A rotating cloud of protogalactic gas would certainly contract more slowly than one without rotation and could only contract along the rotation axes, forming a disk as we see in spiral galaxies. It is plausible to suppose that a non-rotating, freely collapsing protogalaxy would reach higher densities at earlier times, using up its gas rapidly in a time of order a billion years. The environments of differing types of galaxies are also distinct. As we have discussed collisions between galaxies have a much more dramatic effect upon the gas than upon the stars because of the great interstellar distances. Collisions and mergers of galaxies can stimulate intense bursts of star formation as molecular clouds collide.
Collisions can also strip the gas out of galaxies.
It is quite likely that both effects play a role in determining the nature of galaxies. Certainly collisions and mergers must play a role in the formation of cluster elliptical galaxies. It seems almost certain, for example, that the giant elliptical galaxies seen at the centers of rich clusters formed as galaxies gradually accumulated at the center of the cluster's strong gravitational field. Our picture of the formation of galaxies remains hazy, however, and much remains to be understood. Bill Keel's Messier Image Gallery at the University of Alabama.