quenching
/ KWENCH-ing /
A galaxy making stars is like a fire that keeps burning as long as it has fuel. Quenching is what happens when that fire goes out: the galaxy stops forming new stars, more or less permanently. Its color shifts from the blue of hot young stars to the red of an aging population, and it becomes what astronomers vividly call 'red and dead'.
Stars form from cold, dense gas, so to quench a galaxy you have to either remove that gas, heat it so it cannot collapse, stop fresh gas from arriving, or use it all up faster than it is replenished. Several culprits do this. A galaxy can simply consume its gas in a starburst and not get more. Energetic feedback — from a central supermassive black hole (an active nucleus) or from supernovae and stellar winds — can blow gas out or keep it too hot to form stars. And environment matters: a galaxy falling into a dense cluster can have its cold gas stripped away by the hot intracluster medium ('ram-pressure stripping') or cut off from new supplies ('strangulation'). Big galaxies tend to quench from internal feedback; small satellites tend to quench from their environment.
Quenching matters because it explains one of the most striking patterns in the galaxy population: galaxies cluster into two groups — a 'blue cloud' of star-forming galaxies and a 'red sequence' of quenched ones — with relatively few in between, in the so-called 'green valley'. Understanding what flips a galaxy from blue to red, and why the most massive galaxies and those in dense environments are most likely to be quenched, is one of the central questions in galaxy evolution today.
A blue spiral falling into the Virgo cluster can have its cold gas swept out by the cluster's hot gas as it plunges through, like wind stripping leaves off a moving car. Robbed of fuel, it fades over a billion years into a red, quenched lenticular.
The star-making fire goes out — a galaxy turns red and dead.
Quenched does not mean a galaxy went dark; its old stars still shine for billions of years. It means the galaxy stopped making new stars, so its light reddens as the existing population ages and no fresh blue stars replace them.