A comparison of optical and VLA rotation curves for Virgo spirals

From high-resolution optical CCD spectra, were formed rotation curves for eight spiral galaxies in the Virgo Cluster (NGC 4178, 4237, 4388, 4501, 4568, 4569, 4579, and 4654). A comparison with rotation curves formed from VLA observations shows excellent agreement for the galaxies abserved at 21 cm with 15 arcsec resolution. However, for galaxies observed at 45 arcsec resolution, the 21-cm velocities are lower than the optical velocities at virtually all radii covered by the optical velocities. The initial rise of the rotation curve is often well matched by the low-resolution observations, an unexpected result. A computer program that models the VLA observations confirms that observations at low spatial and velocity resolution will produce low rotation velocities, especially in the region of the turnover of the rotation curve. (Author)