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 alpha-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-degrees to 55-degrees in the outer parts, while the inclination is fairly constant at 52-degrees. The rotation curve, derived from this velocity field, reaches a maximum velocity of approximately 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 rho-0 = 0.054 +/- 0.006 M . pc-3. At the last observed point (r = 9.65 kpc), approximately 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(T)0(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.