Molecular gas in the galaxy M 83 - II. Kinematics of the molecular gas

We present the kinematics of the molecular gas in the barred spiral galaxy M 83 (NGC 5236). The study is based on (CO)-C-12(J = 1-0 and 2-1) observations with the Swedish-ESO Submillimetre Telescope (SEST). Iso-velocity maps of the entire optical disk, 10' x 10' or 13 x 13 kpc, are produced. They show the pattern of an inclined, rotating disk, but also the effects of streaming motions along the spiral arms. A dynamical mass of about 6 x 10(10) M-. is estimated by fitting the rotation curve of an exponential disk model to these data. The gas constitutes about 13% of the disk mass. The pattern speed is determined from the residual velocity pattern. The locations of various resonances are discussed. The molecular gas velocity dispersion is determined, and a trend of decreasing dispersion with increasing galactocentric radius is found. A total gas (H-2 + H-1 + He) mass surface density map is presented, and compared to the critical density for star formation of an isothermal gaseous disk. The critical density is exceeded in the spiral arms, but not in the interarm regions. The locations of Giant Molecular Associations (GMAs) and H II regions are consistent with this scenario of dynamically induced star formation.