Necessity for High Accuracy Rotation Curves in Spiral Galaxies

In the last 20 years, rotation curves derived from H I kinematics obtained on radio synthesis instruments were used to probe the dark matter distribution in spiral and dwarf irregular galaxies. It is shown, with the aid of the Sd galaxy NGC 5585, that high resolution 2--D H II kinematics is necessary to determine accurately the mass distribution of spirals. New CFHT Fabry--Perot Hff observations are combined with low resolution Westerbork H I data to study its mass distribution. Using the combined rotation curve and best fit models, it can be seen that the (M/LB) of the luminous disk goes from 0.3, using only the H I rotation curve, to 0.8, using both the optical and the radio data. This reduces the dark--to--luminous mass ratio by 30%.