Figure: Velocity Distributions (or Velocity Profiles)

The top right panel shows a gaussian Velocity Profile (VP) as function of line-of-sight velocity. In this example, there are ~88 stars traveling at 50 km/s, ~60 stars at -100 km/s and ~13 stars at -200 km/s.

The two bottom panels illustrate the effect of changing the mean of the distribution (left) and changing the dispersion, or the width of the distribution (right).

On the left panel, the red group of stars has a mean velocity of +100 km/s, whereas the blue group of stars travels at the same speed but in the opposite direction (-100 km/s). In an inclined or edge-on spiral disk, this happens if you measure the velocity of stars on the major axis, on the right and on the left of the galactic center.

On the right panel, the red group of stars has a smaller velocity dispersion (~width of the distribution) than the blue group. This means that in the blue group some stars travel very fast (v > 200 km/s). Note that stars with similar velocities could not be found in the red group. More mass is required in the blue group to explain how, through gravitational forces, the stars could be accelerated up to such high velocities. If there is not enough visible (stellar) mass, one has to invoke the presence of some dark mass like black holes.

Credit: N. Cretton, Liceo Lugano 2