THE DARK HALO OF POLAR-RING GALAXY NGC 4650A - FLATTENED TOWARDS THE POLAR RING

We propose a new kinematical model for the prototype polar ring galaxy NGC 4650a, and revisit the question of its dark matter halo shape. We show that the visible stellar matter within the lenticular disk (appr oximate to 5 kpc) is sufficient to account for the observed rotation c urve and velocity dispersion along the SO major axis, with a constant mass-to-light ratio M/L(B) of 4, typical of such galaxy type. However, the high polar velocities observed at large radii (r > 5 kpc) require a substantial amount of dark matter (approximate to 50% of the total mass). This dark matter could be confined to a flattened annular regio n, co-existent with the HI component. A simple model where the surface density of dark matter sigma(DM) is proportional to the HI surface de nsity sigma(HI), with a factor 5 to 10, is able to account for all obs ervations. Using new near-infrared photometry of NGC 4650a together wi th existing data, we show that the dynamics of the polar ring itself m akes it similar to a late-type galaxy disk. Dark matter in this framew ork is confined in the outer parts, and the ratio between dark and lum inous surface densities (sigma(DM)/sigma(HI) approximate to 10) is typ ical of spirals.