What is a “quenched” galaxy?

Several galaxies, at some point, stop making new stars. When this star formation is shut down they are known as “quenched” galaxies. 

The processes that cause galaxies to “quench” — that is, cease star formation — are not well understood. Now, using a large sample of around 70,000 galaxies, researchers led by University of California astronomers Behnam Darvish and Bahram Mobasher may have an explanation for why galaxies stop creating stars. 

The team notes that external mechanisms like multiple gravitational encounters with other galaxies and the dense surrounding environment, result in material being stripped away from the galaxy and halt the supply of cold gas to the galaxy, thus strangling the galaxy of the material needed to produce new stars over a prolonged period of time. 

Internal factors like the presence of a black hole could also play a part in this.