Galaxy quenching

Galaxy quenching is the decline or cessation of star formation in a galaxy. It can result from processes that remove, exhaust, or prevent the supply of cold gas.

Ram-pressure stripping: The removal of a galaxy’s gas by pressure from the hot gas through which it moves. Motion through a dense environment can strip the cold gas needed for star formation.
Star formation rate: The mass of new stars formed in a system per unit time. A falling rate is the central observable signature of quenching.
Green valley (astronomy): The region between blue star-forming and red quiescent galaxies in diagrams of galaxy color and brightness. It contains galaxies whose colors suggest they are transitioning toward quiescence.
Starburst: A short-lived period of star formation far above a galaxy’s usual rate. A burst raises star formation, while quenching lowers or ends it.
Strangulation (galaxy evolution): The shutdown of star formation after a galaxy loses access to fresh gas while its existing gas is gradually consumed. It quenches gradually by cutting off replenishment rather than immediately removing all cold gas.
Molecular hydrogen: A molecule of two hydrogen atoms that forms the main cold-gas reservoir for star formation in many galaxies. Its abundance tracks the fuel available for ongoing star formation.
Post-starburst galaxy: A galaxy with evidence of a recent intense burst of star formation followed by a rapid decline. Its stellar spectrum records a rapid quenching event.
Quiescent galaxy: A galaxy with a low star formation rate relative to its stellar mass. Quiescence describes the resulting state, not the process that produced it.
Active galactic nucleus feedback: The influence of energy and momentum released by a galaxy’s central supermassive black hole on its surroundings. Energy from an active nucleus can heat or expel gas that would otherwise form stars.
Stellar mass: The total mass contained in a galaxy’s stars. Quenching likelihood and timing vary systematically with stellar mass.

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