Refined molecular gas mass and star-formation efficiency in NGC 3627

Abstract

We present a (CO)-C-13 (J = 1-0) mapping observation of the nearby barred spiral galaxy NGC3627, covering almost all the stellar disc, with the Nobeyama 45-m telescope. We found that the (CO)-C-13 (J = 1-0) morphology of NGC3627 is similar to the (CO)-C-12 (J = 1-0) map and the (CO)-C-12 (J = 1-0)/(CO)-C-13 (J = 1-0) intensity ratio is 10.7 +/- 0.4 on average. We also found that the ratio varies position-to-position in the galaxy and decreases with the galacto-centric distance. The ratio is especially high, similar to 20, in the bar and this result suggests that the molecular gas mass calculated from the (CO)-C-12 (J = 1-0) intensity with constant conversion factor is overestimated in the bar. As a result, the star-formation efficiency (SFE) in the bar, which is derived from (CO)-C-13 (J = 1-0) intensity, is similar to that in the other regions except for the two bar ends and the centre. We found that the SFE is higher in the bar ends than in the other regions. We discuss plausible reasons for the enhanced SFE in the bar: the kinetic temperature of the molecular gas, the spatial gap between the molecular gas peak and star formation and gravitational instability.