THE GIANT, GAS-RICH, LOW-SURFACE-BRIGHTNESS GALAXY NGC-289

Using Australia Telescope Compact Array neutral hydrogen and Anglo-Aus tralian Telescope optical observations, we investigate the distributio n of luminous and dark matter in the giant, gas-rich, low-surface-brig htness galaxy NGC 289. The observations show NGC 289 to have a high H- I-to-stellar mass ratio (M (HI)/M()approximate to 0.4), and an extrem ely large H I radius (70 kpc, or similar to 13 disk scale-lengths), ma king the HI velocity field an excellent probe of a galaxy dark halo to an unusually large radius. Between similar to 10 kpc and 30 kpc the r otation curve dips by 14% of the maximum velocity. Warped disk and spi ral density wave models are investigated to explain this dip and to de termine the cause of significant velocity deviations in the vicinity o f the H I spiral arms. The best-fit kinematic model has M/L-1=2.1 M./L . for the stellar disk, only slightly lower than the maximum disk valu e of 2.3. A dark matter halo of 3.5x10(11)M. is present, which is simi lar to 3.5 times as massive as the combined stellar and gaseous compon ents at the last measured point on the rotation curve. (C) 1997 Americ an Astronomical Society.