NGC-4254 - A SPIRAL GALAXY WITH AN M=1 MODE AND INFALLING GAS

We present high-resolution H I and Halpha observations of the spiral g alaxy NGC 4254. The observations were obtained with the VLA and the Ma ryland-Caltech Fabry-Perot camera, respectively. NGC 4254 is unusual i n having a grand-design spiral structure with a strong m = 1 component for which there is no obvious cause in optical images. Our observatio ns reveal that, in addition to the usual galactic disk component, ther e are H I clouds superposed on and beyond the H I disk, at velocities up to 150 km s-1 from those established for the disk. The mass in thes e clouds is approximately 2.3 x 10(8) M., and they may be the remnants of an entity that was tidally disrupted by NGC 4254 and is now mergin g with it. The direct effects of the interaction between the cloud gas and the galaxy are limited to the region where the gas appears to be merging with the disk, where it may be causing a warp. But the indirec t effects of the infalling gas appear profound: it is the most likely cause for the unusual spiral structure of NGC 4254. If so, the m = 1 s piral structure of NGC 4254 is recent, and an internal amplification m echanism such as swing amplification has played a major role in its ev olution. Since NGC 4254 does not appear to be exceptionally deficient in dark matter and is apparently a normal Sc galaxy, the nature of the interaction appears important in determining the susceptibility of th e disk to various spiral modes (in particular the m = 1, 3, and 5 mode s of NGC 4254).