H I studies of the Sculptor group galaxies. II - NGC 7793

A study of the H I properties and mass distribution of the Sculptor group galaxy NGC 7793, from VLA observations, is presented. The HI disk extends to~ 1.5 optical diameter. The total H I mass is 6.8 X 10^8^ M_sun_. From the velocity field, a systemic velocity of 227 km s^-1^ and an inclination of 47^deg^ are derived. As for the position angle, it varies from ~290^deg^ at r~5' to ~305^deg^ at the last observed point (r~7.5'). This study confirms that NGC 7793 is one of the very few cases with a truly declining rotation curve. After reaching a maximum of V_rot_ ~116 km s^-1^ at r~4', the velocity at the last observed point (r~7.5') is only V_rot_~88 km s^-1^. While a model with "no dark matter" cannot be totally excluded, adding a dark halo gives a better representation of the observed rotation curve over the whole radius range. The halo, however, is fairly small with r_c_ ~ 2.7 kpc and centrally concentrated with ρ ~0.04 M_sun_ pc^-3^. This central density is a factor 10-15 higher than what is normally seen in other late-type spirals. Finally, the dark matter distribution is shown to be closely related to the H I gas distribution.