Another way astronomers attempt to explain how galaxies transform draws on a correlation between the shape of a galaxy and the density of the galaxy’s environment, that is, the number of neighboring galaxies within a certain distance. It turns out that ellipticals are commonly located in galaxy clusters while spirals are more likely to be found in isolation or on the fringes of clusters. This implies that galaxies are shaped by their surroundings, namely that a high-density environment could be responsible for quenching star formation and aiding in a galaxy’s transition from blue to red.
How might star formation be suppressed in practice? This is where those criminal cosmic acts I mentioned earlier come into play: galaxy harassment, stripping, strangulation, and cannibalism.
The Milky Way is a cannibal.
Galaxies in groups, clusters, and superclusters are constantly interacting via gravity, with the strength of their interactions driven by their relative sizes, motions, and distances from each other. Galaxies harass one another gravitationally in high-speed fly-bys and head-on collisions, each distorting the other’s structure into unrecognizable shapes.
Stripping primarily affects spirals “falling” face on toward the center of a cluster. The motion essentially acts as a wind, blowing the gas and dust within the galaxy in the opposite direction of its path, stripping it of fuel for ongoing star formation.
Strangulation occurs when galaxies fall edge first, rather than disk first. Stars, gas, and dust are pulled out in elongated structures both toward and away from the cluster. With the galaxy’s gas and dust dispersed, it’s much harder for star formation to continue.
Some galaxies take things to the extreme, ripping smaller galaxies apart entirely and accreting their stars and gas, effectively cannibalizing them. The Milky Way is such a cannibal.*
The role of dark matter in shaping the evolution of a galaxy is an entirely separate can of cosmic worms. Some physicists are probing the nature of the dark matter itself, but many astronomers believe we can learn just as much about dark matter by investigating its effects on galaxies. In fact, it was by studying galaxies that we discovered dark matter in the 1930s, when astronomers observed that galaxies rotate much faster than Newton’s Law allowed for, at least based on their visible matter. Current estimates require roughly six times more dark matter than visible matter to keep a given galaxy from flying apart.