The H I distribution and dynamics in two late-type barred spiral galaxies: NGC 925 and NGC 1744

Abstract

We present H I observations of NGC 925 and NGC 1744, two late-type barred spiral galaxies that are quite comparable in mass and optical morphology. NGC 925 is a very asymmetric galaxy with a strong spiral arm in the south and flocculent arms in the north. The rotation curves of the east and west sides of the galaxy are consistent with this asymmetry as well. In addition, NGC 925's dynamical center may be slightly offset (21″.5, or ∼1 kpc) from its optical center. The ratio of the H I to optical diameter of NGC 925 is small compared with other galaxies. We detected a ∼107 M⊙ H I cloud and streamer, with little or no stellar component, apparently interacting with NGC 925. While the interaction between NGC 925 and the cloud may be responsible for the observed asymmetries, given the weakness of the interaction we postulate that NGC 925 has suffered other gravitational encounters over the past few gigayears. NGC 1744, on the other hand, is an unperturbed galaxy having a textbook example of a velocity field for a disk galaxy. It has a symmetric, albeit weak, spiral structure. Both NGC 925 and NGC 1744 have small streaming motions (∼10 km s-1) associated with the bar and spiral arms.