population, not proof that every black hole at some threshold switches off star formation in its galaxy. The survey’s size is specific to that study, not a census of all galaxies.
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AGN hosts when the universe was younger
A NASA Technical Reports Server record of a 2012 Nature study describes observations of AGN host galaxies from a time when the universe was 2–6 billion years old. The study combined submillimeter and X-ray observations. Rapid star formation was common among the hosts, but the most vigorous star formation was not observed around black holes above an X-ray luminosity of 1044 ergs per second in the sample discussed. The paper identified suppression driven by outflows as a key model prediction. The reported threshold is a finding in that sample and observational context, not a universal cutoff at which star formation must stop.
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Jets and bubbles in cool-core clusters
In massive elliptical galaxies and the central galaxies of cool-core clusters, jets provide a particularly clear setting for studying mechanical feedback. X-ray observations show bubbles in the hot gas around some central galaxies. The energy supplied by jets can heat that surrounding gas and reduce how much it cools. If less gas cools into a supply that can form stars, star formation can be limited. Fabian’s 2012 review describes direct evidence for this kinetic or radio-mode process, while noting that direct evidence for luminous quasar-mode feedback had historically been harder to obtain.
Outflows and the scales involved
NASA’s Webb explainer describes gas outflows as relevant to regulating star formation and notes that infrared observations can help researchers investigate their details. Infrared and submillimeter observations are useful because they add information about dusty regions that other wavelengths may not show in the same way. A NASA, ESA, CSA and STScI infographic, published in 2021 and updated in 2025, describes active supermassive black holes as affecting environments across hundreds of thousands of light-years and millions of years. Those are explanatory scales used in the infographic, not a measured boundary or duration that applies to every galaxy.
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Can a black hole stop stars from forming?
It can help suppress star formation, but “stop” is too absolute as a general description. If activity heats gas, makes it harder to cool, displaces it from star-forming regions or removes some of it, the galaxy may have less material able to form stars. Whether that change is sufficient to quench star formation depends on the gas and the system; the observations cited above do not establish a single threshold that applies to all galaxies.
Nor does a relationship between black-hole properties and young-star indicators, by itself, establish that black-hole activity is the only cause of reduced star formation. Galaxy evolution is also shaped by processes such as stellar feedback from supernovae and environmental influences. NASA’s Astro2020 white paper describes both AGN and supernova feedback as factors in galaxy evolution and states that the physics of AGN feedback is not well understood. The safest conclusion is that AGN feedback is an important influence in some systems, acting alongside other processes.
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There is broad agreement that feedback matters to galaxy evolution, but researchers have not settled every part of the mechanism or its quantitative importance. NASA’s program overview describes the exact relationship between black-hole growth and galaxy growth as underdeveloped. The relative contributions of radiation, jets and outflows are not resolved across all types of systems, and observations do not yet provide a single mode-by-mode accounting that can be applied to every galaxy.
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- How much energy couples to the gas? The effect depends not only on energy released but also on how it reaches gas in different phases and locations.
- When does the response occur? A galaxy’s star formation and its black hole’s activity both change over time; an observed association does not by itself specify the sequence or duration.
- Which feedback channel dominates? Jets are well observed in massive ellipticals and cool-core clusters, while direct observations of luminous quasar-mode feedback have been more challenging. This contrast is not a universal ranking for all galaxies.
- How much does feedback explain? AGN activity is one influence among several. The evidence does not justify assigning every transition to a quiet, low-star-forming state to the central black hole.
For that reason, claims about an observed population should stay attached to its sample, epoch and measurement. The GALEX result concerns nearby elliptical galaxies; the 2012 AGN-host result concerns a sample observed in submillimeter and X-ray bands at an earlier cosmic epoch; and X-ray bubbles trace hot gas in particular cluster environments. Together these observations support a role for feedback without proving that one mechanism produces the same outcome everywhere.
Why a small central object can shape a large galaxy
The black hole’s influence is not a matter of its physical size matching the galaxy. The relevant connection is that accretion can release energy near the center, and radiation or outflows can transfer some of that energy to gas on much larger scales. Gas is both raw material for stars and a potential fuel source for future black-hole activity. Altering its temperature or location can therefore affect both parts of a galaxy’s growth.
This is the central idea of co-evolution: the galaxy supplies gas that can feed its black hole, while the active black hole can change the gas available to the galaxy. The broad feedback picture is well established as a framework for understanding galaxy evolution; the strength, timing and detailed balance of its channels remain active questions.
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