Abstract
Star formation in galaxies is quite mysterious, and occurs in various episodes. In large surveys of galaxies, we find that there is a bimodality between red galaxies, which no longer form stars, i.e. are dead, and blue galaxies, which are still very active, known as the "main sequence". This distinction corresponds roughly to Hubble's classification of red early-type elliptical galaxies and bluelate-type spiral galaxies. How is star formation abruptly halted, to explain two well-separated sequences, with a transition populated only by few galaxies, called " green valley " ? When we go back in time, i.e. look far into the Universe, we see that galaxies formed many more stars than today, at least twenty times more. But there was also a blue sequence, the main sequence, and already dead elliptical galaxies, the red sequence.
It has been observed that the fraction of red galaxies increases markedly with mass and environment. Several mechanisms have been put forward to explain the sudden drop in star formation in galaxies. The supply of cold gas from cosmic filaments can be slowly cut off. In very massive halos, the falling gas is heated in shocks, and can no longer form stars. The effect of the galaxy cluster environment can more rapidly smother galaxies, and sweep them of their gas by dynamic pressure, or tidal forces. On the other hand, the formation of galaxy bulges through interaction and merging will stabilize the gas: this is stabilization through morphology. The feedback phenomena of supernovae and active nuclei can also eject gas abruptly, but this is generally not permanent, as the gas returns to the galaxy. Observations have also shown that feedback effects can even be positive, i.e. active core activity can generate star formation.
