We present a new on-the-fly (OTF) mapping of CO(J=3-2) line emission with the Atacama Submillimeter Telescope Experiment (ASTE) toward the 8 x 8 (or 10.5 x 10.5 kpc at the distance of 4.5 Mpc) region of the nearby barred spiral galaxy M 83 at an effe
ctive 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 Halpha luminosity and Spitzer/IRAC 8 um 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 alpha, 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 alpha are 3 -- 10 in the inter-arm region. We found that GMAs with higher $alpha$ tend not to be associated with massive star forming regions, while other virialized GMAs are. Since alpha mainly depends on velocity dispersion of the GMA, we suppose the onset of star formation in these unvirialized GMAs with higher alpha are suppressed by an increase in internal velocity dispersions of Giant Molecular Clouds within these GMAs due to shear motion.
We present aperture synthesis high-resolution (~ 7 x 3) observations in CO(J=1-0) line, HCN(J=1-0) line, and 95 GHz continuum emission toward the central (~ 1.5 kpc) region of the nearby barred spiral galaxy M 83 with the Nobeyama Millimeter Array. O
ur high-resolution CO(J=1-0) mosaic map depicts the presence of molecular ridges along the leading sides of the stellar bar and nuclear twin peak structure. On the other hand, we found the distribution of the HCN(J=1-0) line emission which traces dense molecular gas (nH2 > a few x 10^4 cm^-3) shows nuclear single peak structure and coincides well with that of the 95 GHz continuum emission which traces massive starburst. The peaks of the HCN(J=1-0) line and the 95 GHz continuum emission are not spatially coincident with the optical starburst regions traced by the HST V-band image. This suggests the existence of deeply buried ongoing starburst due to strong extinction (A_v ~ 5 mag) near the peaks of the HCN(J=1-0) line and the 95 GHz continuum emission. We found that the HCN(J=1-0)/CO(J=1-0) intensity ratio R_HCN/CO correlates well with extinction-corrected SFE in the central region of M 83 at a resolution of 7.5 (~ 160 pc). This suggests that SFE is controlled by dense gas fraction traced by R_HCN/CO even on a Giant Molecular cloud Association (GMA) scale. Moreover, the correlation between R_HCN/CO and the SFE in the central region of M 83 seems to be almost coincident with that of the Gao & Solomon (2004a) sample. This suggests that the correlation between R_HCN/CO and the SFE on a GMA (~ 160 pc) scale found in M 83 is the origin of the global correlation on a few kpc scale shown by Gao & Solomon (2004a).
