What is the main reason elliptical galaxies cease forming stars (quenching)?

Answer

Exhaustion or expulsion of cold, molecular gas reservoirs

The central mystery of quenching boils down to the catastrophic exhaustion or expulsion of the cold, molecular gas, which is the essential fuel required for stellar nurseries.

What is the main reason elliptical galaxies cease forming stars (quenching)?
galaxiesastronomystar formationelliptical galaxiesgalaxy evolution