cluster velocity dispersion

Picture a swarm of bees buzzing around a hive. Some fly toward you, some away, some sideways; they do not all move at the same speed. If you could measure how spread out their speeds are, you would learn something about how strongly the hive holds them. A galaxy cluster is a swarm of hundreds or thousands of galaxies orbiting their common center of gravity, and the spread in their speeds — the velocity dispersion — is a direct clue to how much mass is binding them together.

Astronomers measure each galaxy's speed along the line of sight from the Doppler shift of its light, then ask how widely those speeds scatter. A typical rich cluster has a velocity dispersion of roughly 1,000 kilometers per second — the galaxies are racing around at staggering speeds. The faster they move, the stronger the gravity needed to keep them from flying apart. So a large velocity dispersion implies a large total mass, by a relationship rooted in the virial theorem (mass grows roughly with the square of the dispersion).

This was the very first hint of dark matter. In 1933 Fritz Zwicky measured the velocity dispersion of galaxies in the Coma Cluster and found they were moving so fast that the visible galaxies provided only a small fraction of the gravity needed to hold the cluster together. He coined the term 'dunkle Materie' (dark matter) for the missing mass. His estimate was rough and was long ignored, but the basic conclusion has held up: clusters contain far more mass than meets the eye, most of it dark.