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

Abstract

Abstract We present CO ($J = 3$–2) emission observations with the ASTE toward the $5' \times 5'$ (or 6.6 kpc $\times$ 6.6 kpc at the distance $D$ = 4.5 Mpc) region of the nearby barred spiral galaxy M 83. We successfully resolved the major structures, i.e., the nuclear starburst region, bar, and inner spiral arms at a resolution of $22''$ (480 pc), showing a good spatial coincidence between CO and 6 cm continuum emissions. We found a global luminosity, $L'_{\rm CO(3-2)}$, of $5.1 \times 10^8$ K km s$^{-1}$ pc$^2$ within the observed region. We also found $L'_{\rm CO(3-2)}$ in a disk region ($0.5 < r < 3.5$ kpc) of $4.2 \times 10^8$ K km s$^{-1}$ pc$^2$, indicating that ($J = 3$–2) emission in the disk region significantly contributes to the global $L'_{\rm CO(3-2)}$. From a comparison of CO ($J = 3$–2) data with CO ($J = 1$–0) intensities measured with the Nobeyama 45-m telescope, we found that the radial profile of the CO ($J = 3$–2)$/$CO ($J = 1$–0) integrated intensity ratio, $R_{3-2/1-0}$, is almost unity in the central 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 same trend as that of $R_{3-2/1-0}$. At the bar-end ($r \sim 2.4$ kpc), the amounts of molecular gas and the massive stars are enhanced when compared with other disk regions, whereas there is no excess of $R_{3-2/1-0}$ and SFE in that region. This means that a simple summation of the star-forming regions at the bar-end and the disk cannot reproduce the nuclear starburst of M 83, implying that the spatial variation of the dense gas fraction traced by $R_{3-2/1-0}$ governs the spatial variation of SFE in M 83.