Nobeyama Millimeter Array Observations of the Nuclear Starburst of M 83: A GMA Scale Correlation between Dense Gas Fraction and Star Formation Efficiency

We present aperture synthesis high-resolution (∼ 7×3) 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. Our 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 ×10 4 cm) 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 (AV ∼ 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 RHCN/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 RHCN/CO even on a Giant Molecular cloud Association (GMA) scale. Moreover, the correlation between RHCN/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 RHCN/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 1 scale shown by Gao & Solomon (2004a).