HI Observations of the Spiral Arm Pattern Speed in the Late-Type Barred Galaxy NGC 925

摘要

HI observations with the VLA of the late-type barred galaxy NGC 925 showclear streaming motions along four spiral arms close to the minor axis. Atransition from inward streaming to outward streaming is found, presumablymarking the corotation resonance. The realm of inward streaming extends to verylarge radii, corresponding to 1.3R25 and 1.7 bar radii deprojected. Thisimplies that most of the visible disk is inside corotation, although the HIdisk extends beyond, at least to 2R25. As a result, either the bar and thespirals are rotating separately or the bar rotates very slowly compared toearly-type bars. There is apparently no inner Lindblad resonance because of theslowly rising inner rotation curve, and there may be no outer Lindbladresonance if the disk ends first.