Molecular gas in the galaxy M83. I - The molecular gas distribution

We present CO(1-0) and CO(2-1) Swedish-ESO Submillimetre Telescope (SEST) observations of the barred spiral galaxy M83 (NGC5236). The maps cover the entire optical disk. The CO emission is strongly peaked toward the nucleus, which breaks up into two separate components in the CO(2-1) data due to the higher spatial resolution. Emission from the bar is strong, in particular on the leading edges of the bar. The molecular gas arms are clearly resolved and can be traced for more than 360\degr . Emission in the inter-arm regions is detected. The average CO CO(2-1)/CO(1-0) line ratio is 0.77. The ratio is lower than this on the spiral arms and higher in the inter-arm regions. The arms show regularly spaced concentrations of molecular gas, Giant Molecular Associations (GMA's), whose masses are of the order 10^7 Msun. The total molecular gas mass is estimated to be 3.9*10^9 Msun. This mass is comparable to the total HI mass, but H_2 dominates in the optical disk. In the disk, H_2 and HI show very similar distributions, including small scale clumping. We compare the molecular gas distribution with those of other star formation tracers, such as B and H_alpha images.