The Giant, Gas-Rich, Low-Surface-Brightness Galaxy NGC 289

Using Australia Telescope Compact Array neutral hydrogen and Anglo-Australian Telescope optical observations, we investigate the distribution of luminous and dark matter in the giant, gas-rich, low-surface-brightness galaxy NGC 289. The observations show NGC 289 to have a high H 1-to-stellar mass ratio (M_HI/M_* ~ 0.4), and an extremely large H 1 radius (70 kpc, or ~ 13 disk scale-lengths), making the H 1 velocity field an excellent probe of a galaxy dark halo to an unusually large radius. Between ~ 10 kpc and 30 kpc the rotation curve dips by 14% of the maximum velocity. Warped disk and spiral density wave models are investigated to explain this dip and to determine the cause of significant velocity deviations in the vicinity of the H 1 spiral arms. The best-fit kinematic model has M / L_I = 2.1 M_sun/L_sun for the stellar disk, only slightly lower than the maximum disk value of 2.3. A dark matter halo of 3.5x 10(11) M_sun is present, which is ~ 3.5 times as massive as the combined stellar and gaseous components at the last measured point on the rotation curve.