An SDSS image of NGC 3198 has been superposed on the HI intensity map (white contours) at the same scale. Below -- and aligned with the scale -- is the HI rotation curve, with a two component model shown. The disk is assumed to have constant M/L ratio, in which case it clearly cannot generate the rotation curve. Instead, the contribution of a dark halo must be included.
The abstract of the paper by Van Albada et al (1985)