STAR-FORMATION IN THE SPIRAL ARMS OF NGC-4321 .2. H-I DISTRIBUTION AND KINEMATICS

We present H I 21 cm observations of the grand design spiral galaxy NGC 4321, as obtained with the VLA. From our original data, we have obtained data cubes with resolutions ranging from 13'' to 45''. The H I is observed to break up into small condensations at high resolution. These condensations, with typical masses of 3.3 +/- 0.7 x 10(7) M., are not confined to the arm regions. They may be unresolved at 15'' resolution and thus may have physical sizes smaller than 1 kpc. The total H I distribution is mostly confined to the radius of the optical disk, but a large though faint extension is seen in the H I data at 45'' resolution on the SW side of the disk. NGC 4321 is asymmetric in H I and may be called ''lopsided.'' We have derived a rotation curve which agrees fairly well with what was previously published but shows more detail due to the higher resolution of our new observations. The rotation curve does not decline within the radius of the disk, but important differences are seen between the behavior of the approaching and the receding sides. These differences are caused by deviations from circular motions in the outer disk that are probably due to a close passage of the companion galaxy NGC 4322, which may also be the cause of the observed asymmetry in the total H I distribution. Deviations from circular motion due to density wave streaming are seen in the inner disk. From skewing of the velocity contours in the central part of NGC 4321, the presence of a nonaxisymmetric potential is deduced. Near-infrared and Halpha images indicate that a bar is indeed present in this galaxy. The deviations from circular motions seen in the velocity field can be identified with gas streaming around the bar in elongated orbits, in broad agreement with theoretical predictions.