Okay, so, say, you're trying to measure the mass of the very center of the Andromeda galaxy (within 1 arcsecond of the center). You have a central velocity dispersion of 250 km/s and a rotation curve of 100 km/s (1 arcsecond away from the center). Now, from the virial equilibrium equation, M(r) = 5*sigma^2*R/G, where sigma is velocity dispersion. From the rotation curve equation, M(r) = v^2*R/G. Now, from this, I get that the mass in the center is much higher using the dispersion equation. Is this expected? Dispersion velocities tend to be higher in ellipticals than in spirals, and the centers of spiral galaxies (with the bulge) tend to be more elliptical in nature than the lobes of spiral galaxies.