The impact of the bar on dense gas and star formation in M 83

Abstract Although the relationship between molecular gas content and the star formation rate (SFR) has been extensively studied in nearby galaxies, it remains controversial whether the star formation efficiency (SFE) depends on galactic structure. In particular, whether the SFE is suppressed in the bar region compared with other structures, and the physical origin of this suppression, remain poorly understood. In this study, we investigate variations in the SFE and its physical drivers in the bar region of the nearby spiral galaxy M 83, using multiwavelength observations, each covering a single field of view (${\sim }60^{\prime \prime }$) toward the bar and spiral arm regions on a scale of 200 pc observed with ALMA. From the molecular gas surface density derived from $^{12}$CO(J = 2–1) and $^{13}$CO(J = 1–0), the dense molecular gas surface density derived from HCN(J = 1–0), and the SFR surface density determined from extinction-corrected H$\alpha$, we find that the SFEs in the bar region are roughly a factor of two lower than those in the spiral arm, indicating that the SFE is systematically suppressed in the bar. Moreover, we find that the SFEs of dense gas are lower in the bar than in the arm by a factor of about 0.35. These results suggest that not only the efficiency of converting bulk molecular gas into stars reduced in the bar, but the efficiency of star formation from dense gas is also lower. In addition, the CO line widths are systematically larger in the bar region and exhibit a negative correlation with both the SFE and the dense-gas SFE, consistent with the interpretation that enhanced turbulent motions hinder star formation. Although the analysis is limited to small regions of M 83, our results suggest that the suppression of the SFE is related to large-scale dynamical effects on the molecular gas, such as strong shocks induced by cloud–cloud collisions and/or shear, both driven by noncircular motions in the bar.