What is the primary ingredient required for star formation that, when exhausted or ejected, causes a galaxy to quench?

Answer

Vast reservoirs of cold gas

Star formation requires the gravitational collapse of vast reservoirs of cold gas to ignite nuclear fusion. The stopping of this supply leads to the end of star production.

What is the primary ingredient required for star formation that, when exhausted or ejected, causes a galaxy to quench?
Spacegalaxiesastronomycosmologydead galaxies