Bimodal Galaxy Distribution Why do red galaxies tend to be more massive than blue galaxies? Do we have an explanation for this observation?
My understanding is that the matter is not settled, definitively. There is a chain of logic that goes something like this:


Why is any galaxy red? Because it isn't forming any new, high mass, short lived, blue stars.
Why would a galaxy not form stars? Because the gas in the galaxy is either not dense enough or not cold enough to gravitationally collapse into new stars.


There are two main explanations put forward for the inability of the gas in really large galaxies to collapse and form stars. First, the most massive galaxies tend to be found in clusters of galaxies, and the gas in the galaxy clusters is too hot to collapse and form stars. Second, when large galaxies of comparable mass merge simulations suggest that the merger process heats up the gas in the galaxies, cutting off star formation.