Simulation of position-velocity diagrams from assumed rotation curves and gas distribution:
(a) Rotation velocity is assumed to be constant, and therefore, the central velocity is finite.
(b) Four mass components (core, bulge, disk and halo) are assumed, having a very sharp central rise of rotation velocity. Note that the high velocities in the center and steep nuclear rise are hardly detected in PV diagrams with finite resolution. This simple simulation suggests that observed PV diagrams often miss a massive compact component in the center.