quenched galaxy

A quenched galaxy is a galaxy with minimal star formation, as opposed to the typical amount (that of a main sequence galaxy) or the extra formation within a starburst galaxy. Quenched galaxies are determined by color: a lack of the blue evident of early (hot, short-lived) stars is consistent with a lack of recent star formation. An obvious cause is that the galaxy has little gas from which to form molecular clouds, where stars are assumed to be formed. This could be caused by a gas blowout, e.g., from an active galactic nucleus (AGN). Galaxies within clusters are also observed to have less star formation on average, which is attributed to removal of their gas through ram-pressure stripping by the intracluster medium that a cluster galaxy is passing through. Ram pressure can also have such an effect on a satellite galaxy due to its host's galactic halo, which is most pronounced at its closest pass to its host (pericentric passage).

Another possible cause is that the molecular gas is too hot or turbulent (molecular cloud turbulence) to contract into stars, possibly the eventual outcome of a galaxy merger. A measure of the latter is whether the gas motion evident from Doppler shifts matches the stellar motion (misaligned gas).

The quenching (galaxy quenching) process itself may take a long time (slow quenching, with a timescale of a gigayear (Gy) or more, such a process classified as secular evolution) or a relatively short time (rapid quenching, 100 My or smaller timescale). The slower process may be attributed to a reduction in gas entering the galaxy through some means (e.g., if such entry is blocked) or to something preventing the gas from cooling sufficiently.