Question

The morphology-density relation is an empirical law that suggests that the higher the density of galaxies is the higher the fraction of ellipticals becomes in galaxy clusters/groups. In these clusters and groups of galaxies, all member galaxies appear to be bound by gravity that is caused by the presence of all these galaxies, that is, by their masses.

Question: Given that the galaxy groups/clusters are bound by gravity, explain why the morphology-density relation makes sense. Of course you are required to make a physically sound argument.

Answer 1

The early-type galaxies (the ellipticals) are a type of galaxies which are not sustained due to a continous rotation, the stars are all moving in random motions and there is no specific orbit, this randomness is created due to the high density of the galaxies, the gravitational forces are so effective and erratic in different regions that a particular star can't remain in a particular orbit of a star for long, as other set of star's gravitational field will be at play, and this leads to the random movement of stars, and this random movement makes the galaxies elliptic. In the case of spiral galaxies, the galaxies are sustained due to rotation, and this is possible because the galaxy density is less, which makes it possible for a particular planetary body to make a proper orbit around one of its stars, without getting perturbed by the gravity of others. Hence lower density galaxy regions makes the galaxies spiral. This is how the morphology-density relation holds.