ASTE CO(3-2) Mapping toward the Whole Optical Disk of M 83: Properties
  of Inter-arm GMAs
  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 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 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.
