H i Studies of the Sculptor Group Galaxies. VII. Implications on the Distribution and Nature of Dark Matter in Groups

Results from the large-scale mapping of the H I gas in the Sculptor group are presented. From the kinematical analysis of the members, a mean value of (M/L_B_)_global_ = 9 +/- 5 M_sun_/L_sun_ (at the last observed velocity point) is found for the individual galaxies. This is a factor 10 smaller than the (M/L_B_)_dyn_ = 90 +/- 20 M_sun_/L_sun_ derived from a dynamical study of the whole group. Mass models, fitted to the rotation curves, suggest a mean density of dark matter (DM) of ~5x10^-3^M_sun_ pc^-3^ in the halos of the galaxies. This is more than three orders of magnitude greater than the maximum DM density in the intergalactic medium, would all the DM be uniformly distributed in the group. This large density contrast implies that DM is more highly concentrated around the luminous galaxies than uniformly distributed in the group. Under the assumption that the Sculptor group is a virialized system and that all the mass is associated with the galaxies, an upper limit of ~40 kpc is derived for the size of the dark halos present in the five late-type spirals of the group. Finally, the implications for the nature of the DM are explored. It is shown that warm nonbaryonic matter is an unlikely candidate. Different baryonic forms for the dark matter are considered and normal low-mass brown dwarfs or massive black holes remain as the most suitable candidates.