H I observations of the spiral arm pattern speed in the late-type barred galaxy NGC 925

Abstract

H I observations with the VLA of the late-type barred galaxy NGC 925 show clear streaming motions along four spiral arms close to the minor axis. A transition from inward streaming to outward streaming is found, presumably marking the corotation resonance. The realm of inward streaming extends to very large radii, corresponding to 1.2R25 and 3 bar radii deprojected. This implies that most of the visible disk is inside corotation, although the H I disk extends beyond, at least to ∼2R25. As a result, either the bar and the spirals are rotating separately or the bar rotates very slowly compared with early-type bars. There is apparently no inner Lindblad resonance because of the slowly rising inner rotation curve, and there may be no outer Lindblad resonance if the disk ends first. © 1998. The American Astronomical Society. All rights reserved.