Accurate parameters of the mass distribution in spiral galaxies. I. Fabry-Perot observations of NGC 5585

Using the example of the Sd galaxy NGC 5585, it is shown that high-resoluti on, two-dimensional H II kinematical data are necessary to determine accura tely the parameters of the mass (luminous and dark) distribution in spiral galaxies. New Canada-France-Hawaii Telescope Fabry-Perot H alpha observatio ns are combined with low-resolution (20 ") Westerbork H I data to study its mass distribution. Using the combined rotation curve and best-fit models, it can be seen that (M/L-B)* 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 in NGC 5585 by similar to 30% through increasing the dark matter halo core radius by nearly the same amo unt. This shows the importance of the inner, rising part of the rotation cu rve for the accurate determination of the parameters of the global mass (lu minous and dark) distribution and suggests that such a fine tuning of the r otation velocities, using high-resolution, two-dimensional H II kinematics, is necessary to look at correlations between the parameters of the dark ma tter component and other properties of galaxies.