A dark-halo-dominated galaxy - NGC 5585

Studies of the light distribution, the H I kinematics, and the mass distribution of NGC 5585, a late-type spiral galaxy in the M101 group, are presented. From the photometry, an extrapolated central surface brightness B(0)_c_ = 21.94 mag arcsec^-2^ and a scale length α^-1^= 1.4 kpc are derived for the stellar disk. The H I distribution reaches a diameter of 12 arcmin, more than two times the optical diameter. The velocity field reveals a strong warp. The position angle is shown to vary from 45^deg^ to 55^deg^ in the outer parts, while the inclination is fairly constant at 52^deg^. The rotation curve, derived from this velocity field, reaches a maximum velocity of ~90 km s^-1^, extending out to 7 optical scale lengths. Combining the optical and kinematical data, mass models for NGC 5585 were built using both the "best-fit" and the "maximum-disk" approaches. For both models, the dark-halo component is by far the main contributor to the rotation curve and its basic parameters are fairly well constrained with a core radius r_c_= 3.0+/-0.2 kpc and a central density ρ_0_ = 0.054+/- 0.006 M_sun_pc^-3^. At the last observed point (r= 9.65 kpc), ~90% of the total mass is dark. Even the gaseous component is dynamically more important than the stellar disk component. In fact, despite being a fairly bright system with M^0^_T_(B) = - 17.5, NGC 5585 is similar to many fainter dwarf systems where the stellar disk is clearly not self-gravitating even in the inner parts.