1996A&A...305..763C

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 (≃5kpc) is sufficient to account for the observed rotation curve and velocity dispersion along the S0 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>5kpc) require a substantial amount of dark matter (≃50% of the total mass). This dark matter could be confined to a flattened annular region, co-existent with the HI component. A simple model where the surface density of dark matter σ_DM is proportional to the HI surface density σ_HI, with a factor 5 to 10, is able to account for all observations. Using new near-infrared photometry of NGC 4650a together with existing data, we show that the dynamics of the polar ring itself makes it similar to a late-type galaxy disk. Dark matter in this framework is confined in the outer parts, and the ratio between dark and luminous surface densities (σ_DM/σ_HI≃10) is typical of spirals.