Evidence indicates that galaxies live through a life cycle in which they start out gas-rich, consume their gas in star formation, and then stop making stars (quench). Why star formation falls so low is not clear.  It is true that there is less gas falling onto galaxies at late times, but not much gas is needed to keep them blue, yet observed star-formation rates after quenching are very low. Recent observations at z ~ 2 and z ~ 0 have identified sequences of galaxies that appear to be in the process of quenching, and studying these sequences has shed light on the physics of quenching.  Galaxies at z ~ 2 appear to fade on a "fast track" after experiencing feedback from central starbursts and black hole growth triggered by sudden "compaction" events.  Galaxies at z ~ 0 (today) in contrast appear to quench on a "slow track" owing to gradually increasing feedback from a much more slowly growing central BH.  Remarkably, quenching in both cases appears to be associated with reaching nearly the same central threshold stellar density.  Early gas-rich galaxies cross this 

threshold rapidly due to copious gaseous dissipation, whereas late, gas-poor galaxies must depend on slowly acting secular evolution processes, such as bars and spiral arms, that gradually bring gas to the center to make stars and (presumably) build the black hole.  On both tracks, it appears that central stellar buildup and black hole growth proceed together in nearly the same way despite the large difference in speed and physical processes.