Aste CO (3-2) mapping toward the whole optical disk of M 83: Properties of inter-arm giant molecular-cloud associations

We present a new on-the-fly mapping of CO (J = 3-2) line emission with the Atacama Submillimeter Telescope Experiment toward the 8′ × 8′ (or 10.5 × 10.5 kpc at the distance of 4.5 Mpc) region of the nearby barred spiral galaxy M 83 at an effective resolution of 25″. Due to its very high sensitivity, our CO (J = 3 - 2) map can depict not only spiral arm structures but also spur-like substructures extended in inter-arm regions. This spur-like substructures in CO (J = 3-2) emission are well coincident with the distribution of massive star-forming regions traced by Hα luminosity and Spitzer/Infrared Array Camera 8 μm emission. We have identified 54 CO (J = 3-2) clumps as Giant Molecular-cloud Associations (GMAs) employing the CLUMPFIND algorithm, and have obtained their sizes, velocity dispersions, virial masses, and CO luminosity masses. We found that the virial parameter α, which is defined as the ratio of the virial mass to the CO luminosity mass, is almost unity for GMAs in spiral arms, whereas there exist some GMAs whose α are 3-10 in the inter-arm region. We found that GMAs with higher α tend not to be associated with massive star-forming regions, while other virialized GMAs are. Since α mainly depends on velocity dispersion of the GMA, we suppose that the onset of star formation in these unvirialized GMAs with higher α are suppressed by an increase in internal velocity dispersions of giant molecular clouds within these GMAs due to shear motion.