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Galaxy strangulation is a term for one general type of galaxy quenching (the cessation of star formation in a galaxy). Star formation stems from gas that has cooled (cold gas), and the term galaxy strangulation indicates mechanisms that prevent such gas from entering the galaxy, so the galaxy forms some stars from cold gas it already contains, but eventually ceases star formation. The opposing general type of galaxy quenching is star formation feedback, pushing the gas out of the galaxy, which presumably leads to a more rapid decline in star formation. The evidence regarding which has happened is metallicity within quenched galaxies: if many stars have the galaxy's peak metallicity, that is evidence that star formation ceased abruptly as if gas were removed from the galaxy, whereas if a range of metallicities showing fewer stars at each higher metallicity level suggests cold gas wasn't removed but disappeared as stars formed, without being replenished.