How do you calculate the radial velocity of a galaxy?
Rotational speed is directly correlated to a galaxy's luminosity.
Rotation speed is a measure of a spiral galaxy's total mass. This is known as the "Tully-Fisher relation." Radiation from the galaxy where matter is generally approaching us is blueshifted by the Doppler effect. Radiation on the other side is redshifted by a similar amount. The overall effect is that line radiation from the galaxy is "smeared out" or broadened, by a galaxy's rotation. The faster the rotation the greater the amount of broadening.