Stacking analysis of 12CO and 13CO spectra of NGC 3627: Existence of non-optically thick 12CO emission?

We stacked ¹²CO and ¹³CO spectra of NGC 3627 after redefining the velocity axis of each spectrum of the mapping data so that the zero corresponds to the local mean velocity of the ¹²CO spectra. The signal-to-noise ratios of the resulting spectra are improved by a factor of up to 3.2 compared to those obtained with normal stacking analysis. We successfully detect a weak ¹³CO emission from the interarm region where the emission was not detected in the individual pointings. We compare the integrated intensity ratios I_{^{12}{CO}}/I_{^{13}CO} among six characteristic regions (center, bar, bar-end, offset, arm, and interarm). We find that I_{^{12}{CO}}/I_{^{13}CO} in the bar and interarm are higher than those in the other regions by a factor of ∼ 2 and I_{^{12}{CO}}/I_{^{13}CO} in the center is moderately high. These high I_{^{12}{CO}}/I_{^{13}CO} ratios in the bar and center are attributed to a high intensity ratio (T_{^{12}{CO}}/T_{^{13}CO}), and that in the interarm is attributed to a high ratio of the full width at half maximum of spectra (FWHM_{^{12}{CO}}/FWHM_{^{13}CO}). The difference between FWHM_{^{12}CO} and FWHM_{^{13}CO} of the interarm indicates the existence of two components, one with a narrow line width (∼ FWHM_^{13CO}) and the other with a broad line width (∼ FWHM_^{12CO}). Additionally, the T_{^{12}{CO}}/T_{^{13}CO} ratio in the broad-line-width component of the interarm is higher than the other regions. The high T_{^{12}{CO}}/T_{^{13}CO} in the center and bar and of the broad-line-width component in the interarm suggest the existence of non-optically thick ¹²CO components. We find that more than half of the ¹²CO emissions of the interarm are likely to be radiated from the diffuse component. Our result suggests that the use of a universal CO-to-H₂ conversion factor might lead to an overestimation of molecular gas mass and underestimation of star-formation efficiency in the interarm by a factor of a few.