Distribution and Kinematics of Molecular Gas in Barred Spiral Galaxies. I. NGC 3504

We present the results of CO mapping observations of the barred spiral galaxy NGC 3504 with the Nobeyama 45-m telescope. The distribution of the molecular gas shows offset ridges which correspond to the distribution of HII regions along the bar. The velocity perpendicular to the bar decreases abruptly at the ridge. The velocity change implies that the molecular gas changes the direction of its motion to parallel to the bar at the ridge. Since the position angle of the major axis of the bar and the line of nodes are almost the same in NGC 3504, an upper limit to the pattern speed of the bar can be derived directly from the radial velocity. The resultant upper limit is 41 km s _1 kpc -1 , which is much smaller than that derived based on an assumption that the corotation radius is located at the end of the bar (77 km s _1 kpc -1 ). The corotation radius derived from our upper limit is more than two-times larger than the length of the semi-major axis of the bar in NGC 3504. two ILRs in NGC 3504. It is suggested that the intense star formation in NGC 3504 is occurring at the IILR, as suggested by numerical simulations.