H-I STUDIES OF THE SCULPTOR GROUP GALAXIES .2. 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 H I disk extends to ∼1.5 optical diameter. The total H I mass is 6.8 X 108 Script M sign⊙. From the velocity field, a systemic velocity of 227 km s-1 and an inclination of 47° are derived. As for the position angle, it varies from ∼290° at r≃5′ to ∼305° 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 Vrot≃116 km s-1 at r≃4′, the velocity at the last observed point (r≃7.5′) is only Vrot≃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 rc≃2.7 kpc and centrally concentrated with ρ0≃0.04 Script M sign⊙ 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.