AGN feedback is the way an active galactic nucleus changes its host galaxy by heating, stirring, or pushing out gas. In Astrophysics II, it explains why some galaxies stop making stars and how black holes and galaxies grow together.
AGN feedback is the effect a galaxy’s active galactic nucleus has on the gas around it, especially the gas that could otherwise cool and form new stars. In Astrophysics II, you usually meet it as the link between a supermassive black hole’s accretion and the larger galaxy’s evolution.
The core idea is simple: when gas falls toward a supermassive black hole, some of that material does not disappear quietly. A fraction of the infalling energy comes back out as radiation, winds, or jets. That energy and momentum can heat nearby gas, drive it away from the galactic center, or keep it too turbulent to collapse into new stars.
There are two broad ways this shows up. Quasar-mode feedback happens when the black hole is accreting rapidly and the nucleus is extremely luminous. In that case, radiation pressure and fast winds can blast through the interstellar medium. Radio-mode feedback happens at lower accretion rates, often with strong jets that carve channels through the hot gas around the galaxy and galaxy cluster.
The important part is not just that AGNs are bright, but that they change the gas supply for the whole galaxy. If the gas is heated enough, it cannot cool into dense clouds. If the gas is pushed out, the galaxy can lose the raw material for future star formation. That is why AGN feedback is often tied to quenching of star formation.
You can think of it as a control system. Gas feeds the black hole, the black hole releases energy back into the galaxy, and that energy changes the next round of gas inflow and star formation. Astrophysics II uses this feedback loop to explain why galaxies do not keep turning all of their gas into stars forever, and why the growth of the central supermassive black hole seems connected to the growth of the host galaxy itself.
AGN feedback shows up whenever Astrophysics II asks why galaxies look the way they do instead of just asking how stars form. Without it, many galaxy formation models would keep making galaxies that are too massive, too blue, and too full of stars compared with what telescopes actually find.
This term also links small-scale physics to large-scale structure. The accretion disk around a supermassive black hole is tiny compared with a galaxy, but the energy released there can reshape gas across thousands of light-years. That makes AGN feedback one of the best examples of how a compact object can influence an entire galactic environment.
You also need it to explain galaxy evolution over cosmic time. Observations show strong patterns like lower star formation in galaxies with more massive central black holes, and the existence of different galaxy populations that appear to be moving between active and quenched states. AGN feedback is one of the mechanisms used to explain that transition.
For class work, this term often sits at the center of cause-and-effect questions. If a prompt gives you hot halo gas, jets, or a drop in star formation, AGN feedback is usually part of the reasoning chain. If a model or simulation includes black holes, you should ask whether the AGN is heating gas, driving outflows, or suppressing cooling flows.
Keep studying Astrophysics II Unit 8
Visual cheatsheet
view galleryActive Galactic Nucleus (AGN)
AGN feedback starts with an AGN, since the central engine supplies the radiation, winds, and jets that affect the host galaxy. If you are identifying the term in a diagram or case study, first locate the active nucleus and then trace how its output interacts with nearby gas. The feedback is the consequence of AGN activity, not the AGN itself.
Quenching of star formation
AGN feedback is one pathway that can lead to quenching, especially when the gas reservoir is heated or expelled. Quenching is the outcome you look for in galaxy evolution problems, while AGN feedback is one mechanism that can cause it. If a galaxy stops forming stars after a strong outburst, the feedback story is often part of the explanation.
Cooling Flows
Cooling flows describe gas that should cool and move inward, often in galactic centers or clusters. AGN feedback can interrupt that process by reheating the gas or stirring it so it cannot collapse efficiently. In Astrophysics II, this connection is often used to explain why hot gas does not always turn into a star-forming fuel supply.
black hole-galaxy co-evolution
AGN feedback is one of the main physical links in black hole-galaxy co-evolution. The black hole grows by accreting gas, but that growth can change the galaxy’s gas supply, star formation rate, and long-term structure. When a problem asks why black hole mass and galaxy properties seem correlated, this relationship is part of the answer.
A quiz question or short-answer prompt might give you a galaxy with a bright nucleus, jets, and a falling star formation rate, then ask what physical process is at work. You would identify AGN feedback and explain whether the case is more like radiative heating, jet-driven outflow, or suppression of cooling gas.
In a problem set or data-analysis question, you may compare galaxies with different star formation rates and infer how AGN activity could be changing the gas supply. If a simulation output shows hot halo gas staying hot after an outburst, AGN feedback is the mechanism you use to justify the result. In discussion or essay work, the best move is to trace the chain from black hole accretion to energy release to changes in the interstellar medium to reduced star formation.
AGN heating is the thermal part of the process, where AGN energy raises the temperature of surrounding gas. AGN feedback is broader, because it includes heating plus momentum transfer, jets, winds, and the resulting changes in star formation and gas flow. If the prompt emphasizes the whole galaxy-wide effect, use AGN feedback. If it focuses only on gas temperature, AGN heating may be the tighter term.
AGN feedback is the way an active galactic nucleus changes the gas in its host galaxy through radiation, winds, or jets.
The main effect is to heat, stir, or remove gas, which makes it harder for that gas to cool and form new stars.
Quasar-mode feedback comes from very high accretion rates, while radio-mode feedback is tied to jets and lower accretion states.
This term connects supermassive black hole growth with galaxy evolution, especially quenching and galaxy-galaxy scaling trends.
If a galaxy has less star formation than expected, AGN feedback is one of the first mechanisms to check.
AGN feedback is the process where an active galactic nucleus affects its host galaxy by releasing energy and momentum into the surrounding gas. That energy can heat gas, push it outward, or keep it from cooling into new stars. In Astrophysics II, it is a major idea in galaxy evolution and black hole-galaxy co-evolution.
It can stop star formation by heating the gas so it cannot collapse, or by driving the gas out of the galaxy before it becomes dense enough to form stars. Even if some gas stays in the galaxy, turbulence from jets or winds can keep it from settling into cold clouds. The result is often quenching of star formation.
Quasar-mode feedback happens during high accretion and strong radiation output, so winds and radiation pressure do a lot of the work. Radio-mode feedback is usually associated with jets and lower accretion rates, and it often affects hot gas in the galaxy or cluster environment. Both are feedback, but they act through different energy channels.
No. AGN heating is one piece of AGN feedback, because it refers specifically to raising the temperature of nearby gas. AGN feedback is the bigger idea that includes heating, outflows, jets, and the resulting changes in star formation and gas supply. If you see a broader galaxy-evolution question, AGN feedback is usually the better term.