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

Abstract

We present aperture synthesis high-resolution (∼7'' × 3'') observations in CO (J = 1--0), HCN (J = 1--0), and 95GHz 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 a nuclear twin peak structure, whereas the distributions of the HCN (J = 1--0) emission that traces dense molecular gas (n_H2 > a few × 10⁴cm^-3) and the 95GHz continuum emission that traces massive starburst show nuclear single-peak structures. The HCN (J = 1--0) and the 95GHz continuum peaks are not spatially coincident with the optical starburst regions traced by the HST V-band image, suggesting the existence of deeply buried ongoing starburst due to strong extinction (A_V ∼ 5 mag) near these peaks. We found that the HCN (J = 1--0)/CO (J = 1--0) intensity ratio, R_HCN/CO, correlates well with the extinction-corrected star formation efficiency (SFE) in the central region of M 83 at a resolution of 7''.5 (∼160pc). This suggests that SFE is controlled by a 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 SFE in M 83 seems to be almost coincident with that among the Gao and Solomon (2004a, ApJ, 606, 271) sample. This suggests that the correlation between R_HCN/CO and the SFE on a GMA (∼160pc) scale found in M 83 is the origin of the global correlation on a few kpc scale shown by Gao and Solomon (2004a).