ASTE CO (3–2) Observations of the Barred Spiral Galaxy M 83: I. Correlation between CO (3–2)/CO (1–0) Ratios and Star Formation Efficiencies

We present CO(J=3-2) emission observations with the Atacama SubmillimeterTelescope Experiment (ASTE) toward the 5' x 5' (or 6.6 x 6.6 kpc at thedistance D = 4.5 Mpc) region of the nearby barred spiral galaxy M 83. Wesuccessfully resolved the major structures, i.e., the nuclear starburst region,bar, and inner spiral arms in CO(J=3-2) emission at a resolution of 22'' (or480 pc), showing a good spatial coincidence between CO(J=3-2) and 6 cmcontinuum emissions. We found a global CO(J=3-2) luminosity L'_CO(3-2) of 5.1 x10^8 K km s^-1 pc^2 within the observed region. We also found L'_CO(3-2) in thedisk region (0.5 < r < 3.5 kpc) of 4.2 x 10^8 K km s^-1 pc^2, indicating thatCO(J=3-2) emission in the disk region significantly contributes to the globalL'_CO(3-2). From a comparison of a CO(J=3-2) data with CO(J=1-0) intensitiesmeasured with Nobeyama 45-m telescope, we found that the radial profile ofCO(J=3-2)/CO(J=1-0) integrated intensity ratio R_3-2/1-0 is almost unity in thecentral region (r < 0.25 kpc), whereas it drops to a constant value, 0.6--0.7,in the disk region. The radial profile of star formation efficiencies (SFEs),determined from 6 cm radio continuum and CO(J=1-0) emission, shows the sametrend as that of R_3-2/1-0. At the bar-end (r ~ 2.4 kpc), the amounts ofmolecular gas and the massive stars are enhanced when compared with other diskregions, whereas there is no excess of R_3-2/1-0 and SFE in that region. Thismeans that a simple summation of the star forming regions at the bar-end andthe disk cannot reproduce the nuclear starburst of M 83, implying that thespatial variation of the dense gas fraction traced by R_3-2/1-0 governs thespatial variation of SFE in M 83.