| Abstract | The gas-rich S0 galaxy NGC 4203 was mapped in the 21-cm H I line with a spatial resolution of 24"x44" (αxδ) and a velocity resolution of 33 km s^-1^. The H I gas distribution shows an, almost circular, inner ring, in circular rotation in the plane of the optical disk, and an extended outer component. A warped-disk model for the H I distribution is adopted, which contains an intrinsically circular outer pseudo-ring, rotating in a plane inclined 34^deg^ (or 87^deg^) to the optical disk. Such a model gives an essentially flat rotation curve out to 10 disk scale lengths. The total mass- to-light ratio for a simple spherical mass model is M_T_/L_B_= 8 M_sun_/L_sun_ within R=23 kpc. A detailed disk/bulge/halo mass model fit to the rotation curve shows the presence of a dark halo and a dark to luminous mass ratio of ~4. Various scenarios for the origin and evolution of the gaseous component in NGC 4203 are discussed. No firm conclusion is reached, but the preferred scenario is an external origin for the H I gas in NGC 4203, i.e. accretion from another galaxy. |
